Language Fundamentals OverView

1. Introduction
2. Identifiers
   • Rules to define java identifiers:
3. Reserved words
   • Reserved words for data types: (8)
   • Reserved words for flow control:(11)
   • Keywords for modifiers:(11)
   • Keywords for exception handling:(6)
   • Class related keywords:(6)
   • Object related keywords:(4)
   • Void return type keyword
   • Unused keywords
   • Reserved literals
   • Enum
   • Conclusions
4. Data types
   • Integral data types
     • Byte
     • Short
     • Int
     • long
   • Floating Point Data types
   • boolean data type
   • Char data type
   • Java is pure object oriented programming or not ?
   • Summary of java primitive data type
5. Literals
   • Integral Literals
   • Floating Point Literals
   • Boolean literals
   • Char literals
   • String literals
   • 1.7 Version enhansements with respect to Literals
     • Binary Literals
     • Usage of _ (underscore)symbol in numeric literals
6. Arrays
   1. Introduction
   2. Array declaration
      • Single dimensional array declaration
      • Two dimensional array declaration
      • Three dimensional array declaration
   3. Array construction
      • Multi dimensional array creation
   4. Array initialization
   5. Array declaration, construction, initialization in a single line.
   6. length Vs length() method
   7. Anonymous arrays
   8. Array element assignments
   9. Array variable assignments
7. Types of variables
   • Primitive variables
   • Reference variables
   • Instance variables
   • Static variables
   • Local variables
   • Conclusions
8. Un initialized arrays
   • Instance level
   • Static level
   • Local level
9. Var arg method
   • Single Dimensional Array Vs Var-Arg Method
10. Main method
    • 1.7 Version Enhansements with respect to main()
11. Command line arguments
12. Java coding standards
    • Coding standards for classes
    • Coding standards for interfaces
    • Coding standards for methods
    • Coding standards for variables
    • Coding standards for constants
    • Java bean coding standards
      • Syntax for setter method
      • Syntax for getter method
    • Coding standards for listeners
      • To register a listener
      • To unregister a listener
13. Various Memory areas present inside JVM

Language Fundamentals

Identifier :

Rules to define java identifiers:

1)	a to z
2)	A to Z
3)	0 to 9
4)	_ (underscore)
5)	$

1)	total_number-------valid
2)	Total#------------------invalid

1)	ABC123---------valid
2)	123ABC---------invalid

class Test{
int number=10;
int Number=20;
int NUMBER=20;	we can differentiate with case. 
int NuMbEr=30;
}

 int if=10; --------------invalid

class Test
{
public static void main(String[] args){
int String=10;
System.out.println(String);
}}
Output:
10

class Test
{
public static void main(String[] args){
int Runnable=10;
System.out.println(Runnable);
}}
Output:
10

Reserved words:

Reserved words for data types: (8)

1)	byte
2)	short
3)	int
4)	long
5)	float
6)	double
7)	char
8)	boolean

Reserved words for flow control:(11)

1)	if 
2)	else 
3)	switch 
4)	case 
5)	default 
6)	for 
7)	do 
8)	while 
9)	break 
10)	continue
11)	return

Keywords for modifiers:(11)

1)	public 
2)	private 
3)	protected
4)	static
5)	final 
6)	abstract 
7)	synchronized 
8)	native 
9)	strictfp(1.2 version) 
10)	transient
11)	volatile

Keywords for exception handling:(6)

1)	try
2)	catch
3)	finally 
4)	throw
5)	throws
6)	assert(1.4 version)

Class related keywords:(6)

1)	class
2)	package
3)	import
4)	extends
5)	implements
6)	interface

Object related keywords:(4)

1)	new
2)	instanceof 
3)	super 
4)	this

Void return type keyword:

1)	void

Unused keywords:

Reserved literals:

1)	true	values for boolean data type.
2)	false
3)	null----------------- default value for object reference.

Enum:

Example:
enum Beer
{
KF, RC, KO, FO;
}

Conclusions :

1. All reserved words in java contain only lowercase alphabet symbols.
2. New keywords in java are:

   strictfp-----------1.2v
   assert-------------1.4v
   enum--------------1.5v
   

3. In java we have only new keyword but not delete because destruction of useless objects is the responsibility of Garbage Collection.

4. instanceof  but not  instanceOf
   strictfp    but not  strictFp
   const      but not  Constant
   syncronized but not  syncronize
   extends      but not  extend
   implements  but not  implement
   import      but not  imports
   int          but not  Int
   

Which of the following list contains only java reserved words ?

