Java Notes

Learn Java www.shareittips.com

OOPS Object Oriented Programming Structure Features of OOPS Objects / Instance Classes Inheritance Polymorphism ●

●

Overloading / Static Polymorphism / Compile-time polymorphism Overriding / Dynamic polymorphism / Run-time polymorphism

Encapsulation Abstraction

OOPS Objects – Real time entity Classes – Blueprint of an object. It gives structure to the objects Inheritance – Deriving base class properties and behaviour to the child class Polymorphism - One object in different forms Encapsulation - Hiding the irrelevant details to the irrelevant entity Abstraction – Revealing the relevant details to the relevant entity.

Structure of Java Compiler

Source File

Parser

Type Checker

Class File Writer Class File

class C { @NonNull Object field; C(@NonNull Object p) { field = p; } @NonNull Object get() { return field; }

Comments

}

Error

History of Java The original name of Java was Oak, and it was developed as a part of the Green project at Sun Microsystems. Java was conceived by James Gosling, Patrick Naughton, Chris Warth, Ed Frank, and Mike Sheridon at Sun Microsystems in 1991. Sun formally announced the Java SunWorld in 1995.

Features of JAVA Object Oriented Programming language Platform Independent Robust Portable Scalable Multithreaded Architecturally neutral Secured

Components of Java

JDK – Java Development Kit JRE – Java Run-time Environment JVM - Java Virtual Machine

Java Virtual Machine Ø

The Java Virtual Machine provides a platformindependent way of executing code, by abstracting the differences between operating systems and CPU architectures.

Ø

JVM is Write Once-Run Anywhere (WORA) software.

Ø

JVM forms part of large system JRE.

Ø

Ø

JVM's main job is interpreting java byte code and translating this into actions or OS calls. JVM is OS dependent which makes java source code as machine independent.

JRE and JVM

Purpose of Features of OOPS Classes - Classification Encapsulation - maintainability, flexibility and extensibility. Polymorphism – one method will behave differently. Inheritance – Reusability, Easier updates, Do not break what is already working.

Structure of JAVA Program Package declaration; Import statements Class declaration { Variable declaration/ definition; method declaration / definition; }

Main method class { public static void main(String[] args) { // Object instantiation m = new (); } }

CamelCase Convention Variables Method Class Package Constants

- myVariable - myMethod() MyClass - mypackage - MYCONSTANT

Types of Variable

Local variable (variable inside method or block) Class Variable (Static Variable) Instance Variable ( variable inside the class) Note: Local variables require explicit initialization. Instance variables are initialized automatically.

Variable Initialization Variable Value byte 0 short 0 int 0 long 0L float 0.0F double 0.0D char '\u0000' boolean false All reference types null

Constructor It is a Spl. Method Purpose: To initialize the class members Features: Same name as that of the class name No return type including VOID Can have access specifier Can be overloaded Constructors are NOT inherited Invoked automatically whenever the object is created. The no. of time of invocation depends on no. of object created

Arrays Group data objects of the same type, in a contiguous block of memory. An array is an object; it is created with new. Can be declared as a primitive type or Class type. Array index starts with 0. You cannot resize an array. You can use the same reference variable to refer to an entirely new array.

Array Declaration and Instantiation Array declaration [] ; int[] myArray; • Constructing an array = new []; myArray = new int[5];

Initializing an Array Explicit initialization in one line. [] = { }; Primitive array: ***************** int[] myArray = { 1 , 2 , 3 , 4 , 5 } ; Reference Array: ******************** Object[] objArr = { new Pizza(), new Pizza(), null };

Initializing an Array Explicit initialization can also be done using array subscripts. int[] myArray = new int[3]; myArray [0] = 10; myArray [1] = 20; myArray [2] = 30;

Inheritance Deriving the parent class properties and methods to the child class. Types of inheritance: Single level inheritance - one super and one sub class Multilevel inheritance - The sub class of one level forms the super class of another level Multiple inheritance [ not supported by java] - many super and one sub class

Hierarchical inheritance - one super and many sub classes

Hybrid inheritance - multiple and multi level combined.

Inheritance Two important concepts Generalization - Up the hierarchy Specialization - Down the hierarchy Purpose : Reusability (without changing its identity) Syntax: class extends { * }

IS-A & HAS-A relationship When you want to know if one thing should extend another, use the IS-A test. Eg : Triangle IS-A Shape Do not apply inheritance if the subclass and super class do not pass the IS-A test. Is-a relationship can be described in Java keyword extends. The IS-A relationship – Unidirectional

IS-A & HAS-A relationship When two classes are related, but not through inheritance, (for example, one class has a reference to another class) then you say that the two classes are joined by HAS-A relationship. Has-a relationship can be described in Java code as member fields. Note: Code reuse is also best achieved by aggregation when there is no is-a relationship

IS-A & HAS-A relationship class Car { } class BMW extends Car => IS-A R/S { boolean auto_gear = “true” => Has-A R/S }

Polymorphism Polymorphism (from Greek, meaning “many forms”) is a feature that allows one interface to be used for a general class of actions that is one interface with multiple methods. Types: Overloading => Ad-Hoc Polymorphism - Same method name with different set of parameters. - Early Binding Overriding => True polymorphism - Same method name with same set of parameters. - Late Binding

Types of Overloading

Function Overloading Constructor Overloading NO operator Overloading in Java Rules : No. of parameter should change Datatype of the parameter should change Sequence of passing the paramter should change.

