Pattern Program in java Part-3

Posted by: Instanceofjava Posted date: September 13, 2015 / comment : 0


 Program #1:  java program to print pyramid of stars using for loop in below format


*
**
***
****
*****
******
*******
********
*********
**********




  1. package com.learnJavaOnline;
  2. public class PrintStarsFormat {
  3.  
  4. public static void main(String[] args) {
  5.   
  6. int  numberOfStars=10;
  7.  
  8. for (int row = 1; row <= 10; row++) {
  9.  
  10.     for(numberOfStars = 1; numberOfStars <= row; numberOfStars++) {
  11.       System.out.print("*");
  12.     }
  13.  
  14.     System.out.println(); 
  15.  }
  16.  
  17. }
  18.  
  19. }
Output:
  1. *
  2. **
  3. ***
  4. ****
  5. *****
  6. ******
  7. *******
  8. ********
  9. *********
  10. **********

            

Program #2: java program to print pyramid of stars using for loop in below format



**********
*********
********
*******
******
*****
****
***
**
*



  1. package com.learnJavaOnline;
  2. public class PrintStarsPyramid {
  3.  
  4. public static void main(String[] args) {
  5.   
  6. int  numberOfStars=10;
  7.  
  8. for(int i=numberOfStars; i>0 ;i--){
  9.  
  10.     for(int j=0; j < i; j++){
  11.  
  12.           System.out.print("*");
  13.     }
  14.  
  15.     System.out.println("");
  16. }
  17.  
  18. }
  19.  
  20. }
Output:
  1. **********
  2. *********
  3. ********
  4. *******
  5. ******
  6. *****
  7. ****
  8. ***
  9. **
  10. *



Program #3 java program to print pyramid of stars using for loop in below format


*
**
***
****
*****
******
*******
********
*********
**********
**********
*********
********
*******
******
*****
****
***
**
*




  1. package com.learnJavaOnline;
  2. public class PrintStarsPyramid {
  3.  
  4. public static void main(String[] args) {
  5.   
  6. int  numberOfStars=10;
  7.  
  8.  for(int i=1; i<= numberOfStars ;i++){
  9.  
  10. for(int j=0; j < i; j++){
  11.  
  12.   System.out.print("*");
  13.  
  14. }
  15.  
  16. System.out.println("");
  17.  
  18. }
  19.  
  20. for(int i=numberOfStars; i>0 ;i--){
  21.  
  22. for(int j=0; j < i; j++){
  23.   System.out.print("*");
  24. }
  25.  
  26. System.out.println("")}
  27.  
  28. }
  29.  
  30. }
Output:
  1. *
  2. **
  3. ***
  4. ****
  5. *****
  6. ******
  7. *******
  8. ********
  9. *********
  10. **********
  11. **********
  12. *********
  13. ********
  14. *******
  15. ******
  16. *****
  17. ****
  18. ***
  19. **
  20. *




Print Pascals triangle using java program


Pattern Program in java Part-2

Posted by: Instanceofjava Posted date: September 06, 2015 / comment : 0

#4 Java Program to print Numbers in Below pattern

1 2 3 4 5 6 7 8 9
 1 2 3 4 5 6 7 8
  1 2 3 4 5 6 7
   1 2 3 4 5 6
    1 2 3 4 5
     1 2 3 4
      1 2 3
       1 2
        1
        




  1. package com.learnJavaOnline;
  2. public class NumbersFormat {
  3. public static void main(String[] args) {
  4.  
  5. int r, c1,c2;
  6.  
  7. for (r = 1; r <= 10; r++) {
  8.  
  9. for (c1 = 1; c1 <r; c1++) {
  10.   System.out.print(" ");
  11. }
  12.  
  13. for (c2 = 1; c2 <= 10 - r; c2++) {
  14.     System.out.print(c2 + " ");
  15. }
  16.  
  17. System.out.println("");
  18. }
  19.  
  20. }
  21.  
  22. }


Output:
  1. 1 2 3 4 5 6 7 8 9  
  2. 1 2 3 4 5 6 7 8 
  3.   1 2 3 4 5 6 7 
  4.    1 2 3 4 5 6 
  5.     1 2 3 4 5 
  6.      1 2 3 4 
  7.       1 2 3 
  8.        1 2 
  9.         1
            
#5 Java Program to Print Numbers in Below pattern:

        1
       1 2
      1 2 3
     1 2 3 4
    1 2 3 4 5
   1 2 3 4 5 6
  1 2 3 4 5 6 7
 1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9 10




  1. package com.learnJavaOnline;
  2. public class NumbersFormat {
  3. public static void main(String[] args) {
  4.  
  5. int r, c1,c2;
  6.  
  7. for (r = 1; r <= 10; r++) {
  8.  
  9. for (c1 = 1; c1 <10-r; c1++) {
  10.   System.out.print(" ");
  11. }
  12.  
  13. for (c2 = 1; c2 <= r; c2++) {
  14.     System.out.print(c2 + " ");
  15. }
  16.  
  17. System.out.println("");
  18. }
  19.  
  20. }
  21.  
  22. }