1. final, finally, finalize (invalid) //here finalize is a method in Object class.
2. throw, throws, thrown(invalid) //thrown is not available in java
3. break, continue, return, exit(invalid) //exit is not reserved keyword
4. goto, constant(invalid) //here constant is not reserved keyword
5. byte, short, Integer, long(invalid) //here Integer is a wrapper class
6. extends, implements, imports(invalid) //imports keyword is not available in java
7. finalize, synchronized(invalid) //finalize is a method in Object class
8. instanceof, sizeOf(invalid) //sizeOf is not reserved keyword
9. new, delete(invalid) //delete is not a keyword
10. None of the above(valid)

Which of the following are valid java keywords?

1. public(valid)
2. static(valid)
3. void(valid)
4. main(invalid)
5. String(invalid)
6. args(invalid)

Data types:

Java is pure object oriented programming or not?

Integral data types :

Byte:

Size: 1byte (8bits)
Maxvalue: +127
Minvalue:-128
Range:-128to 127[-27 to 27-1]

• The most significant bit acts as sign bit. "0" means "+ve" number and "1" means "–ve" number.
• "+ve" numbers will be represented directly in the memory whereas "–ve" numbers will be represented in 2's complement form.

Example:
byte b=10;
byte b2=130;//C.E:possible loss of precision
                     found : int
                     required : byte
byte b=10.5;//C.E:possible loss of precision
byte b=true;//C.E:incompatible types
byte b="ashok";//C.E:incompatible types
                         found : java.lang.String
                         required : byte

Short:

Size: 2 bytes
Range: -32768 to 32767(-215 to 215-1)

short s=130;
short s=32768;//C.E:possible loss of precision
short s=true;//C.E:incompatible types

Int:

Size: 4 bytes
Range:-2147483648 to 2147483647 (-231 to 231-1)

int i=130;
int i=10.5;//C.E:possible loss of precision
int i=true;//C.E:incompatible types

long:

long l=f.length();//f is a file
Size: 8 bytes
Range:-263  to 263-1

Floating Point Data types:

boolean data type:

Size: Not applicable (virtual machine dependent)
Range: Not applicable but allowed values are true or false.

Which of the following boolean declarations are valid?

boolean b=true;
boolean b=True;//C.E:cannot find symbol
boolean b="True";//C.E:incompatible types
boolean b=0;//C.E:incompatible types

Char data type:

Size: 2 bytes
Range: 0 to 65535

char ch1=97;
char ch2=65536;//C.E:possible loss of precision

Summary of java primitive data type:

Literals:

Integral Literals:

• The allowed digits are 0 to 9, A to Z.
• For the extra digits we can use both upper case and lower case characters.
• This is one of very few areas where java is not case sensitive.
• Literal value should be prefixed with ox(or)oX.

Which of the following are valid declarations?

1. int x=0777; //(valid)
2. int x=0786; //C.E:integer number too large: 0786(invalid)
3. int x=0xFACE; (valid)
4. int x=0xbeef; (valid)
5. int x=0xBeer; //C.E:';' expected(invalid) //:int x=0xBeer; ^// ^
6. int x=0xabb2cd;(valid)

int x=10;
int y=010;
int z=0x10;
System.out.println(x+"----"+y+"----"+z); //10----8----16

int x=10;(valid)
long l=10L;(valid)
long l=10;(valid)
int x=10l;//C.E:possible loss of precision(invalid)
               found : long
               required : int

byte b=127;(valid)
byte b=130;//C.E:possible loss of precision(invalid)
short s=32767;(valid)
short s=32768;//C.E:possible loss of precision(invalid)

Floating Point Literals:

float f=123.456;//C.E:possible loss of precision(invalid)
float f=123.456f;(valid)
double d=123.456;(valid)

double d=123.456D;

double d=123.456;(valid)
double d=0123.456;(valid)  //it is treated as decimal value  but not octal 
double d=0x123.456;//C.E:malformed floating point literal(invalid)

Which of the following floating point declarations are valid?