Overriding The overridden method in the superclass is NOT inherited by the subclass, and the new method in the subclass must uphold the following rules of method overriding: The new method definition must have the same method signature (i.e., method name and parameters) and the same return type. Overridden Methods Cannot Be Less Accessible. A subclass cannot override fields of the superclass, but it can hide them. Works only with inheritance. Constructors cant be Overridden. Super keyword is used to invoke an overridden method in the superclass.

this() and super() call for constructor this() construct is used to implement local chaining of constructors in the class when an instance of the class is created. The this() call invokes the constructor with the corresponding parameter list. super() method is used to invoke the IMMEDIATE base class constructor. This allows the subclass to influence the initialization of its inherited state when an object of the subclass is created. this() and super() call must occur as the first statement in a constructor.

Example : this() and super() class GParent { int a,b,c; GParent() { System.out.println("From gparent"); } GParent(int a,int b) { //this(a,b,100); this(); System.out.println("a= "+a+" b = "+ b); } GParent(int a,int b,int c) { this.a=a; this.b=b; this.c=c; System.out.println("a= "+a+" b = "+ b + " c= "

Example : this() and super() class Parent extends GParent { int x,y; Parent() { System.out.println("From parent"); } Parent(int x,int y) { super(x,y); this.x=x; this.y = y; System.out.println("x= "+x+" y = "+ y); }

Example : this() and super() class Child extends Parent { Child() { super(23,343); System.out.println("From child"); } } class SuperEx { public static void main(String[] a) { //Parent p = new Parent(12,23); Child d = new Child(); }

instanceof operator Use instanceof to test the type of an object. Restore full functionality of an object by casting. Example: public void doSomething(Employee e) { if ( e instanceof Manager ) { Manager m = (Manager) e; } // rest of operation }

Static Keyword It’s a Access Modifier The static keyword is used as a modifier on variables, methods, and nested classes. The static keyword declares the attribute or method is associated with the class as a whole rather than any particular instance of that class. Thus static members are often called class members, such as class attributes or class methods.

Static Keyword A static method can access only the static variable. But the normal variable can access both static and normal variable. Static members will get loaded into the memory only once. Static members are subjected to change common for all the instance. NO NEED FOR OBJECT to access the static member.

Static Variable Example class StatEx { int i=10; static int j = 20; public void normalMethod() { System.out.println("Instance var = " + i++); System.out.println("Static var = " + j++); }

}

public static void main(String arg[]) { StatEx s1 = new StatEx(); StatEx s2 = new StatEx(); s1.normalMethod(); s2.normalMethod(); }

Static Method Example class StatEx { int i=10; static int j = 20; public static void staticMethod() { //System.out.println("Instance var = " + i++); //illegal System.out.println("Static var = " + j++); }

}

public static void main(String arg[]) { staticMethod(); staticMethod(); }

Static Initializer Example class StatEx1 { static int counter; //static initializer static { counter=10; System.out.println("Static block invoked "+counter); } public static void sMethod() { System.out.println("Static method" + counter++); }

Static Initializer Example class StatEx { public static void main(String arg[]) { System.out.println("from main"); StatEx1.sMethod(); StatEx1.sMethod(); } }

Final Keyword Variable become Constant Method cant be Overridden Class cant be inherited Note: All final variable need Explicit initialization

Wrapper Class Conversion of primitive types to the object equivalent done through wrapper classes. Allow objects to be created from primitive types. Wrapped values are immutable (Cant modify) . To wrap another value, you need to create another object. Wrapper class are present in java.lang package All the wrapper classes are declared final.

Primitive Data Types and Corresponding Wrapper Primitive Data Classes Wrapper Constructor Type

Class

Arguments

boolean

Boolean

boolean or String

byte

Byte

byte or String

char

Character

char

short

Short

short or String

int

Integer

int or String

long

Long

long or String

float

Float

double or float or String

double

Double

double or String

All the wrapper classes except Boolean and Character are subclasses of an abstract class called Number, whereas Boolean and Character are derived directly from the Object class.

Boxing and Unboxing Converting a value type to a reference type is known as Boxing. Converting a reference type to a value type is known as UnBoxing. int x=10; Integer n = new Integer(x); //Boxing int y = n.intValue(); //UnBoxing

AutoBoxing and AutoUnboxing Example: int x=10; Integer n = x; int y = n;

//AutoBoxing //AutoUnBoxing

Methods to Extract the Wrapped Values Method

Class

public boolean booleanValue()

Boolean

public char charValue()

Character

public byte byteValue()

Byte, Short, Integer, Long, Float, Double

public short shortValue()

Byte, Short, Integer, Long, Float, Double

public int intValue()

Byte, Short, Integer, Long, Float, Double

public long longValue()

Byte, Short, Integer, Long, Float, Double

public float floatValue()

Byte, Short, Integer, Long, Float, Double

public double doubleValue()

Byte, Short, Integer, Long, Float, Double

Methods to Convert Strings to Primitive Types Wrapper Class

Method Signature

Method Arguments

Boolean

static boolean parseBoolean(…)

String

Character

Not available

Byte

static byte parseByte(…)

String, or String and radix

Short

static short parseShort(..)