Output:
  1.         1 
  2.        1 2 
  3.       1 2 3 
  4.      1 2 3 4 
  5.     1 2 3 4 5 
  6.    1 2 3 4 5 6 
  7.   1 2 3 4 5 6 7  
  8. 1 2 3 4 5 6 7 8 
  9. 1 2 3 4 5 6 7 8 9 
  10. 1 2 3 4 5 6 7 8 9 10
            

Print Pascals triangle using java program




Pattern Program in java Part-1

Posted by: Instanceofjava Posted date: September 06, 2015 / comment : 0

#1 Java Program To print Numbers in Below pattern:

              1
             2 3
            4 5 6
           7 8 9 10
         11 12 13 14 15




  1. package com.javatutorial;
  2. public class NumbersFormat {
  3.  
  4.  public static void main(String[] args) {
  5.  
  6.  int num=15;
  7.  int temp=1;
  8.      
  9.  for (int i = 1; i <= num; i++)
  10.  {
  11.  
  12.   for (int k = i; k <num; k++)
  13.    System.out.print(" ");
  14.    for (int j =1; j <= i; j++){
  15.  
  16.     System.out.print("" +temp+ " ");
  17.      temp++;
  18.  
  19.  if(temp>15){
  20.        break;
  21.  }
  22.  
  23.  }
  24.  
  25.   System.out.println();
  26.  
  27. if(temp>15){
  28.      break;
  29.   }
  30.  
  31. }
  32.  
  33. }

  35. }
Output:

  1.               1
                 2 3
                4 5 6
               7 8 9 10
              11 12 13 14 15





# Java Program to print Numbers in Below Format:

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
1 2 3 4 5 6
1 2 3 4 5 6 7
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9 10


  1. package com.learnJavaOnline;
  2. public class NumbersFormat {
  3. public static void main(String[] args) {
  4.  
  5. int r, c;
  6.  
  7. for (r = 1; r <= 10; r++) {
  8.  
  9. for (c = 1; c <= r; c++) {
  10.  
  11. System.out.print(c + " ");
  12.  
  13. }
  14.  
  15. System.out.println("");
  16. }
  17.  
  18. }
  19.  
  20. }
Output:
  1. 1 
  2. 1 2 
  3. 1 2 3 
  4. 1 2 3 4 
  5. 1 2 3 4 5 
  6. 1 2 3 4 5 6 
  7. 1 2 3 4 5 6 7 
  8. 1 2 3 4 5 6 7 8 
  9. 1 2 3 4 5 6 7 8 9 
  10. 1 2 3 4 5 6 7 8 9 10


#3 Java Program to print numbers in below format

1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7
1 2 3 4 5 6
1 2 3 4 5
1 2 3 4
1 2 3
1 2
1


  1. package com.learnJavaOnline;
  2. public class NumbersFormat {
  3. public static void main(String[] args) {
  4.  
  5. int r, c;
  6.  
  7. for (r = 1; r <= 10; r++) {
  8.  
  9. for (c = 1; c <= 10-r; c++) {
  10.  
  11. System.out.print(c + " ");
  12.  
  13. }
  14.  
  15. System.out.println("");
  16. }
  17.  
  18. }
  19.  
  20. }

Output:
  1. 1 2 3 4 5 6 7 8 9 
  2. 1 2 3 4 5 6 7 8 
  3. 1 2 3 4 5 6 7 
  4. 1 2 3 4 5 6 
  5. 1 2 3 4 5 
  6. 1 2 3 4 
  7. 1 2 3 
  8. 1 2 
  9. 1



Print Pascals triangle using java program

Return type / return statement in java example

Posted by: Instanceofjava Posted date: June 27, 2015 / comment : 3

Return type in java: return statement in java

  • Basically return type is used in java methods.
  • Method signature includes this return type.
  • public int show(){ // }
  • we will use methods to do a particular task after completion of task if we want to return something to the calling place these return types will be used.
  • Based on the type of data to be returned will mention it as int , char , float double etc as return type in method signature and return statement should be the last statement of the method body.
  • In Java, the return statement is used to exit a method and return a value to the calling method. The value returned can be of any data type that is specified in the method's return type. 
  • For example, if a method has a return type of int, it can return an integer value. Here's an example of a simple method that uses the return statement to return an int value:


public int add(int a, int b) {
    int sum = a + b;
    return sum;
}

  • In this example, the add method takes two int parameters, a and b, and returns the sum of the two values. The method uses the return statement to return the value of the sum variable back to the calling method.
  • You can also use return statement without returning any value, in this case method should have void return type.


public void printHelloWorld() {
    System.out.println("Hello, World!");
    return;
}

  • In this case the method printHelloWorld doesn't return any value and the calling method doesn't need to assign the return to any variable, just invoking the method will do the job.
  • It is also worth noting that when a return statement is executed, it immediately exits the current method, regardless of where it is in the method's execution flow. So, any code after a return statement will not be executed.