1. float f=123.456; //C.E:possible loss of precision(invalid)
2. float f=123.456D; //C.E:possible loss of precision(invalid)
3. double d=0x123.456; //C.E:malformed floating point literal(invalid)
4. double d=0xFace; (valid)
5. double d=0xBeef; (valid)

double d=0xBeef;
System.out.println(d);//48879.0

int x=10.0;//C.E:possible loss of precision

double d=10e2;//==>10*102(valid)
System.out.println(d);//1000.0
float f=10e2;//C.E:possible loss of precision(invalid)
float f=10e2F;(valid)

Boolean literals:

1. boolean b=true;(valid)
2. boolean b=0;//C.E:incompatible types(invalid)
3. boolean b=True;//C.E:cannot find symbol(invalid)
4. boolean b="true";//C.E:incompatible types(invalid)

Char literals:

1. char ch='a';(valid)
2. char ch=a;//C.E:cannot find symbol(invalid)
3. char ch="a";//C.E:incompatible types(invalid)
4. char ch='ab';//C.E:unclosed character literal(invalid)

1. char ch=97; (valid)
2. char ch=0xFace; (valid)
   System.out.println(ch); //?
3. char ch=65536; //C.E: possible loss of precision(invalid)

1. char ch='\ubeef';
2. char ch1='\u0061';
   System.out.println(ch1); //a
3. char ch2=\u0062; //C.E:cannot find symbol
4. char ch3='\iface'; //C.E:illegal escape character
5. Every escape character in java acts as a char literal.

1) char ch='\n';  //(valid)
2) char ch='\l';  //C.E:illegal escape character(invalid)

Which of the following char declarations are valid?

1. char ch=a; //C.E:cannot find symbol(invalid)
2. char ch='ab'; //C.E:unclosed character literal(invalid)
3. char ch=65536; //C.E:possible loss of precision(invalid)
4. char ch=\uface; //C.E:illegal character: \64206(invalid)
5. char ch='/n'; //C.E:unclosed character literal(invalid)
6. none of the above. (valid)

String literals:

1.7 Version enhansements with respect to Literals :

1. Binary Literals
2. Usage of '_' in Numeric Literals

Binary Literals :

1. Decimal
2. Octal
3. Hexa decimal

int x = 0b111;
System.out.println(x);  // 7

Usage of _ symbol in numeric literals :

double d = 123456.789;  //valid

double d = 1_23_456.7_8_9;  //valid
double d = 123_456.7_8_9;   //valid

We can use more than one underscore symbol also between the digits.
Ex : double d = 1_23_ _456.789;

double d=_1_23_456.7_8_9;    //invalid
double d=1_23_456.7_8_9_;    //invalid
double d=1_23_456_.7_8_9;    //invalid

double d='a';
System.out.println(d); //97
  integral  data types

float f=10L;
System.out.println(f); //10.0
  floating-point  data types

Arrays

Introduction

The main advantage of arrays is we can represent multiple values with the same name so that readability of the code will be improved.

But the main disadvantage of arrays is:
Fixed in size that is once we created an array there is no chance of increasing or decreasing the size based on our requirement that is to use arrays concept compulsory we should know the size in advance which may not possible always.

Array declarations:

Single dimensional array declaration:

int[] a;//recommended to use because name is clearly separated from the type
int []a;
int a[];

int[] a;//valid
int[5] a;//invalid

Two dimensional array declaration:

int[][] a;
int [][]a;
int a[][];      All are valid.(6 ways)
int[] []a;
int[] a[];
int []a[];

Three dimensional array declaration:

int[][][] a;
int [][][]a;
int a[][][];
int[] [][]a;
int[] a[][];      All are valid.(10 ways)
int[] []a[];
int[][] []a;
int[][] a[];
int []a[][];
int [][]a[];

1)	int[] a1,b1; //a-1,b-1 (valid)
2)	int[] a2[],b2; //a-2,b-1 (valid)
3)	int[] []a3,b3; //a-2,b-2 (valid)	
4)	int[] a,[]b; //C.E:<identifier> expected (invalid)

Array construction:

int[] a=new int[3];

int[] a=new int[3];
int[] a=new int[];//C.E:array dimension missing

int[] a=new int[0];
System.out.println(a.length);//0

int[] a=new int[-3];//R.E:NegativeArraySizeException

int[] a=new int['a'];//(valid)
byte b=10;
int[] a=new int[b];//(valid)
short s=20;
int[] a=new int[s];//(valid)
int[] a=new int[10l];//C.E:possible loss of precision//(invalid)
int[] a=new int[10.5];//C.E:possible loss of precision//(invalid)

int[] a1=new int[2147483647];(valid)
int[] a2=new int[2147483648];
  //C.E:integer number too large: 2147483648(invalid)

Multi dimensional array creation:

int[][] a=new int[2][];
a[0]=new int[3];
a[1]=new int[2];

int[][][] a=new int[2][][];
a[0]=new int[3][];
a[0][0]=new int[1];
a[0][1]=new int[2];
a[0][2]=new int[3];
a[1]=new int[2][2];