String, or String and radix

Integer

static int parseInt(…)

String, or String and radix

Long

static long parseLong(…)

String, or String and radix

Float

static float parseFloat(…)

String

Double

static double parseDouble(…)

double or String

Wrapper Conversion methods Primitive xxxValue() To convert Wrapper to primitive •

Primitive parseXxx(String)

To convert a String to a primitive •

Wrapper valueOf(String)

To convert a String to a Wrapper

Object Class Root class of Java => Object equals() method = > Check only values toString() method =>Check value & reference hashCode() => return the address of the object Object Class is in java.lang package.

Abstract Class Class which have a abstract method (method without definition) is abstract class. Can have normal method and variable Cant be instantiated Methods may or may not be implemented by the child class. Use abstract keyword to declare a class as abstract. Abstract method cannot be private or final A class can inherit only one abstract class. NEED RELATIONSHIP between classes

Interface Interface is to support multiple inheritance in Java. Interfaces should be implemented by the child class Can have only abstract method. Interface contain only constants.NO Variables. All the fields are public static final in nature. Interfaces cant be instantiated A class can implement many interfaces. All the methods should be implemented by the child class. NO NEED FOR RELATIONSHIP

enum Assigning a integral constant to a symbolic name => enum Use enum when you want a variable to hold only a predetermined set of values. You use the keyword enum and not class to declare an enum. Just like a class, an enum can have constructors, methods, and fields. An enum cannot be declared within a method. You cannot instantiate an enum with the new operator.

enum The enums do not participate in class hierarchy: they cannot extend and they cannot be extended. You cannot directly call an enum constructor. An enum may have a main() method and therefore can be executed by name from the command line like an application.

Enum Example1 enum Edge { TOP,BOTTOM,LEFT,RIGHT }; class MyClass { public static void main(String[] a) { Edge e = Edge.TOP; int i = e.ordinal(); System.out.println(e); System.out.println(i); } }

Enum Example2

enum Edge { TOP,BOTTOM,LEFT,RIGHT;

}

public static void main(String[] a) { Edge e = Edge.TOP; int i = e.ordinal(); System.out.println(e); System.out.println(i); }

Enum Example3 public enum Day { MONDAY(8,true), TUESDAY(8,true), WEDNESDAY(8,true), THURSDAY(8,true), FRIDAY(8,true), SATURDAY(4,false), SUNDAY(0,false); private int hours; private boolean weekday;

Enum Example3 Day(int whours,boolean wday) { hours=whours; wday=weekday; } public int getHours() { return hours; } public boolean isWeekDay() { return weekday; }

Enum Example3 public static void showDay(Day d) { if(d.isWeekDay()) { System.out.println(d +" is a weekday and has "+ d.getHours() +" hours working hours"); } else { System.out.println(d +" is a not weekday and has "+ d.getHours() +" hours working hours"); } }

Enum Example3

public static void main(String[] ar) { Day day; day = Day.SUNDAY; showDay(day); } }

Inner Class A class that is declared within another class or interface, is called a nested class. There are four categories of nested classes Regular class - class within the class Method-local class – class within the method of the outer class Static nested class - inner classes marked with the static modifier (top-level nested class) Anonymous class - part of a method argument. All inner classes are nested classes, but not all nested classes are inner classes.

Example for Regular InnerClass class MyOuter {

int x =7; class MyInner { public void InnerMethod() { System.out.println("x == " + x); } } public void OuterMethod() { MyInner inn = new MyInner(); inn.InnerMethod(); }

Example for Regular InnerClass public static void main(String[] a) { MyOuter mo = new MyOuter(); MyOuter.MyInner mi = mo.new MyInner(); mi.InnerMethod(); mo.OuterMethod(); //mi.OuterMethod(); illegal //mo.InnerMethod(); illegal } }

Method-local inner class A method-local inner class can be instantiated only within the method where the inner class is defined. Can access the outer class level variable. CANT access the variable inside the method in which the inner class is created except a final variable. Method-local inner class can be declared abstract and final. method-local inner class can't use any access specifiers.

Method-local inner class class MouterClass { int x =10; public void OuterMethod() { final int j=90; class MinnerClass { public void minnerMethod() { System.out.println("Hello ..." + x + j); } } MinnerClass mic = new MinnerClass(); mic.minnerMethod(); } public static void main(String[] a) {

Static nested class Static nested classes are inner classes marked with the static modifier. A static nested class is not an inner class, it's a top-level nested class. A static nested class cannot access non-static members of the outer class.

Static nested class class OuterClass { static int i =10; public void method() { System.out.println("i == " + ++i); } static class InnerClass { public void display() { System.out.println("i == " + i); } }

Static nested class public static void main(String[] a) { OuterClass.InnerClass ic = new OuterClass.InnerClass(); ic.display();

}

OuterClass oc = new OuterClass(); oc.method(); }

Anonymous Inner Classes Anonymous inner classes have no name. Anonymous inner classes cannot have constructor.