Type of declaration of methods based on return type and arguments:

1.Method with out return type  and without arguments: return statement in java


  1. package com.instanceofjava;
  2. class sample{
  3.  
  4. public void add(){
  5.  
  6. int a=40;
  7. int b=50;
  8. int c=a+b;
  9. System.out.println(c);
  10.  
  11. }

  13. public static void main(String args[]) // ->method prototype.
  14. { 
  15.  
  16. sample obj= new sample();
  17. obj.add();
  18.  
  19.  
  20.  }
  21. }

2.Method with out return type and with arguments.

  1. package com.instanceofjava;
  2. class sample{
  3.  
  4. public void add(int a, int b){
  5.  
  6. int c=a+b;
  7. System.out.println(c);
  8.  
  9. }

  11. public static void main(String args[]) // ->method prototype.
  12. { 
  13.  
  14. sample obj= new sample();
  15. obj.add(13,24);
  16.  
  17.  
  18.  }
  19. }

3.Method with return type  and without arguments.


  1. package com.instanceofjava;
  2. class sample{
  3.  
  4. public int add(){
  5. int a=40;
  6. int b=50;
  7. int c=a+b;
  8. return c;
  9. }

  11. public static void main(String args[]) // ->method prototype.
  12. { 
  13.  
  14. sample obj= new sample();
  15. int x=obj.add(); 
  16. System.out.println(x);
  17.  
  18.  }
  19. }

4.Method with return type and with arguments.

 

  1. package com.instanceofjava;
  2. class sample{
  3.  
  4. public int add(int a, int b){
  5.  
  6. int c=a+b;
  7. return c;
  8. }

  10. public static void main(String args[]) // ->method prototype.
  11. { 
  12.  
  13. sample obj= new sample();
  14.  
  15. int x=obj.add(1,2);
  16. System.out.println(x);
  17.  
  18.  }
  19. }


If Else Statment in Java

Posted by: Instanceofjava Posted date: June 27, 2015 / comment : 0

If statement :
  • if statement in java is decision making statement in java.
  • if statement will have a condition and if that condition is true then the corresponding block will be executed.
  • if(condition){ //  }
If condition syntax:



  1. if(condition){
  2. //  statements
  3. }
 Sample program on if statement:
  1. package com.instanceofjava; 
  2. import java.lang.*; 
  3.  
  4. public class ifExample { 
  5.  
  6. public static void main(String[] args) {
  7.  
  8. if(true){
  9.  
  10.  System.out.println("if condition executed");
  11.  
  12. }
  13.  
  14. }

Output:
  1. if condition executed

Java program to compare two numbers in java
  1. package com.instanceofjava; 
  2. import java.lang.*; 
  3.  
  4. public class simpleIfExample { 
  5.   
  7. public static void main(String[] args) {
  8.  
  9.   int x= 37;
  10.   int y= 37;
  11.                
  12. if(x==y){ 
  13.     System.out.println(x+ " is equal to " + y);
  14. }
  15.  
  16. if(x>y){
  17.     System.out.println(x+ " is greater than " + y);
  18. }
  19.  
  20. if(x<y){
  21.     System.out.println(x+ " is less than " + y);
  22. }
  23. }

Output:
  1. 37 is equal to 37



 Else Statement:

  •  if(condition){ //} else { // }

Java program to compare two numbers in java using if else
  1. package com.instanceofjava; 
  2. import java.lang.*; 
  3.  
  4. public class ifElseExample { 
  5.   
  7. public static void main(String[] args) {
  8.  
  9.   int x= 37;
  10.   int y= 37;
  11.                
  12. if(x==y){
  13.     System.out.println(x+ " is equal to " + y);
  14. }else if(x>y){
  15.     System.out.println(x+ " is greater than " + y);
  16. }else{
  17.     System.out.println(x+ " is less than " + y);
  18. }
  19. }



Output:
  1. 37 is equal to 37


Java program to find leap year or not in java using if else
  1. package com.instanceofjava; 
  2. import java.lang.*; 
  3.  
  4. public class ifElseExample { 
  5.   
  7. public static void main(String[] args) {
  8.  
  9.     int year = 2015;                      
  10.   //if year is divisible by 4, it is a leap year

  12.  if((year % 400 == 0) || ((year % 4 == 0) && (year % 100 != 0)))
  13.  System.out.println("Year " + year + " is a leap year");
  14.  else
  15.       System.out.println( year + " is not a leap year");
  16.   
  17.  
  18. }

Output:
  1. 2015 is not a leap year


Java program to find even or odd using if else statment
  1. package com.instanceofjava; 
  2. import java.lang.*; 
  3.  
  4. public class ifElseEvenOddExample { 
  5.  
  6. public static void main(String[] args) {
  7.   
  8. int a=10;
  9. if((a%2)==0){
  10.  
  11.  System.out.println(a+"is even number");
  12.  
  13. }else{ 

  15.     System.out.println(a+"is odd number");
  16.  
  17. }
  18.  
  19. }

Output:
  1. 10 is even number

Static block in Java

Posted by: Instanceofjava Posted date: May 23, 2015 / comment : 0 ,

  • Static variables are class level variables and without instantiating class we can access these variables.

    1. Static int a;


  • Class loading time itself these variables gets memory
  • Static methods are the methods with static keyword are class level. without creating the object of the class we can call these static methods.