1)	int[] a=new int[]//C.E: array dimension missing(invalid)
2)	int[][] a=new int[3][4];(valid)
3)	int[][] a=new int[3][];(valid)
4)	int[][] a=new int[][4];//C.E:']' expected(invalid)
5)	int[][][] a=new int[3][4][5];(valid)
6)	int[][][] a=new int[3][4][];(valid)
7)	int[][][] a=new int[3][][5];//C.E:']' expected(invalid)

Array Initialization:

int[] a=new int[3];
System.out.println(a);//[I@3e25a5
System.out.println(a[0]);//0

System.out.println(a);//[[I@3e25a5
System.out.println(a[0]);//[I@19821f
System.out.println(a[0][0]);//0

int[][] a=new int[2][];
System.out.println(a);//[[I@3e25a5
System.out.println(a[0]);//null
System.out.println(a[0][0]);//R.E:NullPointerException

int[] a=new int[4];
a[0]=10;
a[1]=20;
a[2]=30;
a[3]=40;
a[4]=50;//R.E:ArrayIndexOutOfBoundsException: 4
a[-4]=60;//R.E:ArrayIndexOutOfBoundsException: -4

Declaration, construction and initialization of an array in a single line:

char[] ch={'a','e','i','o','u'};(valid)
String[] s={"balayya","venki","nag","chiru"};(valid)

int[][] a={{10,20,30},{40,50}};

int[][][] a={{{10,20,30},{40,50}},{{60},{70,80},{90,100,110}}};

int[][][] a={{{10,20,30},{40,50}},{{60},{70,80},{90,100,110}}};
System.out.println(a[0][1][1]);//50(valid)
System.out.println(a[1][0][2]);//R.E:ArrayIndexOutOfBoundsException: 2(invalid)
System.out.println(a[1][2][1]);//100(valid)
System.out.println(a[1][2][2]);//110(valid)
System.out.println(a[2][1][0]);//R.E:ArrayIndexOutOfBoundsException: 2(invalid)
System.out.println(a[1][1][1]);//80(valid)

• If we want to use this short cut compulsory we should perform declaration, construction and initialization in a single line.
• If we are trying to divide into multiple lines then we will get compile time error.

length Vs length():

length:

1. It is the final variable applicable only for arrays.
2. It represents the size of the array.

int[] x=new int[3];
System.out.println(x.length());//C.E: cannot find symbol
System.out.println(x.length);//3

length() method:

1. It is a final method applicable for String objects.
2. It returns the no of characters present in the String.

String s="bhaskar";
System.out.println(s.length);//C.E:cannot find symbol
System.out.println(s.length());//7

int[][] a=new int[6][3];
System.out.println(a.length);//6
System.out.println(a[0].length);//3

There is no direct way to find total size of multi dimentional array but indirectly we can find as follows
x[o].length +x[1].length + x[2].length + .......

Anonymous Arrays:

• Sometimes we can create an array without name such type of nameless arrays are called anonymous arrays.
• The main objective of anonymous arrays is "just for instant use".
• We can create anonymous array as follows.

  new int[]{10,20,30,40};(valid)
  new int[][]{{10,20},{30,40}};(valid)
  

• At the time of anonymous array creation we can't specify the size otherwise we will get compile time error.

new int[3]{10,20,30,40};//C.E:';' expected(invalid)
new int[]{10,20,30,40};(valid)

int[] a=new int[]{10,20,30,40};(valid)  

class Test 
{
	public static void main(String[] args) 
	{
	 System.out.println(sum(new int[]{10,20,30,40}));//100
	}
	public static int sum(int[] x)
	{
		int total=0;
		for(int x1:x)
		{
			total=total+x1;
		}
		return total;
	}
}

Array element assignments:

int[] a=new int[10];
a[0]=97;//(valid)
a[1]='a';//(valid)
byte b=10;
a[2]=b;//(valid)
short s=20;
a[3]=s;//(valid)
a[4]=10l;//C.E:possible loss of precision

Object[] a=new Object[10];
a[0]=new Integer(10);//(valid)
a[1]=new Object();//(valid)
a[2]=new String("bhaskar");//(valid)

Number[] n=new Number[10];
n[0]=new Integer(10);//(valid)
n[1]=new Double(10.5);//(valid)
n[2]=new String("bhaskar");//C.E:incompatible types//(invalid)

Runnable[] r=new Runnable[10];
r[0]=new Thread();
r[1]=new String("bhaskar");//C.E: incompatible types

Array variable assignments:

• Element level promotions are not applicable at array object level.
• Ex : A char value can be promoted to int type but char array cannot be promoted to int array.

