Syntax

Hello World

public class HelloWorld{
 public static void main(String[] args){
  System.out.println("Hello World");
 }
}

Rules

1. Outer Class - Always public or default
2. There can be only one public class in a java file.
3. File name = (public,if any) class name
4. Every function has to be a member function of a class. Even main function.
5. Main function’s prototype is fixed public static void main(String[] args)
6. No semicolon after class definition.
7. Semicolon after every statement.

Naming Conventions

• PascalCase for class names.
• camelCase for methods, objects and variable names.
• UPPERCASE for constants

Comments

/*Multiple Line*/
//Single Line
/** Documentation Style*/

Variables

Declaration

Initialization

Both at once

Constants

Prefixing final keyword to the declaration+ initialization.

Blank final variable

Final variable not initialised during declaration

All final variables must be initialised, or else => Compile time error

=> These have to be initialized later, in constructor or somewhere.

It might not sound correct to call it variable since by definition, final means constant, so by behaviour, it doesn't vary. But these variables can vary once.

Ways to initialise

• Can be initialised within an Instance Initializer Block
• Can be initialised within a constructor
• If there are multiple constructors
• Initialise in all of them, or else => Compile time error

Blank static final member/variable

Blank final variable which is Static.

Initialisation

Only within static block.

Data types

Primitives

• byte
  • 8 bits and signed
• short
  • 16 bits and signed
• char
  • 16 bits and unsigned, so that it may represent Unicode characters
• int
  • 32 bits and signed
• long
  • 64 bits and signed
• float
  • 32 bits and signed
• double
  • 64 bits and signed
• boolean
  • it's not numeric, may only have true or false values

Suffixing for Non-default Primitive Data Types

• int and long

  Default type for integer literals is int. That is, unless specified otherwise, 35 is an int. So if we assign an integer directly to a long variable, it will result in compilation error.

  //Compilation error
  long x = 232342;
  

  Thus, we have to specify explicitly that the value 232342 is a long.

  This can be done by suffixing l or L to the value. For example. 232342L.

• float and double

  Similarly, double is the default value for floating point literals. Unless explicitly stated, 9.5 is a double not a float.

  float a = 3.7 // 3.7 is a double, ‘a’ is float => Error
  float a = 3.7f // a is a float
  

Prefixing for Different Bases

• Decimal: 127
• Hexadecimal: 0x7f (Prefix - 0x)
• Octal: 0177 (Prefix - 0)
• Binary: 0b10011001 (Prefix - 0b)

Compound

Array

Declaration

int[] arr;

int a[];

Array can be defined in java in the above two ways. However, unlike C++ defining an array actually means that we are defining a pointer, which will point to an array.

Initialization

int arr[] = new int(3)

Strings

Declaration

String s;

Initialization

s = new String();

s = new String("string");

s = "string";

Both at once

String s = new String();

String s = new String("string");

String s = "string";

Length

s.length();

Looping/character at index

s.charAt(i);

Structures

Interfaces

Classes

Creating objects

Box smallBox;

Unlike C++, the above statement won’t create an object. It will just create a reference variable. The reference variable has to be given an object.

Box smallBox= new Box();

Now smallBox contains address of an object of Box class. That is, it points to the object. However, the object itself has no name.

Using objects

Through its reference.

class Example 
 {
 public static void main(String[] args)
  {
   Box smallBox=new Box();
   smallBox.setDimension(12,10,5);  
   smallBox.showDimension; 
  /*From the perspective of C++, 
  it looks like smallBox is the name of an object. 
  Though its not.*/
   
   smallBox= new Box();
  /*smallBox will now refer to a new object 
  and the older object will become ‘Garbage Block’, 
  which will get destroyed 
  when Garbage collector will run.*/

  smallBox.showDimension();
  /*Will print 0,0,0 since no value is assigned. 
  The default value is 0.*/
  }
}

Operators

Flow

Conditionals

Loops

Functions/Methods

public static void main(String[] args)

• public: So that JVM can call main function from outside of the class body.

• static: Static member functions can be called without an object.

• void: Always — no return type

• Argument: String[] args. Array of strings