    1. public static void show(){ 
    2.  
    3. }

  • Now its time to discuss about static blocks.
  • Static block also known as static initializer
  • Static blocks are the blocks with static keyword.
  • Static blocks wont have any name in its prototype.
  • Static blocks are class level.
  • Static block will be executed only once.
  • No return statements.
  • No arguments.
  • No this or super keywords supported.
    •  
    1. static{ 
    2.  
    3.  }

     

What is the need of static block?

  • Static blocks will be executed at the time of class loading.
  • So if you want any logic that needs to be executed at the time of class loading that logic need to place inside the static block so that it will be executed at the time of class loading. 
  • To initialize the static variables while class loading itself we need static block.
  • If we have multiple classes then if you want to see the order of those classes loading then static block needed.

When to use static blocks?

  • If you want to access static variables before executing the constructor.
  • If you want to execute your code only once even any number of objects created.
  • If you want to initialize static constants.

When and where static blocks will be executed?

  • Static blocks will be executed at the time of class loading by the JVM  by creating separate stack frames in java stacks area (Please refer JVM Architecture  to know about stacks area).
  • Static blocks will be executed in the order they defined from top to bottom.



  1. package com.instanceofjava;
  2.  
  3. class StaticBlockDemo 
  4. {
  5.  
  6. static{
  7.       System.out.println("First static block executed");
  8. }
  9.  
  10. static{
  11.      System.out.println("Second static block executed");
  12. }
  13.  
  14. static{
  15.      System.out.println("Third static block executed");
  16. }
  17.  
  18. }
     
Output:
  1. First static block executed
  2. Second static block executed
  3. Third static block executed

Order of execution of static block and main method:

  1. package com.instanceofjava;
  2.  
  3. class StaticBlockDemo 
  4. {
  5.  
  6. public static void main (String[] args) throws java.lang.Exception
  7. {
  8.         System.out.println("Main method executed");
  9. }
  10.  
  11. static{
  12.       System.out.println("First static block executed");
  13. }
  14.  
  15. static{
  16.      System.out.println("Second static block executed");
  17. }
  18.  
  19. static{
  20.      System.out.println("Third static block executed");
  21. }
  22.  
  23. }
     
Output:
  1. First static block executed
  2. Second static block executed
  3. Third static block executed
  4. Main method executed


Order of execution of Static block and constructor:

 

  1. package com.instanceofjava;
  2.  
  3. class StaticBlockDemo 
  4. {
  5.  
  6. StaticBlockDemo(){
  7.  
  8.   System.out.println("Constructor executed");
  9.  
  10. }
  11. public static void main (String[] args) throws java.lang.Exception
  12. {
  13.  
  14.     System.out.println("Main method executed");
  15.     StaticBlockDemo obj = new StaticBlockDemo();
  16.  
  17. }
  18.  
  19. static{
  20.       System.out.println("First static block executed");
  21. }
  22.  
  23. static{
  24.      System.out.println("Second static block executed");
  25. }
  26.  
  27. static{
  28.      System.out.println("Third static block executed");
  29. }
  30.  
  31. }
     


Output:
  1. First static block executed
  2. Second static block executed
  3. Third static block executed
  4. Main method executed
  5. Constructor executed

What is the output of following program:


  1. package com.instanceofjava;
  2.  
  3. class StaticBlockDemo 
  4. {
  5.  
  6.   static int a=getNum();

  8. public static int getNum(){
  9.  
  10. System.out.println("In method m1()");
  11. return 10;
  12.  
  13. }

  15. public static void main (String[] args) throws java.lang.Exception
  16. {
  17.  
  18.     System.out.println("Main method executed");
  19.   
  20.  
  21. }
  22.  
  23. static{
  24.       System.out.println("First static block executed");
  25. }
  26.  
  27. static{
  28.      System.out.println("Second static block executed");
  29. }
  30.  
  31. static{
  32.      System.out.println("Third static block executed");
  33. }
  34.  
  35. }
     
Output:
  1. In method m1()
  2. First static block executed
  3. Second static block executed
  4. Third static block executed
  5. Main method executed


Static methods in java example

Posted by: Instanceofjava Posted date: April 20, 2015 / comment : 0

  • A method which has static keyword in its definition is called static method.
  1. static void print(){
  2.  
  3. }

  • JVM will not execute static methods by default. They are executed only if they are called explicitly by developer either from main method or from static variable as its assignment statement or from static block.

Static method called from main method:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  
  5. static void show(){
  6. System.out.println("static method"):
  7. }
  8.  
  9. public static void main(String[] args)
  10. {
  11.  
  12. show();
  13.  
  14. }
  15. }
Output:

  1. static method

Static method called from variable assignment:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  static int x=show();
  5. static int show(){
  6. System.out.println("static method called"):
  7. return 10;
  8. }
  9.  
  10. public static void main(String[] args)
  11. {
  12.  
  13. System.our.println(x);
  14.  
  15. }
  16. }
Output:

  1. static method called
  2. 10

Static method called from static block:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  static{
  5.  show();
  6. }

  8. static void show(){
  9. System.out.println("static method called"):
  10. }
  11.  
  12. }
Output:

  1. static method called

Order of Execution:

  •  Static methods are executed in the order of they are called, not in the order of they are defined.
  • All static methods are executed in java stack area by creating separate stack frame.
  • When a method is called from main method, JVM creates stack frame in main thread for that method execution.
  • The stack frame is destroyed immediately after method execution is completed.