int[] a={10,20,30};
char[] ch={'a','b','c'};
int[] b=a;//(valid)
int[] c=ch;//C.E:incompatible types(invalid)

String[] s={"A","B"};
Object[] o=s;

int[] a={10,20,30,40,50,60,70};
int[] b={80,90};
a=b;//(valid)
b=a;//(valid)

int[][] a=new int[3][];
a[0]=new int[4][5];//C.E:incompatible types(invalid)
a[0]=10;//C.E:incompatible types(invalid)
a[0]=new int[4];//(valid)

int[][] a=new int[3][2];
a[0]=new int[3];
a[1]=new int[4];
a=new int[4][3];

Total how many objects created?
Ans: 11
How many objects eligible for GC: 6

class Test 
{
	public static void main(String[] args) 
	{
		String[] argh={"A","B"};
		args=argh;
		System.out.println(args.length);//2
		for(int i=0;i<=args.length;i++)
		{
			System.out.println(args[i]);
		}
	}
}
Output:
java Test x y
R.E: ArrayIndexOutOfBoundsException: 2
java Test x
R.E: ArrayIndexOutOfBoundsException: 2
java Test
R.E: ArrayIndexOutOfBoundsException: 2

class Test 
{
	public static void main(String[] args) 
	{
		String[] argh={"A","B"};
		args=argh;
		System.out.println(args.length);//2
		for(int i=0;i<args.length;i++)
		{
			System.out.println(args[i]);
		}
	}
}
Output:
2
A
B

class Test 
{
	public static void main(String[] args) 
	{
                   String[] argh={"A","B"};
                    args=argh;
		
                    for(String s : args) {
                      System.out.println(s);
	}
}
Output:
A
B

Types of Variables

1. Primitive variables
2. Reference variables

Primitive variables:

Reference variables:

1. Instance variables
2. Static variables
3. Local variables

Instance variables:

• If the value of a variable is varied from object to object such type of variables are called instance variables.
• For every object a separate copy of instance variables will be created.
• Instance variables will be created at the time of object creation and destroyed at the time of object destruction hence the scope of instance variables is exactly same as scope of objects.
• Instance variables will be stored on the heap as the part of object.
• Instance variables should be declared with in the class directly but outside of any method or block or constructor.
• Instance variables can be accessed directly from Instance area. But cannot be accessed directly from static area.
• But by using object reference we can access instance variables from static area.

class Test 
{
	int i=10;
	public static void main(String[] args) 
	{
		//System.out.println(i);
//C.E:non-static variable i cannot be referenced from a static context(invalid)
		Test t=new Test();
		System.out.println(t.i);//10(valid)
		t.methodOne();
	}
	public void methodOne()
	{
		System.out.println(i);//10(valid)
	}
}

class Test 
{
	boolean b;
	public static void main(String[] args) 
	{
		Test t=new Test();
		System.out.println(t.b);//false
	}
}

Static variables:

• If the value of a variable is not varied from object to object such type of variables is not recommended to declare as instance variables. We have to declare such type of variables at class level by using static modifier.
• In the case of instance variables for every object a separate copy will be created but in the case of static variables for entire class only one copy will be created and shared by every object of that class.
• Static variables will be crated at the time of class loading and destroyed at the time of class unloading hence the scope of the static variable is exactly same as the scope of the .class file.
• Static variables will be stored in method area. Static variables should be declared with in the class directly but outside of any method or block or constructor.
• Static variables can be accessed from both instance and static areas directly.
• We can access static variables either by class name or by object reference but usage of class name is recommended.
• But within the same class it is not required to use class name we can access directly.

1. Start JVM.
2. Create and start Main Thread by JVM.
3. Locate(find) Test.class by main Thread.
4. Load Test.class by main Thread.     // static variable creation
5. Execution of main() method.
6. Unload Test.class     // static variable destruction
7. Terminate main Thread.
8. Shutdown JVM.

class Test 
{
	static int i=10;
	public static void main(String[] args) 
	{
		Test t=new Test();
		System.out.println(t.i);//10
		System.out.println(Test.i);//10
		System.out.println(i);//10
	}
}

class Test 
{
	static String s;
	public static void main(String[] args) 
	{
		System.out.println(s);//null
	}
}

class Test 
{
	int x=10;
	static int y=20;
	public static void main(String[] args) 
	{
		Test t1=new Test();
		t1.x=888;
		t1.y=999;
		Test t2=new Test();
		System.out.println(t2.x+"----"+t2.y);//10----999
	}
}

Local variables:

Some times to meet temporary requirements of the programmer we can declare variables inside a method or block or constructors such type of variables are called local variables or automatic variables or temporary variables or stack variables.

