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CORE JAVA 1.6

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mCLASS Class is a template for multiple objects with

similar features and It is a blue print for objects. It defines a type of object according to the

data the object can hold and the operations the object can perform.

Classes are the fundamental units in an object-oriented programming.

We use a class to create objects( instances ). Each instance carries its own data.

Class Instance

Rubber stamp Stamped image

Photographic negative Printed photo

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mOBJECT Instance of class. Have its own state. Have access to all the behaviors of the class.

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mSTATE Instance variables Attributes of class

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mBEHAVIOR Methods in class Functionalities of object.

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mOBJECT CLASS All classes in JavaTM technology are directly or

indirectly derived from the Object class. Some of the subclasses of Object class are -

Boolean, Number, Void, Math, String, StringBuffer etc.

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mIDENTIFIER Three important points to be noted about

identifiers. Legal Identifiers

Naming Rules Code Conventions

Coding Standards Java Beans Naming Standards.

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mLEGAL IDENTIFIER Start with a letter, a currency symbol($) or

connecting characters( _ ). Don’t use keywords. Case sensitive. Example

Legal Illegal

int _a; int :b;

int _____2_w; int -b;

int $c; int e#;

int _$; int .f;

int 7g;

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mJAVA KEYWORDS

abstract boolean break continue

char class const final

double else extends implements

for goto if native

int interface long return

private protected public synchronized

strictfp super switch void

throws transient try case

assert enum byte default

finally import new short

this volatile catch do

float instanceof package static

throw while

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mSOURCE FILE DECLARATION RULES

Only one public class per source file. Comments can appear any where in tile. Public class name should be the filename. Java is a package centric language so the

package declaration should be the first line. A file belongs to only one package. Import should be given after package

declaration. A file can contain more than one non-public

class. File with no public class can have a file name

does not match any of the classes in the file.

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m

CLASS DECLARATIONS AND MODIFIERS

Modifiers Access Modifiers

public private protected default

Non-Access Modifiers final abstract strictfp

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mDEFAULT OR PACKAGE LEVEL ACCESS

A Class can use public and default access modifiers only.

Has no modifier proceeding in the declaration.

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mDEFAULT OR PACKAGE LEVEL ACCESS

Default access in method level.

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mPUBLIC ACCESS

All classes from all packages can access to the public class.

ie all class in Java Universe can have access to public class.

Three ways to access a method Invoking a method declared in the same class. Invoking a method using a reference of the class. Invoking an inherited method.

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mPRIVATE ACCESS

Members marked private can’t be accessed by code in any class other than the class in which the private members is declared.

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mPROTECTED ACCESS

Protected and default are almost identical with one difference ie protected member can have access to other package classes (through inheritance)

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mACCESS MODIFIERS

Visibility Public Protected Private Default

Same Class Yes Yes Yes Yes

Any Class of Same pack

Yes Yes No Yes

Subclass in same pack Yes Yes No Yes

Subclass outside same pack

Yes Yesinheritance

No No

Not subclass outside pack

Yes No No No

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mNON – ACCESS MODIFIERS

final abstract transient synchronized native – Platform dependent. strictfp – IEEE 754 static.

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mVARIABLE DECLARATIONS

Primitive Variables: Variables with primitive data types such as int or

long. Stores data in the actual memory location of

where the variable is present  Reference Variables:

Variables that store the address in the memory location Points to another memory location where the actual data is present

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mPRIMITIVE VARIABLES

Type Bits Bytes Min Range

Max Range

byte 8 1 -27 27 -1

short 16 2 -215 215 -1

int 32 4 -231 231 -1

long 64 8 -263 263 -1

float 32 4 n/a n/a

double 64 8 n/a n/a

Char 16 bit Unicode

Boolean True or false.

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mINSTANCE VARIABLE DECLARATIONS

Defined inside class and outside methods. Initialized when the class is instantiated. Can use any access level. Can be marked as

final transient static volatile

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mLOCAL VARIABLE DECLARATIONS

Variable declaration within a method. Declared, initialized inside the method ie the life

time of local variable will be inside method. Local variable will be on stack not on heap. In case of reference variable it will be created in

heap not in stack. Can not be marked as

public transient volatile abstract static.

Compiler reject if any local variable has not been assigned a value.

If local and instance variable has same name we call it as shadowing.

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mARRAY DECLARATIONS

Object that store multiple variables of the same type.

Can hold primitives or references Array will be as an object on the heap.

Examples

int[ ] key; // recommended

int key[ ]; // less readable

String [ ] [ ] [ ] name;

String [ ] manager[ ];

int [5] scores; // not legal.

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mFINAL VARIABLES

Makes it impossible to reinitialize once it has been initialized with an explicit value.(not default value)

For primitive Once a variable is assigned a value it can’t be

altered. For Reference

Data within the object can be modified. Reference variable cannot be changed.

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mTRANSIENT VARIABLES

If a variable is declared as transient means we are telling the JVM to skip serialization for particular object.

Serialization means writing objects state into streams.

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mVOLATILE VARIABLES

Tells the JVM that a thread accessing the variable must always have its won private copy of the variable with master copy in memory.

Only applied to instance variable.

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mSTATIC VARIABLES Create variables independent of any instance.

Static members exits before making a new instance of class.

Only one copy of static member regardless of member variable.