Example program on order of execution of static methods:


  1. package com.instanceofjava;
  2. class StaticMehodDemo
  3. {
  4.  
  5. static void show(){
  6.  
  7. System.out.println("show method called");
  8.  
  9. }
  10.  
  11. static void print(){
  12.  
  13. System.out.println("print method called");
  14.  
  15. }
  16.  
  17. static void display(){
  18.  
  19. System.out.println("display method called");
  20.  
  21. }
  22.  
  23.  
  24. public static void main(String[] args)
  25. {
  26.  
  27. System.our.println("main method called");
  28.  
  29. show();
  30. print();
  31. display();
  32.  
  33.  
  34. }
  35. }


Output:

  1. main method called
  2. print method called
  3. show method called
  4. display method called

Variable initialization with same variable:

  •  We can initialize variable with same variable name. this assignment is valid. In this case the variable value is replaced with same value.
  1. int x=20;
  2. x=x; // valid statement
Example program:



  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  static int a=10;

  6. public static void main(String[] args){
  7.   
  8. int a=20;
  9. a=a;
  10. System.out.println("a="+a);
  11.  System.out.println("StaticDemo.a="+StaticDemo.a);
  12.  
  13. }
  14.  
  15. }
Output:

  1. 20
  2. 10

 

Local preference with parameters:

  • Parameters are also treated as local variables.
  • Hence if parameter declared with same static variable name and if we want to access static variable in presence of parameter or if we want to initialize static variable with parameter name we must refer variable with class name.

Example program:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  static int x=10;
  5.  
  6. static void m1(int x){ 
  7.  
  8.  System.out.println(x);
  9.  System.out.println(StaticDemo.x);

  11. }

  13. public static void main(String[] args){
  14.   
  15.  m1(37);
  16.  System.out.println(x);
  17.  
  18. }
  19.  
  20. }
Output:

  1. 37
  2. 10

 

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Static Members in java

Posted by: Instanceofjava Posted date: April 19, 2015 / comment : 2

  • The class level members which have static keyword in their definition are called static members.

Types of Static Members:

  •  Java supports four types of static members
  1. Static Variables
  2. Static Blocks
  3. Static Methods
  4. Main Method (static method)

  • All static members are identified and get memory location at the time of class loading by default by JVM in Method area.
  • Only static variables get memory location, methods will not have separate memory location like variables.
  • Static Methods are just identified and can be accessed directly without object creation.

1.Static Variable:

  • A class level variable which has static keyword in its creation statement is called static variable.

  1. package com.instanceofjava;
  2. class Demo{
  3.  
  4. static int a=10;
  5. static int b=20;
  6.  
  7. }

  • We can not declare local variables as static it leads to compile time error "illegal start of expression".
  • Because being static variable it must get memory at the time of class loading, which is not possible to provide memory to local variable at the time of class loading.

  1. package com.instanceofjava;
  2. class Demo{
  3.  
  4. static int a=10;
  5. static int b=20;
  6.  public static void main(String [] args){
  7.  
  8.    //local variables should not be static
  9.  static int a=10;// compile time error: illegal start of expression

  11. }


  • All static variables are executed by JVM in the order of they defined from top to bottom.
  • JVM provides individual memory location to each static variable in method area only once in a class life time.

Life time and scope:

  •  Static variable get life as soon as class is loaded into JVM and is available till class is removed from JVM or JVM is shutdown.
  • And its scope is class scope means it is accessible throughout the class.

  1. package com.instanceofjava;
  2. class StaticDemo{
  3.  
  4. static int a=10;
  5. static int b=20;
  6.  
  7.  public static void main(String [] args){
  8.  
  9.    System.out.println("a="+a);
  10.    System.out.println("a="+b);
  11.    show();

  13.  }
  14.  
  15. public static void show(){
  16.  
  17.    System.out.println("a="+a);
  18.    System.out.println("a="+b);
  19. }

  21. }

static methods JVM architecture

Duplicate Variables:

  • If multiple variables are created with same name are considered as duplicate variables.
  • In the same scope we can not create multiple variables with same name.
  • If we create it leads to compile time error "variable is already defined".
  • Even if we change data type or modifier or its assigned value we can not create another variable with same name.

  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4.  
  5. static int a=10;
  6. int a=30;// Compile time error 
  7.  
  8. }

  • But it is possible to create multiple variables with same name in different scopes.

  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=10;
  5. int a=30;// Compile time error
  6.  
  7. public static void main(String [] args){
  8.   
  9. // it is allowed to define "a" in this method.
  10. int a=20;
  11.  
  12. }
  13. }

Shadowing:

  • It is possible to create local variables or parameters with same variable name.
  • The concept is called shadowing . It means local variable is a shadow of class level variable.
  • It means when you access it in side method , you will get local variables value but not  from class level.