The local variables will be created as part of the block execution in which it is declared and destroyed once that block execution completes. Hence the scope of the local variables is exactly same as scope of the block in which we declared.

class Test 
{
	public static void main(String[] args) 
	{
		int i=0;
		for(int j=0;j<3;j++)
		{
			i=i+j;
		}


			
}
}

class Test 
{
	public static void main(String[] args) 
	{
		try
		{
			int i=Integer.parseInt("ten");
		}
		catch(NullPointerException e)
		{


					
                  }
	}
}

• The local variables will be stored on the stack.
• For the local variables JVM won't provide any default values compulsory we should perform initialization explicitly before using that variable.

class Test 
{
	public static void main(String[] args) 
	{
		int x;
		if(args.length>0)
		{
			x=10;
		}
		System.out.println(x);
                  //C.E:variable x might not have been initialized
	}
}

class Test 
{
	public static void main(String[] args) 
	{
		int x;
		if(args.length>0)
		{
			x=10;
		}
		else
		{
			x=20;
		}
		System.out.println(x);
	}
}
Output:
java Test x 
10
java Test x y
10
java Test
20

• It is never recommended to perform initialization for the local variables inside logical blocks because there is no guarantee of executing that block always at runtime.
• It is highly recommended to perform initialization for the local variables at the time of declaration at least with default values.

class Test 
{
	public static void main(String[] args) 
	{
		
		public int x=10; //(invalid)
		private int x=10; //(invalid)
		protected int x=10; //(invalid)    C.E: illegal start of expression
		static int x=10; //(invalid)
		volatile int x=10; //(invalid)
		transient int x=10; //(invalid)
		
		final int x=10;//(valid)
	}
}

Conclusions:

1. For the static and instance variables it is not required to perform initialization explicitly JVM will provide default values. But for the local variables JVM won't provide any default values compulsory we should perform initialization explicitly before using that variable.
2. For every object a separate copy of instance variable will be created whereas for entire class a single copy of static variable will be created. For every Thread a separate copy of local variable will be created.
3. Instance and static variables can be accessed by multiple Threads simultaneously and hence these are not Thread safe but local variables can be accessed by only one Thread at a time and hence local variables are Thread safe.
4. If we are not declaring any modifier explicitly then it means default modifier but this rule is applicable only for static and instance variables but not local variable.

Un Initialized arrays

class Test 
{
	int[] a;
	public static void main(String[] args) 
	{
		Test t1=new Test();
		System.out.println(t1.a);//null
		System.out.println(t1.a[0]);//R.E:NullPointerException
	}
}

Instance level:

int[] a;
System.out.println(obj.a);//null
System.out.println(obj.a[0]);//R.E:NullPointerException

int[] a=new int[3];
System.out.println(obj.a);//[I@3e25a5
System.out.println(obj.a[0]);//0

Static level:

static int[] a;
System.out.println(a);//null
System.out.println(a[0]);//R.E:NullPointerException

static int[] a=new int[3];
System.out.println(a);//[I@3e25a5
System.out.println(a[0]);//0

Local level:

int[] a;
System.out.println(a); //C.E: variable a might not have been initialized
System.out.println(a[0]);

int[] a=new int[3];
System.out.println(a);//[I@3e25a5
System.out.println(a[0]);//0

Every variable in java should be either instance or static or local.
Every variable in java should be either primitive or reference

Var- arg methods (variable no of argument methods) (1.5)

• Until 1.4v we can't declared a method with variable no. Of arguments.
• If there is a change in no of arguments compulsory we have to define a new method.
• This approach increases length of the code and reduces readability.
• But from 1.5 version onwards we can declare a method with variable no. Of arguments such type of methods are called var-arg methods.

class Test 
{
	public static void methodOne(int... x)
	{
		System.out.println("var-arg method");
	}
	public static void main(String[] args)
	{
		methodOne();
		methodOne(10);
		methodOne(10,20,30);
	}
}
Output:
var-arg method
var-arg method
var-arg method

class Test 
{
 public static void sum(int... x)
 {
    int total=0;
    for(int i=0;i<x.length;i++)
    {
	total=total+x[i];
        }
         System.out.println("The sum :"+total);
}
public static void main(String[] args)
{
 sum();
 sum(10);
 sum(10,20);
 sum(10,20,30,40);
 }
}
Output:
The sum: 0
The sum: 10
The sum: 30
The sum: 100

class Test 
{
 public static void sum(int... x)
 {
    int total=0;
    for(int x1 : x)
    {
	total=total+x1;
        }
         System.out.println("The sum :"+total);
}
public static void main(String[] args)
{
 sum();
 sum(10);
 sum(10,20);
 sum(10,20,30,40);
 }
}
Output:
The sum: 0
The sum: 10
The sum: 30
The sum: 100