Area to use

Methods

Variables

Inner classes

Initialization Blocks

Not to use in

Constructors

Classes

Interfaces

Method local inner classes

Inner class methods and variables

Local variables.

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mENUMS Should be declared as separate class or a class member

not inside methods.

Enum cannot be private or protected.

Equalent Java Class

Never invoke an enum directly

Constructor overloading is also possible in enum

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mENUMS

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mENUMS

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mENUMS

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mENCAPSULATION

Encapsulation is the technique of making the fields in a class

private and providing access to the fields via public methods.

If a field is declared private, it cannot be accessed by anyone

outside the class, thereby hiding the fields within the class.

For this reason, encapsulation is also referred to as data hiding.

Encapsulation can be described as a protective barrier that

prevents the code and data being randomly accessed by other

code defined outside the class.

The main benefit of encapsulation is the ability to modify our

implemented code without breaking the code of others who use our

code.

With this feature Encapsulation gives maintainability, flexibility and

extensibility to our code.

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mENCAPSULATION

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mCONSTRUCTOR

Objects are constructed with the help of constructors.

You cannot make a new object without constructor.

Constructors are the code that runs whenever you use

the keyword new.

Every class including abstract class must have a

constructor.

Typically constructors are used to initialize instance

variables.

Two Key Points

No return type

Name should be same as class

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mRULES FOR CONSTRUCTOR

Can use any access modifier

Must match the name of the class

Must not have a return type

If you don’t type a constructor a default constructor will be

automatically generated by the compiler

The default constructor is always a no-args constructor.

Every constructor has its first statement either

this() ->overloaded constructor

super() ->super class constructor

Interface don’t have constructor

Abstract class have constructor that activates in concrete class.

Don’t call a constructor inside any methods.

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mDETERMINE WHETHER DEFAULT CONSTRUCTOR WILL BE CREATED

No!

No!

Yes!

Yes!

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mCHECK WHETHER LEGAL IN CONSTRUCTORS

Legal !

IlLegal !

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mSTATIC VARIABLES & METHODS

In static variable or method the behavior has no dependency on

its instance.

Variables or methods declared as static belongs to class not to

particular instance.

We can use static method or variable without instance of class.

Non static instance variable cannot be referenced inside static

methods because there is no instance.

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mSTATIC VARIABLES & METHODS

If the same program written without static

Here the JVM does not know about frogCount

as it is not static.

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mACCESSING STATIC VARIABLES & METHODS

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mCHECK WHETHER LEGAL IN STATICS

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m

LITERAL VALUES FOR ALL PRIMITIVES

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mINTEGER LITERALS Three ways to represent integer values in

Java Decimal ( base 10 ) Octal ( base 8 ) Hexadecimal ( base 16 )

Decimal Literal Normal integers

Octal Integers Use only digits 0 to 7 We can have up to 21 digits in an octal number

not including the leading 0

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mINTEGER LITERALS Hexadecimal Literal

Constructed using 16 digit symbols Numbers ( 0 to 9 ) Alphabets ( a to f )

All integer literals are defined in int but they can be used in long with the suffix “F”

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mFLOATING POINT LITERALS Floating points are defined as a number, a

decimal and more numbers specifying fraction.

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mBOOLEAN AND CHARACTER LITERALS Boolean Literal

Can hold true or false. Character Literal

Characters are 16 bit unsigned integers It fill fit unsigned int range ( 65535 or less )

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mPRIMITIVE CASTING We can assign a primitive by value and result

of an expression with the help of assignment operator.

Casting lets you convert primitive values from one type to another.

Types of casting Implicit Explicit

Implicit Casting Done by the compiler If we assign a byte value into int implicit casting

is done.

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mPRIMITIVE CASTING Explicit Casting

Look at the following example Rule :

Result of an expression involving anything int-sized or smaller is always int.

Ex, Multiply int and short gives an int. Divide short and byte gives an int.

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mPRIMITIVE CASTING Casting

Saying the JVM I am responsible for any data loss while placing it here.

Why we need casting in this example ?

Byte can hold only up to 127 and here it is 128

Then How come -126 ?

Bit Value of 128 is 10000000As 128 is greater than 127 it’s a integer. Integer has 32 bit so add 24 bits 000000000000000000000000100

00000Once after type cast remove the 24 prefix bits and we will get 10000000The left most bit is for signTo find out the value of a negative using twos complement notationFlip all of the bits and add one gives 01111111 gives -128

Adding 1 in 01111111 give 128 againS

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mARRAY Arrays are objects in java that store multiple

values of same type. Declaring Array of primitives

Declaring Array of References

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mCONSTRUCTING ARRAY Array of primitives

Array of References

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mCONSTRUCTING ARRAY Multi dimensional Array

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mINITIALIZATION BLOCKS

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mWRAPPER CLASS Wrapper class is a wrapper around a primitive data type. It represents primitive data types in their corresponding

class instances These classes will be in java.lang package

Primitive type

Wrapper class

Constructor Arguments

byte Byte byte or String

short Short short or String

int Integer int or String

long Long long or String

float Float float, double or String

double Double double or String

char Character char

boolean Boolean boolean or String

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mWRAPPER CLASS Class to min and max values of primitives

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mWRAPPER CLASS Class to print default values of primitives

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mWRAPPER CLASS

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mWRAPPER CLASS – AUTO BOXING