  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=10;

  6.  
  7. public static void main(String [] args){
  8.   

  10.  int a=20;
  11.  System.out.println("a="+a); // prints 20

  13. }
  14. }

 Local preference:

  • When we call a variable compiler and JVM will search for its definition in that method first, if its definition is not found in that method then will search for its definition at class level.
  • If its definition not found at class level also then compiler throws error: can not find symbol.
  • This phenomenon is called local preference.


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=10;

  6.  
  7. public static void main(String [] args){
  8.  
  9.  System.out.println("a="+a); // prints 10 : method level no definition so access class level

  11.  int a=20;
  12.  System.out.println("a="+a); // prints 20 :method level variable found

  14. }
  15. }


  • By using class name we can get class level variables value.


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=10;

  6.  
  7. public static void main(String [] args){
  8.  
  9.  System.out.println("a="+a); // prints 10 : method level no definition so access class level

  11.  int a=20;
  12.  System.out.println("a="+a); // prints 20 :method level variable found

  14.  System.out.println("a="+StaticDemo.a); // prints 10 : class level variable
  15.  
  16. }
  17. }


Order of execution of static variables and main method:

  • First all static variables are executed in the order they defined from top to bottom then main method is executed.
Example Program:


  1. package com.instanceofjava;
  2. class StaticDemo
  3. {
  4. static int a=m1();
  5.  
  6. static int m1(){
  7.  
  8. System.out.println("variable a is created");
  9. return 10;
  10.  
  11. } 
  12.  
  13. static int b=m2();
  14.  
  15. static int m2(){
  16.  
  17. System.out.println("variable b is created");
  18. return 20;

  20. } 
  21.  
  22. public static void main(String [] args){
  23.   
  24.  System.out.println("in main method");
  25.  System.out.println("a="+a);
  26.  System.out.println("b="+b);


  29.  
  30. }
  31. }

Output:
  1. variable a is created
  2. variable b is created
  3. in main method
  4. 10
  5. 20

  • we can access static fields using objects but static fields should be accessed in a static way because static fields are class level so even if we change field value using particular object that will change the original value 


  1. package com.instanceofjava;
  2.  
  3. public class StaticDemo {
  4.  
  5.  static int x=10;
  6.  
  7. public static void main(String[] args) {
  8.  
  9.  StaticDemo obj=new StaticDemo();
  10.  
  11. System.out.println(x);
  12.  
  13. obj.x=30;
  14.  
  15. System.out.println(x);
  16.  
  17. StaticDemo obj2=new StaticDemo();
  18.  System.out.println(obj2.x); 
  19.  
  20. System.out.println(StaticDemo.x); // Recommended way to access static variables

  22. }
  23. }

Output:

  1. 10
  2. 30
  3. 30
  4. 30

Final Static Variables:

  • Final static variables are constants.
  • Declaration itself we need to assign value to the final static variables otherwise compiler error will come: "The blank final field  may not have been initialized".
  • And we can not change this value if we try then compiler error will come: The final field  cannot be assigned.


  1. package com.instanceofjava;
  2. class Demo{
  3.  
  4.  final static int MAX_VALUE=10;

  6.  public static void main(String [] args){
  7.  
  8.    System.out.println(MAX_VALUE);

  10. }



Java Variables and Types of Variables

Posted by: Instanceofjava Posted date: April 14, 2015 / comment : 0


Variable:

  • Variable is a named memory location used to store data temporarily.
  • During program execution we can modify that data

Limitation of Variable:

  •  It can only store single value at a time.
  • It means , always it returns latest modified value.

How can a variable be created?

  • A variable can be created by using data type.
  • As we have two types of data types  we can create two types of variables.
    1.Primitive Variable
    2.Referenced variable
  • The difference between primitive and referenced variable is "primitive variable stores data directly , where referenced variables stores reference/address of object, not direct values"

  1. package com.instanceofjava;
  2.  
  3. class Example
  4. {
  5.  
  6.  int x=10;
  7.  int y=20;
  8.  
  9. }


  1. package instanceofjava;
  2. class Test
  3. {
  4.  
  5. public int get(){
  6. return 10:
  7. }
  8.  
  9. public static void main(String [] args){
  10.  
  11. //primitive variables
  12. int s=50;
  13. int t=get(); 
  14.  
  15. //reference variables
  16. String str="a";
  17. String str1=new String("a");
  18. Example e= new Example();

  20. }
  21. }



What is an object?

  • Technically object means collection of all non static variables and methods memory locations.
  • Object is created using new keyword

Defining a variable:

  • Variable creation with value is called defining a variable
  • <Accessibility modifier><Modifier><Data type><Variable name>=<Value> ;

  1. package instanceofjava;
  2. public class Demo{
  3. public static void main(String[] args){
  4.   
  5. public static int x=10;
  6. public static Example e= new Example();

  8. }
  9. }

Declaring a variable:

  • Variable creation without value is called declaring a variable.
  • <Accessibility modifier><Modifier><Data type><Variable name>;


  1. package instanceofjava;
  2. public class Demo{
  3. public static void main(String[] args){
  4.   
  5. public static int a;
  6. public static Example e;

  8. }
  9. }


Initializing/Assigning a variable:

  • Storing a value in a variable at the time of its creation is called initializing a variable.
  • Storing a value in a variable after its creation is called assigning a value to a variable.
  • <Variable_Name>=<Value>;

  1. package instanceofjava;
  2. public class Demo{
  3. public static void main(String[] args){
  4.   
  5. //declaring a variable
  6.  int a;
  7.  int x;
  8.  
  9. //assigning a variable
  10.  a=20;
  11.  x=10;
  12.  
  13. // re initialization/ re assignment
  14.  a=30;
  15.  x=20;

  17. }
  18. }


Calling a variable:

  • Reading a value from a variable is called calling a variable.


