Java Notes

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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

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

Source File Class File_Writer

Class File Error Type Checker Comments

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

ØThe Java Virtual Machine provides a platform-

independent way of executing code, by abstracting the differences between operating systems and CPU

architectures.

ØJVM is Write Once-Run Anywhere (WORA) software. ØJVM forms part of large system JRE.

ØJVM's main job is interpreting java byte code and translating this into actions or OS calls.

Ø JVM is OS dependent which makes java source code as machine independent.

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 <classname> {

public static void main(String[] args) {

// Object instantiation

<classname> m = new <classname>(); }

CamelCase Convention

Variables - myVariable Method - myMethod() Class - MyClass Package - mypackage Constants - 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

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

<element type>[] <array name>; int[] myArray;

• Constructing an array

<array name> =

new <element type> [<array size>]; myArray = new int[5];

Initializing an Array

Explicit initialization in one line.

<element type>[] <array name> = { <array initialize list> };

Primitive array: ***************** int[] myArray = { 1 , 2 , 3 , 4 , 5 } ; Reference Array: ********************

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

Inheritance

Two important concepts

Generalization - Up the hierarchy

Specialization - Down the hierarchy

Purpose : Reusability (without changing its identity)

Syntax:

<modifier> class <name> extends <superclass> { <declaration>*

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.

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

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

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= " +c); } www.shareittips.co m

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:

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

Classes

Primitive Data

Type Wrapper Class Constructor 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

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; //AutoBoxing int y = n; //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

Wrapper Conversion methods

Primitive xxxValue()

To convert Wrapper to primitive • Primitive parseXxx(String) To convert a String to a primitive

• Wrapper valueOf(String)

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.

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;

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

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.

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

The Exception Class

Hierarchy

Object Throwable Exception Error RuntimeException Others… Others… 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.

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

vThe 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

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

User Defined Exception

Create User-Defined Exception as a Class that EXTENDS Exception Class.

• Instantiate the created Exception and use it in

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).

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.

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; String word1 = “Java";_{String word2 = new String(word1);}

word 1 OK Less efficient: wastes memory “Java" “Java" “Java" word 2 word 1 word 2

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.

Not the same as an uninitialized String.

String word1 = "";

String word2 = new 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.

String word1 = new String(“Java”);

String word2 = new String(word);

word1 word2

“Java" “Java"

Copy Constructor: Each variable points to a different copy of the String.

String word1 = “Java”; String word2 = word;

word1

“Java"

word2

Assignment: Both variables point to the same String.

Other Constructors

Most other constructors take an array as a parameter to create a String.

String index starts with 0 like arrays.

char[] letters = {‘J’, ‘a’, ‘v’, ‘a’};

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 16-character capacity.

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

vThe 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.

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

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.

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.

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 insertion

Not sorted No

TreeSet Set No Sorted either by

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

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

Generics

For checking the type of object during the compilation time.

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:

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:

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 THREAD DATA THREAD TEXT SHARED MEMOR Y

l All threads are parts of a process hence communication

easier and simpler.

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

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.

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.

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. If a process does a wait (p) on S, and if the semaphore is free, then S is decremented (S.counter = S.counter – 1;)

2. 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.

3. 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).

4. 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

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 output - stream Device Program input - stream Device

MEM CPU HDD keyboard monitor terminal console standard input stream standar doutput stream

Streams

How does information

travel across?

MEM CPU HDD keyboard monitor terminal console standard input stream standar doutput stream file input strea m LOAD READ file outpu t strea m SAVE

Streams

files

How does

information travel

across?

IOStreams

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

Binary Streams, containing 8 – bit information

I ‘ M A S T R I N G \n Progra m Device 0110100 1 Progra m 1110110₁ 0000000₀ Device

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. Reader: text-input 2. Writer: text-output

3. InputStream: byte-input 4. OutputStream: byte-output

Binary vs. TextFiles

pro

con

Binary

(input

&output

stream)

Efficient in terms of time and space

Preinformation

about data needed to understand content

Text(reader

and writer)

Human readable, contains redundant information Not efficient

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

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

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

Click to edit Master text styles Second level

● Third level ● Fourth level

● Fifth level

ØJava application calls the JDBC library. JDBC

loads a driver which talks to the database. We can change database engines without changing database code.

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 :

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

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

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

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

CheckerClass FileWriter Type Checker Error Error Source File Program with annotations Class File

Annotation Types

Marker

Single-Element

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,

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 Run-time 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.

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