1. methodOne(int... x) (valid)
2. methodOne(int ...x) (valid)
3. methodOne(int...x) (valid)
4. methodOne(int x...) (invalid)
5. methodOne(int. ..x) (invalid)
6. methodOne(int .x..) (invalid)

methodOne(int a,int... b)        //valid
methodOne(String s,int... x)	//valid

methodOne(int a,int... b)        //valid
methodOne(int... a,int b)   //(invalid)

methodOne(int... a,int... b)  //(invalid)

class Test 
{
	public static void methodOne(int i)
	{
		System.out.println("general method");
	}
	public static void methodOne(int... i)
	{
		System.out.println("var-arg method");
	}
	public static void main(String[] args)
	{
		methodOne();//var-arg method
		methodOne(10,20);//var-arg method
		methodOne(10);//general method
	}
}

class Test 
{
		
{
		System.out.println("var-arg method");
	}	
	public static void main(String[] args)
	{
		methodOne(new int[]{10,20,30});//var-arg method
	}
}

class Test 
{
	public void methodOne(int[] i){}
	public void methodOne(int... i){}
}
Output:
Compile time error.
Cannot declare both methodOne(int...) and methodOne(int[]) in Test

Single Dimensional Array Vs Var-Arg Method:

class Test 
{
	public static void main(String... args)
	{
		System.out.println("var-arg main method");//var-arg main method
	}
}

1. methodOne(int... x)
   we can call this method by passing a group of int values and x will become 1D array. (i.e., int[] x)
2. methodOne(int[]... x)
   we can call this method by passing a group of 1D int[] and x will become 2D array. ( i.e., int[][] x)

class Test 
{
	public static void methodOne(int[]... x)
	{
		for(int[] a:x)
		{
			System.out.println(a[0]);
		}
	}
	public static void main(String[] args)
	{
		int[] l={10,20,30};
		int[] m={40,50};
		methodOne(l,m);
		
	}
}
Output:
10
40

Main Method

class Test 
{}
Output:
javac Test.java
java Test R.E: NoSuchMethodError: main

1. The order of modifiers is not important that is instead of public static we can take static public.
2. We can declare string[] in any acceptable form
   • String[] args
   • String []args
   • String args[]
3. Instead of args we can use any valid java identifier.
4. We can replace string[] with var-arg parameter.
   Example: main(String... args)
5. main() method can be declared with the following modifiers.
   final, synchronized, strictfp.

   class Test { 
    static final syncronized strictfp public void  main(String... ask){
     System.out.println("valid  main method");
    }
   }
   output :
   valid main method
   

Which of the following main() method declarations are valid ?

1. public static void main(String args){} (invalid)
2. public synchronized final strictfp void main(String[] args){} (invalid)
3. public static void Main(String... args){} (invalid)
4. public static int main(String[] args){} //int return type we can't take //(invalid)
5. public static synchronized final strictfp void main(String... args){}(valid)
6. public static void main(String... args){}(valid)
7. public void main(String[] args){}(invalid)

In which of the above cases we will get compile time error ?

class Test 
{
	public static void main(String[] args)
	{
	System.out.println("String[] array main method");      //overloaded methods
	}
	public static void main(int[] args)
	{
	System.out.println("int[] array main method");
	}
}
Output:
String[] array main method

The other overloaded method we have to call explicitly then only it will be executed.

Case 2:
Inheritance concept is applicable for static methods including main() method
hence while executing child class if the child class doesn't contain main() method then the parent class main() method will be executed.

class Parent
{
	public static void main(String[] args)
	{
		System.out.println("parent main");          //Parent.java
	}
}
class Child extends Parent
{}

class Parent
{
	public static void main(String[] args)
	{
		System.out.println("parent main");         // Parent.java
	}
}
class Child extends Parent				     
{
	public static void main(String[] args)
	{
		System.out.println("Child main");
	}
}

1.7 Version Enhansements with respect to main() :

• Untill 1.6v if our class doesn't contain main() method then at runtime we will get Runtime Exception saying NosuchMethodError:main
• But from 1.7 version onwards instead of NoSuchMethodError we will get more meaning full description

class Test {
}
 
 1.6 version :
javac Test.java
java Test
RE: NoSuchMethodError:main

1.7 version :
javac Test.java
java Test
Error: main method not found in class Test, please  define the main  method as
public static void main(String[]  args)

class Test {
 static {
  System.out.println("static block");
 }
}
 
 1.6 version :
javac Test.java
java Test
output :
static block
RE: NoSuchMethodError:main