  1. package instanceofjava;
  2. public class Demo{
  3. public static void main(String[] args){
  4.   
  5.  int x=10;
  6.   
  7.  //calling x variable for printing
  8. System.out.println(x);
  9.  
  10. // calling x variable for initializing y
  11.  int y=x;


  14. }
  15. }



Types of Variables:

  • Local
  • Static
  • Non static
  • Final
  • Transient
  • Volatile

1.Local Variables:

  • The variables created inside a method or block are called local variables.

Rules:

  • While working with local variables , we must follow below 3 rules.

Rule #1:

  •  Local variables can not be accessed from another method. Because its scope is restricted only within its method.
  • Also we can not guarantee that variable creation , because that method may or may not be called. It leads to compile time Exception : can not find symbol.
 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  
  5. public void show(){
  6.  System.out.println("a:"+a); Compile time error: can not find symbol a.
  7. }
  8.  
  9. public static void main(String [] args){
  10.  
  11.  int a=10;
  12.  System.out.println("a:"+a);
  13. }
  14. }


Rule #2:

  • Local variable should not be accessed without initialization.

 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4. public static void main(String [] args){
  5.  
  6.  int a=10;
  7.  int b;
  8.  System.out.println("a:"+a);
  9.  
  10.  System.out.println("b:"+b);//Error
  11.  // Above statement throws Compile time Error: variable b might not have been initialized
  12.  
  13.  b=20;
  14. System.out.println("b:"+b);
  15.  
  16. }
  17. }


Rule #3:

  • Local variable must be accessed only after its creation statement. because method execution is sequential execution from top to bottom. If we access before  its creation statement it leads compile time Error : Can not find symbol.


 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  
  5.  static void show(){
  6.  System.out.println("a:"+a);// Compile time Error
  7.  int a;
  8.  System.out.println("a:"+a); // Compile time Error
  9.   
  10.  a=10
  11.  
  12.  System.out.println("a:"+a);
  13.  
  14. }
  15. }

2. Static variables :

  • Static variables gets life when class is loaded.
  • It is destroyed either if class is unloaded from JVM or JVM is destroyed.
  • Its scope is throughout the class it means where class is available there static variable is available provided it is public.

 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  static int a=10;

  6. public static void show(){
  7.  System.out.println("a:"+a);
  8. }
  9.  
  10. public static void main(String [] args){
  11.  
  12.  System.out.println("a:"+a);
  13.  
  14. }
  15. }

3. Non Static variables:

  • Non static variable gets life when object is created. It is destroyed when object is destroyed.
  • Object is destroyed when it is unreferenced.
  • Its scope is the scope of the object, object is available only if its referenced variable is available

 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  
  5.  int a=10;

  7. public void show(){
  8.   test t= new test();
  9.  System.out.println("a:"+t.a);
  10. }
  11.  
  12. public static void main(String [] args){
  13.  
  14.   test t= new test();
  15.  System.out.println("a:"+t.a);
  16.  
  17. }
  18. }


4.Final variables:

  • The class level or local variable that has final keyword in its definition is called final variable.

Rule:

  •  Once it is initialized by developer its value can not be changed. If we try to change its value it leads to Compile tile error.


 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  static final int a=10;
  5. static final int b=20;

  7. public static void main(String [] args){
  8.  
  9.  a=20;//Error : variable might be already have been assigned
  10.  b=30; //Error : variable might be already have been assigned
  11.  
  12. }
  13. }


5. Transient variable:

  • The class level variable that has transient keyword in its definition is called transient variable.

Rule:

  • Local variable can not be declared as transient.
  • It leads to Compile time error: Illegal start expression


 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  static transient int a=10;
  5. static transient  int b=20;

  7. public static void main(String [] args){
  8.  
  9.  transient int x=10; // compile time error : illegal start expression
  10.  
  11. }
  12. }

  • We declare variable as transient to tell to JVM that we do not want to store variable value in a file in object serialization. Since local variable is not part of object , declaring it as transient is illegal.
  • Refer IOStreams for more details

6.Volatile Variable:

  • The class level variable that has volatile keyword in its definition. 

Rule:

  • Local variable can not be declared as transient.
  • It leads to Compile time error: Illegal start expression


 Example program:

  1. package com.instanceofjava;
  2. class test
  3. {
  4.  static volatile int a=10;
  5. volatile int b=20;

  7. public static void main(String [] args){
  8.  
  9.  volatile int x=10; // compile time error : illegal start expression
  10.  
  11. }
  12. }

  • We declare variable as volatile to tell to JVM that we do not want to modify variable value concurrently by multiple threads.
  • If we declare variable as volatile multiple threads are allowed to change its value in sequence one after another one.