Anonymous Inner Classes import java.awt.*; import java.awt.event.*; class FrameExample { private Frame f; public FrameExample() { f = new Frame("Hello .....!"); } public void launchFrame() { f.setSize(170,170); f.setBackground(Color.blue); f.setVisible(true);

Anonymous Inner Classes // Add a window listener f.addWindowListener(new WindowAdapter(){ public void windowClosing(WindowEvent evt) { System.exit(0); } }); //Anonymous Inner Classes }

}

public static void main(String args[]) { FrameExample f = new FrameExample(); f.launchFrame(); }

Exception Handling An exception in Java is a signal that indicates the occurrence of some important or unexpected condition during execution. Error Types: happens due to problems originating from the execution environment. (Error Class) happens due to problems originating inside the application itself. (Exception Class) •

Exception Should be Handled or Thrown to the exception handler.

Exceptions Errors (represented by subclasses of Error) occur in the Java virtual machine (JVM) and not in the application itself. The exceptions (represented by subclasses of Exception), on the other hand, generally originate from within the application. Types: Checked Exception Unchecked Exception

Checked Exception Checked exceptions are generally related to how the program interacts with its environment. This is the category of exceptions for which the compiler checks (hence the name checked exceptions) to ensure that your code is prepared for them. The programmer is required to write code to deal with checked exceptions. The compiler checks that such code exists. It MUST be thrown programmatically or Handled.

Unchecked Exception Occur due to program bugs. Runtime exceptions are not checked by the compiler. Write the correct code to avoid the runtime exceptions than write the code to catch them but it is not illegal to catch them. Runtime exceptions and errors combined are also called unchecked exceptions and they are mostly thrown by the JVM.

The Exception Class Hierarchy Object Throwable

Exception

Others… RuntimeException Others…

Error

Others…

Exception-handling mechanism

Contains five keywords: try - catch – throw - throws – finally Method throws ExceptionName{ try{ --risky code goes here }catch(ExceptionClassName ObjectName){ -- Exception handler block code throw Exception_Instance //Ducking it } finally{ -- cleanup your code goes here } }

About try-catch-finally A try block should be followed by at least one catch block. The code inside try block is called as protected code. Can have one or more catch block. If you have multiple catch block, make sure that the last catch block contain the super most class in the hierarchy. You may also write an optional “finally” block. This block contains code that is ALWAYS executed, either after the “try” block code, or after the “catch” block code. The catch block may or may not contain throw keyword. The try block can also be nested.

Example 1 class PrintStack { public static void main(String args[]) { int Num1= 30 , Num2 = 0; try { int Num3=Num1/Num2; } catch(ArithmeticException obj) { System.out.println("Exception"+obj); obj.printStackTrace(); } } }

Rules in Exception The Declare or Handle Rule Handle the exception by using the try-catch-finally block. Declare that the code causes an exception by using the throws clause. You do not need to declare runtime exceptions or errors. You can choose to handle runtime exceptions.

Passing the exception In any method that might throw an exception, you may declare the method as “throws” that exception, and thus avoid handling the exception yourself Example ●

public void myMethod throws IOException { … normal code with some I/O }

Throws clause class UncheckedThrows { public void show() throws ArithmeticException { System.out.println("Hai I am not handled"); }

}

public static void main(String[] arg) { new UncheckedThrows().show(); }

Method Overriding and Exceptions The overriding method can throw: v No exceptions v One or more of the exceptions thrown by the overridden method. v One or more subclasses of the exceptions

v

thrown by the overridden method. v

The overriding method cannot throw: v Additional exceptions not thrown by the overridden method. v Super classes of the exceptions thrown by the overridden method

User Defined Exception Create User-Defined Exception as a Class that EXTENDS Exception Class. • Instantiate the created Exception and use it in the catch block as a handler.

Example 2 import java.io.*; class MyException extends Exception { MyException() { System.out.println("UserDefined Error occured"); }

}

public String toString() { return "MyException thrown"; }

Example 2 cont… class UserExceptions { public void valid() { try { String str1,str2; BufferedReader br=new BufferedReader(new InputStreamReader(System.in)); System.out.println("Enter Login id"); str1=br.readLine(); System.out.println("Enter password"); str2=br.readLine(); if(str1.equals(str2)) System.out.println("Hai welcome"); else throw new MyException(); }

Example 2 cont … catch(MyException e) { System.out.println("Sorry U r not a valid user" + e); valid(); } catch(IOException ioe){} } public static void main(String[] arg) throws IOException { UserExceptions e1=new UserExceptions(); e1.valid(); } }

String Class Facts An object of the String class represents a string of characters. The String class belongs to the java.lang package, which does not require an import statement. Like other classes, String has constructors and methods. Unlike other classes, String has two operators, + and += (used for concatenation). String class is declare final , therefore immutable.

Literal Strings are anonymous objects of the String class are defined by enclosing text in double quotes. “This is a literal String” don’t have to be constructed. can be assigned to String variables. can be passed to methods and constructors as parameters. have methods you can call.