1.7 version :
javac Test.java
java Test
Error: main method not found in class Test, please  define the main  method as
public static void main(String[]  args)

class Test {
 static {
  System.out.println("static block");
  System.exit(0);
 }
}
 
 1.6 version :
javac Test.java
java Test
output :
static block

1.7 version :
javac Test.java
java Test
Error: main method not found in class Test, please  define the main  method as
public static void main(String[]  args)

class Test {
 static {
  System.out.println("static block");
 }
 public static void main(String[] args) {
  System.out.println("main method");
 }
}
 
 1.6 version :
javac Test.java
java Test
output :
static block
main method

1.7 version :
javac Test.java
java Test
output :
static block
main method

Command line arguments:

class Test
{
	public static void main(String[] args)
	{
		for(int i=0;i<=args.length;i++)
		{
			System.out.println(args[i]);
		}
	}
}
Output:
java Test x y z
ArrayIndexOutOfBoundsException: 3

class Test
{
	public static void main(String[] args)
	{
	 String[] argh={"X","Y","Z"};
	  args=argh;
		for(String s : args)
		{
			System.out.println(s);
		}
	}
}
Output:
java Test A B C
X
Y
Z

java Test A B 
X
Y
Z

java Test 
X
Y
Z

class Test
{
	public static void main(String[] args)
	{
		System.out.println(args[0]+args[1]);
	}
}
Output:
E:\SCJP>javac Test.java
E:\SCJP>java Test 10 20
1020

class Test
{
	public static void main(String[] args)
	{
		System.out.println(args[0]);
	}
}
Output:
E:\SCJP>javac Test.java
E:\SCJP>java Test "Sai Charan"
Sai Charan

Java coding standards

• Whenever we are writing java code , It is highly recommended to follow coding standards , which improves the readability and understandability of the code.
• Whenever we are writing any component(i.e., class or method or variable) the name of the component should reflect the purpose or functionality.

Coding standards for classes:

• Usually class names are nouns.
• Should starts with uppercase letter and if it contains multiple words every inner word should starts with upper case letter.

Coding standards for interfaces:

• Usually interface names are adjectives.
• Should starts with upper case letter and if it contains multiple words every inner word should starts with upper case letter.

1. Serializable
2. Runnable
3. Cloneable

Coding standards for methods:

• Usually method names are either verbs or verb-noun combination.
• Should starts with lowercase character and if it contains multiple words every inner word should starts with upper case letter.(camel case convention)

Coding standards for variables:

• Usually variable names are nouns.
• Should starts with lowercase alphabet symbol and if it contains multiple words every inner word should starts with upper case character.(camel case convention)

length
name
salary            	        nouns
age
mobileNumber

Coding standards for constants:

• Usually constants are nouns.
• Should contain only uppercase characters and if it contains multiple words then these words are separated with underscore symbol.
• Usually we can declare constants by using public static final modifiers.

MAX_VALUE	
MIN_VALUE                           nouns
NORM_PRIORITY

Java bean coding standards:

Syntax for setter method:

1. Method name should be prefixed with set.
2. It should be public.
3. Return type should be void.
4. Compulsory it should take some argument.

Syntax for getter method:

1. The method name should be prefixed with get.
2. It should be public.
3. Return type should not be void.
4. It is always no argument method.

Coding standards for listeners:

To register a listener:

1. public void addMyActionListener(MyActionListener l) (valid)
2. public void registerMyActionListener(MyActionListener l) (invalid)
3. public void addMyActionListener(ActionListener l) (invalid)

To unregister a listener:

1. public void removeMyActionListener(MyActionListener l) (valid)
2. publc void unregisterMyActionListener(MyActionListener l) (invalid)
3. public void removeMyActionListener(ActionListener l) (invalid)
4. public void deleteMyActionListener(MyActionListener l) (invalid)

Various Memory areas present inside JVM :

1. Class level binary data includung static variables will be stored in method area.
2. Objects and corresponding instance variables will be stored in Heap area.
3. For every method the JVM will create a Runtime stack all method calls performed by that Thread and corresponding local variables will be stored in that stack.
   Every entry in stack is called Stack Frame or Action Record.
4. The instruction which has to execute next will be stored in the corresponding PC Registers.
5. Native method invocations will be stored in native method stacks.