Literal String Example //assign a literal to a String variable String name = “Priya”; //calling a method on a literal String char Initial = “Priya”.charAt(0); //calling a method on a String variable char Initial = name.charAt(0);

Immutability Once created, a string cannot be changed: none of its methods changes the string. Such objects are called immutable. Immutable objects are convenient because several references can point to the same object safely: there is no danger of changing an object through one reference without the others being aware of the change.

Advantages Of Immutability Uses less memory String word1 = "Java"; String word2 = word1;

word 1

word 1 word 2

String word1 = “Java"; String word2 = new String(word1);

“Java"

OK

“Java"

“Java" word 2 Less efficient: wastes memory

Disadvantages of Immutability Less efficient — you need to create a new string and throw away the old one even for small changes. String word = “Java”; char ch = Character.toUpperCase(word.charAt (0)); word = ch + word.substring (1); word

“java" “Java"

Empty Strings An empty String has no characters. It’s length is 0. String word1 = ""; String word2 = new String();

Empty strings

Not the same as an uninitialized String.

private String errorMsg;

errorMsg is null

Copy Constructors Copy constructor creates a copy of an existing String. Also rarely used. Not the same as an assignment. Copy Constructor: Each variable points to a different copy of the String. word1 String word1 = new String(“Java”); word2 String word2 = new String(word); Assignment: Both variables point to the same String. word1 String word1 = “Java”; word2 String word2 = word;

“Java" “Java"

“Java"

Other Constructors Most other constructors take an array as a parameter to create a String. char[] letters = {‘J’, ‘a’, ‘v’, ‘a’}; String word = new String(letters);//”Java”

String index starts with 0 like arrays.

Methods in String Class char charAt(i) => Returns the char at position i. int length(); => Returns the number of characters in the string. String substring() => Returns a substring object substring(i,k) substring(i) “television".substring (2,5); “television".substring (2); television i

k

television i

Methods in String Clas indexOf() => returns the index position of the character. equals() equalsIgnoreCase() compareTo() compareToIgnoreCase() trim() replace() toUpperCase() toLowerCase()

StringBuffer Class String Buffers are mutable strings. StringBuffer is a final class. They can be created empty, from a string or with a capacity. An empty StringBuffer is created with 16character capacity. Can grow dynamically in size without bounds.

Methods in String Buffer length() capacity() ensureCapacity() setLength() charAt() Append() setCharAt() Insert() deleteCharAt() replace() reverse()

StringBuilder Class Same like StringBuffer Class StringBuilder’s methods are not synchronized. StringBuilder methods should run faster than StringBuffer methods.

Collections A collection allows a group of objects to be treated as a single unit. Arbitrary objects can be stored, retrieved, and manipulated as elements of collections. Provided in the java.util package. The collections framework comprises three main parts. Interfaces => Collection Classes => Collections Algorithms

The Collections Interfaces The root of the hierarchy of the collections interfaces is the Collection interface. v There is another kind of collections called maps, which are represented by the super interface Map. v

Both a Map object and a Set collection cannot contain duplicates data items. while a List collection can contain duplicates.

The Collections Interfaces A collection has no special order and does not reject duplicates. (java.util.Collection) A list is ordered and accept duplicates. (java.util.List). A set has no special order but rejects duplicates. (java.util.Set) A map supports searching on a key field, values of which must be unique. (java.util.Map)

Collection Classes ArrayList, LinkedList, and Vector are the classes that implement the List interface. HashMap and HashTable are examples of classes that implement the Map interface. HashSet and LinkedHashSet are examples of classes that implement the Set interface.

List import java.util.*; class ListExample { public static void main(String[] args) { List list = new ArrayList(); list.add("one"); list.add("second"); list.add("3rd"); list.add(new Integer(4)); list.add(new Float(5.0F)); list.add("second"); // duplicate, is added list.add(new Integer(4)); // duplicate, is added System.out.println(list); } }

Set import java.util.*; class SetExample { public static void main(String[] args) { Set set = new HashSet(); set.add("one"); set.add("second"); set.add("3rd"); set.add(new Integer(4)); set.add(new Float(5.0F)); set.add("second"); // duplicate, not added set.add(new Integer(4)); // duplicate, not added System.out.println(set); } }

Collection API - Storage

The storage associated with any one collection can be implemented in many ways, but the Collections API implements the four methods that are most widely used: Array: supports insertion, deletion, but growing the store is more difficult. ArrayList: grow in number of elements. Search is faster. But not insertion and deletion. Vector(provides synchronization) Linked list: supports insertion, deletion, and growing the store, but makes indexed access slower. Use when insertions and deletions happen frequently. Tree: supports insertion, deletion, and growing the list. Indexed access is slow, but searching is faster. Hash table: supports insertion, deletion, and growing the store. Indexed access is slow, but searching is particularly fast. However, hashing requires the use of unique keys for storing data elements.

Set Classes HashSet : provides the faster access to a data item. no guarantee that the items will be ordered. does not offer synchronization. Tree Set: presents sorted data items. performance is not as good as HashSet. does not offer synchronization. LinkedHashSet: Similar to HashSet that maintains a doubly linked list. It is an ordered collection, ordered by insertion, but not sorted. does not offer synchronization

Map Classes HashTable: implementation is based on the hashtable data structure. No ordering. implementation is synchronized HashMap: based on the hashtable data structure. No ordering allows null and is unsynchronized LinkedHashMap: maintains a doubly linked list. TreeMap: implements the SortedMap interface Sorted and unsynchronized.

Class

Interface

Duplicates Allowed?

Ordered/ Sorted

Synchronized

ArrayList

List

Yes

Ordered by index Not sorted

No

LinkedList

List

Yes

Ordered by index Not sorted

No

Vector

List

Yes

Ordered by index Not sorted

Yes

HashSet

Set

No

Not ordered Not sorted

No

LinkedHashSet

Set

No

Ordered by insertionNo Not sorted

TreeSet

Set

No

Sorted either by No natural order or by your comparison rules

Class

Interface

Duplicates Allowed?

Ordered/ Sorted

Synchronized

HashMap

Map

No

Not ordered Not sorted

No

LinkedHashMap

Map

No

Ordered

No

Hashtable

Map

No

Not ordered Not sorted

Yes

TreeMap

Map

No

Sorted either by No natural order or by your comparison rules

Collection Advantages and Disadvantages Advantages Can hold different types of objects. Resizable Disadvantages Must cast to correct type Cannot do compile-time type checking.

Generics For checking the type of object during the compilation time. Enclose the type within angular brackets .

Date Class The java.text.DateFormat class provides several methods for formatting the date/time for a default or a specific location, and yet you can keep your code completely independent of the locale conventions for months, days of the week, days of the months, and so on. You create a locale object by using the Locale class

Process and Thread A process is a program that is currently executing. Every process has at least one thread running within it. Threads are referred to as lightweight processes. A thread is a path of code execution through a program, and each thread has its own local variables, program counter (pointer to the current instruction being executed), and lifetime.

Threads A thread is not an object A thread is a flow of control A thread is a series of executed statements A thread is a nested sequence of method calls

MultiThreading and MultiTasking Multitasking is a mechanism to run many Heavyweight processes simultaneously in a different address space so context switch or intercommunication between processes is much expensive. Multithreading is a mechanism of running various lightweight processes under single process within its own space Multiprocessing there will be more than one processor and each thread will be handled by a different processor.

Creation of a Thread By extending Thread class By implementing Runnable interface. Even a non-multithreaded program has one thread of execution, called the main thread. Call the start() method to start the thread. When a thread is started, it calls the run() method to make our thread to perform useful work.

1st Method: Extending the Thread class

class MyThread extends Thread { public void run() {

// thread body of execution } } Creating thread: MyThread thr1 = new MyThread(); Start Execution: thr1.start();

2nd method: Threads by implementing Runnable interface class ClassName implements Runnable{ public void run() { // thread body of execution } } Creating Object: ClassName myObject = new ClassName(); Creating Thread Object: Thread thr1 = new Thread( myObject ); Start Execution: thr1.start();

Thread scheduling Thread scheduling is implementation dependent and cannot be relied on to act in the same way on every JVM The two approaches to scheduling are Preemptive - will be applied for thread with highest and lowest priority Time-Sliced (Round-Robin) Scheduling – will be applied when more than one thread has the same priority.

Threads within a process

THREAD STACK

SHARED MEMOR Y THREAD DATA THREAD TEXT l

l

All threads are parts of a process hence communication easier and simpler. Independent executables

Thread States A thread can in one of several possible states: 1.Running Currently running In control of CPU 2.Ready to run Can run but not yet given the chance 3.Resumed Ready to run after being suspended or block 4.Suspended Voluntarily allowed other threads to run 5.Blocked Waiting for some resource or event to occur

Thread Priorities Why priorities? Determine which thread receives CPU control and gets to be executed first Definition: – Integer value ranging from 1 to 10 – Higher the thread priority → larger chance of being executed first – Example: ● Two threads are ready to run ● First thread: priority of 5, already running ● Second thread = priority of 10, comes in while first thread is running

Thread Synchronization Done in two ways To method ● ●

public synchronized void method() { }

To block synchronized(this)

● ●

●

{ }

Wait() and notify() Wait() and notify should be used to restrict the thread before doing an operation without a notification from the other thread. Should be used along with the synchronized block

wait() When a thread enters a wait state, it does nothing until it is notified by another thread. It also gives up it’s lock on the object when wait is called. public synchronized blah() { wait(); … // do something }

notify() To awaken a thread, a different thread which has a lock on the same object must call notify. When notify is called, the block that had the lock on the object continues to have it’s lock it releases it. Then a thread is awakened from its wait() and can grab the lock and continue processing. There are two versions - notify() and notifyAll(). Notify is safe only under 2 conditions: When only 1 thread is waiting, and thus guaranteed to be awakened. When multiple threads are waiting on the same condition, and it doesn’t matter which one awakens. In general, use notifyAll()

Thread Group You can include thread in a set of threads by adding it to an instance of ThreadGroup ThreadGroups can contain not only threads but also other ThreadGroups.

Semaphore Semaphore is a synchronization mechanism, which implements mutual exclusion among processes to avoid race condition to access any shared resource. Semaphore maintains a counter to implement locking and unlocking. It avoids busy waiting. If a critical section is in use, the calling process will be removed from a run queue and put into a sleep state. Java 5 comes with semaphore implementations in the java.util.concurrent package so you don't have to implement your own semaphores. A mutex is really a semaphore with value 1.

Semaphores Semaphores have two purposes Mutex: Ensure threads don’t access critical section at same time Scheduling constraints: Ensure threads execute in specific order • A semaphore is an IPC mechanism that is implemented conceptually with at least these two components – a counter (int) variable – a wait queue of processes • And has at least these two operation – wait for the semaphore to be free (p) – signal that the semaphore is now free (v)

Semaphore The semaphore has at least these possible states: Free, or available, or not in use Not free, or unavailable, or in use Interpretation of the counter variable: If the counter is positive, then the semaphore is free. If the counter is zero (or negative), then the semaphore is in use (not free).

Semaphore •

Cases using a semaphore S 1.

2.

3.

4.

If a process does a wait (p) on S, and if the semaphore is free, then S is decremented (S.counter = S.counter – 1;) If a process does a wait (p) on S and if S is not free, then the process is blocked and put in S’s wait queue. If a process does a signal (v) on S and if there is no process in the wait queue for S, then the semaphore is set to free by incrementing its counter (to positive). If a signal (v) on S and there is a process in the S queue, then the process at the head of the queue is removed and unblocked (and can continue to execute)

IOStreams Usual Purpose: storing data to ‘nonvolatile‘ devices, e.g. harddisk Classes provided by package java.io Data is transferred to devices by ‘streams‘

Program Program

output - stream input - stream

Device Device

keyboard standard input stream

monitor terminal console

standar doutput stream

How does information travel across? Streams

CPU MEM

HDD

keyboard standard input stream

monitor terminal console

standar doutput stream

How does information travel across?

Streams

file input strea m LOAD READ

CPU MEM

HDD files

file outpu t strea m SAVE

IOStreams JAVA distinguishes between 2 types of streams: Text – streams, containing ‘characters‘

Progra m

Progra m

Binary Streams, containing 8 – bit information

I ‘ M

0110100 1

A

S T R I N G \n

1110110 1

0000000 0

Devic e

Devic e

IOStreams Streams in JAVA are Objects, having 2 types of streams (text / binary) and 2 directions (input / output) Results in 4 base-classes dealing with I/O: 1. 2. 3. 4.

Reader: text-input Writer: text-output InputStream: byte-input OutputStream: byte-output

Binary vs. TextFiles pro

con

Binary (input &output stream)

Efficient in terms of Preinformation about data needed time and space

Text(reader and writer)

Human readable, Not efficient contains redundant information

to understand content

Binary vs. TextFiles When use Text- / BinaryFiles ? ALWAYS use TextFiles for final results Binary Files might be used for non-final interchange between programs Binary Files are always used for large amount of data (images, videos etc.)

Serialization Serialization: process of saving objects to a stream i.e. in-memory object to a byte stream. Each object is assigned a serial number on the stream If the same object is saved twice, only serial number is written out the second time When reading, duplicate serial numbers are restored as references to the same object The objects must be read from the stream in the same order in which they were written.

Serialization Why isn’t everything serializable? Security reasons – may not want contents of objects printed out to disk, then anyone can print out internal structure and analyze it Could also have temporary variables that are useless once the program is done running.

Serialization basics The requirements for serialization are straightforward: Only class instances rather than primitive types can be serialized. For an object to be serializable, its class or some ancestor must implement the empty Serializable interface. An empty interface is called a marker interface. The syntax for serialization is straightforward: An object is serialized by writing it to an ObjectOutputStream. An object is deserialized by reading it from an ObjectInputStream.

Serialization code => Writing objects to a file FileOutputStream out = new FileOutputStream( “save.txt” ); ObjectOutputStream oos = new ObjectOutputStream( out ); oos.writeObject( new Date() ); oos.close();

Deserialization code => Reading objects from a file FileInputStream in = new FileInputStream( “save.txt” ); ObjectInputStream ois = new ObjectInputStream( in ); myObject d = (myObject_type) ois.readObject(); ois.close();

Conditions for serializability If an object is to be serialized: The class must be declared as public The class must implement Serializable The class must have a no-argument constructor All fields of the class must be serializable: either primitive types or serializable objects The Serializable interface does not define any methods! Question: What possible use is there for an interface that does not declare any methods? Answer: Serializable is used as flag to tell Java it needs to do extra work with this class

Object Serialization (cont’d) writeObject() will throw an Error if the object passed to it is not Serializable. You can control serialization by implementing the Externalizable interface. readObject() returns something of type Object, so it needs to be cast.

Serialization and primitive types Technically, primitive types cannot be serialized or deserialized. However, the ObjectOutputStream implements the DataOutput interface, which declares methods such as writeInt to write primitive types to streams. ObjectInputStream implements DataInput for reading primitive types

transient and static fields A field marked as transient is not impacted by serialization. During deserialization, transient fields are restored to their default values (e.g., transient numeric fields are restored to zero). static fields are not impacted by serialization.

JDBC Java DataBase Connectivity The JDBC ( Java Database Connectivity) API defines interfaces and classes for writing database applications in Java by making database connections. JDBC provides RDBMS access by allowing you to embed SQL inside Java code

JDBC Architecture Java application calls the JDBC library. JDBC loads a driver which talks to the database. We can change database engines without changing database code. Ø

Click to edit Master text styles Second level ● Third level ● Fourth level ● Fifth level

Ø

import the java.sql package.

Steps in JDBC To register the Driver: Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); To Get the connecttion: Connection con = DriverManager. getConnection("jdbc:odbc:Deepi","sa","pass@123"); To create a SQL statement: Statement st=con.createStatement(); To execute it: st.execute(“DDL Query”); st.executeUpdate(“DML Query”); st.executeQuery(“select query”);

Garbage Collector (GC) Provides automated memory management. Deletes the unused objects in the memory. Only the JVM decides when to run the GC, you can only suggest it. An object becomes eligible for Garbage Collection when its last live reference disappears.

Garbage collection and Performance How Memory is allocated: Object creation Object is constructed either on a memory heap or on a stack. Memory heap When new keyword is called memory is allocated in the heap and returned when the reference is made null Stack During method calls, objects are created for method arguments and method variables. These objects are created on stack. Such objects are eligible for garbage-collection when they go out of scope.

Garbage Collection Advantages of Garbage Collection : More productivity Program Integrity Disadvantages of Garbage Collection : program performance

Finalize() method Finalize() Class Object has a finalize() method. Before gc happens the finalize() method is called It is called only once Finalize method can be overridden by the user. Finalize can be used to make an object not to be garbage collected

Classical Algorithms Three classical algorithms Mark-sweep Reference counting Semispace Tweaks Generational garbage collection (JAVA DEFAULT) Out of scope Parallel –perform GC in parallel Concurrent –run GC at same time as app Real-time–ensure bounded pause times

Mark-Sweep Start with roots Global variables, variables on stack& in registers Recursively visit every object through pointers Markevery object we find (set mark bit) Everything not marked = garbage Can then sweep heap for unmarked objectsand free them

Annotations Annotations in Java is all about adding meta-data facility to the Java Elements like package declarations, class, constructors, methods, fields, variables and etc An annotation indicates that the declared element should be processed in some special way by a compiler, development tool, deployment tool, or during runtime. Annotations are defined using an @ syntax

Structure of Java5 Compiler Source File

Parser

Type Annotation Class File Checker Checker Writer Class File

class C { @NonNull Object field; C(@NonNull Object p) { field = p; } @NonNull Object get() { return field; } }

Program with annotations

Error

Error

Annotation Checker Plugins

Annotation Types Marker Single-Element Full-value or multi-value

Marker Marker annotations take no parameters. They are used to mark a Java element to be processed in a particular way. Example: public @interface MyAnnotation { } Usage: @MyAnnotation public void mymethod() { .... }

Single-Element Single-element, or single-value type, annotations provide a single piece of data only. This can be represented with a data=value pair or, simply with the value (a shortcut syntax) only, within parenthesis. Example: ● ● ●

public @interface MyAnnotation { String doSomething(); }

Usage: ● ● ● ●

@MyAnnotation ("What to do") public void mymethod() { .... }

Full-value or multi-value Full-value type annotations have multiple data members. Example: public @interface MyAnnotation { String doSomething(); int count; String date(); } Usage: @MyAnnotation (doSomething= "What to do", count=1, date="09-09-2005") public void mymethod() { .... }

The Built-In Annotations Java defines seven built-in annotations. Four are imported from java.lang.annotation ● ● ● ●

@Retention, @Documented, @Target, and @Inherited.

Three are included in java.lang. @Override, @Deprecated, and @SuppressWarnings.

The Target annotation @Target(ElementType.TYPE) can be applied to any element of a class

@Target(ElementType.FIELD) can be applied to a

field or property

@Target(ElementType.METHOD) can be applied to a

method level annotation

@Target(ElementType.PARAMETER) can be applied to the parameters of a method

@Target(ElementType.CONSTRUCTOR) can be applied to constructors

@Target(ElementType.LOCAL_VARIABLE) can be applied to local variables

@Target(ElementType.ANNOTATION_TYPE) indicates

that the declared type itself is a

Reflection When we have some Annotations defined in the source code and have a mechanism through which we can say that to what extent the Annotations should be retained. The three possible ways of telling this are, Retain the Annotation in the Source Code only Retain the Annotation in the Class file also. Retain the Annotation Definition during the Runtime so that JVM can make use of it. The Annotation that is used to achieve this is @Retention and it takes a possible values of SOURCE, CLASS and RUNTIME defined in RetentionPolicy Enumeration.

Need of Annotation Less coding Easier to change Smarter development. Providing information to the Compiler. Providing information to the tools. Providing information to the Runtime System

www.shareittips.com www.bcahub.shareittips.com