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Accessibility modifiers examples

Posted by: Instanceofjava Posted date: March 09, 2015 / comment : 0


  • The keywords which define accessibility permissions are called accessibility modifiers.
  • Java supports four accessibility modifiers to define accessibility permissions at different levels.

Accessibility modifier keywords: 

 1.private

 2.protected

 3.public

 4.default(no keyword)

1.private:

  • The class members which have private keyword in its creation statement are called private members. Those members are only accessible within that class.
  • If we declare any variable or method with private accessibility modifier then those variables and methods are accessible only within that class , not accessible out side the class.

private variable are accessible within the class :


  1. package com.instanceofjava;
  2.  
  3. public class PrivateDemo {
  4.  
  5.     private String first_name;
  6.     private String last_name;
  7.  
  8. void show(){
  9.  
  10.   System.out.println("First Name:="+first_name);
  11.   System.out.println("Last Name:="+last_name);
  12.  
  13. }
  14.  
  15. public static void main(String[] args) {
  16.  
  17.        PrivateDemo obj= new PrivateDemo ();
  18.  
  19.         obj.first_name="James";
  20.         obj.last_name="Goosling";
  21.         obj.show();
  22.  
  23.     }
  24. }

Output:

  1. First Name:=James
  2. Last Name:=Goosling

private variable are not  accessible out side the class :

  1. package com.instanceofjava;
  2.  
  3. public class PrivateDemo {
  4.  
  5.     private String first_name;
  6.     private String last_name;
  7.  
  8. void show(){
  9.  
  10.   System.out.println("First Name:="+first_name);
  11.   System.out.println("Last Name:="+last_name);
  12.  
  13. }
  14.  
  15. public static void main(String[] args) {
  16.  
  17.        PrivateDemo obj= new PrivateDemo ();
  18.  
  19.         obj.first_name="James";
  20.         obj.last_name="Goosling";
  21.       
  22.  
  23.     }
  24. }

  1. package instanceofjava;
  2.  
  3. class Demo {
  4.  
  5. public static void main(String[] args){
  6.  
  7. PrivateDemo obj= new PrivateDemo ();
  8.  
  9.         obj.first_name="James"; // ERROR: The field PrivateDemo.first_name is not visible
  10.  }
  11.  
  12. }

private methods are accessible within the class :

  1. package com.instanceofjava;
  2.  
  3. public class PrivateDemo {
  4.  
  5.     private String first_name;
  6.     private String last_name;
  7.  
  8. private void show(){
  9.  
  10.   System.out.println("First Name:="+first_name);
  11.   System.out.println("Last Name:="+last_name);
  12.  
  13. }
  14.  
  15. public static void main(String[] args) {
  16.  
  17.        PrivateDemo obj= new PrivateDemo ();
  18.  
  19.         obj.first_name="James";
  20.         obj.last_name="Goosling";
  21.         obj.show();
  22.  
  23.     }
  24. }

Output:

  1. First Name:=James
  2. Last Name:=Goosling

private variable are not  accessible out side the class :

  1. package com.instanceofjava;
  2.  
  3. public class PrivateDemo {
  4.  
  5.     private String first_name;
  6.     private String last_name;
  7.  
  8. void show(){
  9.  
  10.   System.out.println("First Name:="+first_name);
  11.   System.out.println("Last Name:="+last_name);
  12.  
  13. }
  14.  
  15. public static void main(String[] args) {
  16.  
  17.        PrivateDemo obj= new PrivateDemo ();
  18.  
  19.         obj.first_name="James";
  20.         obj.last_name="Goosling";
  21.       
  22.  
  23.     }
  24. }

  1. package instanceofjava;
  2.  
  3. class Demo {
  4.  
  5. public static void main(String[] args){
  6.  
  7. PrivateDemo obj= new PrivateDemo ();
  8.  
  9.         obj.add(); // ERROR: The method add() from the type PrivateDemo is not visible
  10.  }
  11.  
  12. }



  • private variables and methods are accessible inside that class only. If we declare any variable or method as private , not accessible outside the class.

2.protected

  • The class members which have protected keyword in its creation statements are called protected members. Those members can be accessible with in package from all classes, but from out side package only in subclass that too using subclass name or its object.
  • protected variables accessible inside the package anywhere. Outside package accessible only in sub classes.

Same package anywhere:

  1. package com.instanceofjava;
  2.  
  3. public class ProtectedDemo {
  4.  
  5.     protected int a;
  6.     protected int b;
  7.  
  8. protected void show(){
  9.  
  10.   System.out.println("a="+a);
  11.   System.out.println("b="+b);
  12.  
  13. }
  14.  
  15. public static void main(String[] args) {
  16.  
  17.        ProtectedDemo obj= new ProtectedDemo ();
  18.  
  19.         obj.a=12;
  20.         obj.b=13;
  21.         obj.show();
  22.  
  23.     }
  24. }

Output:

  1. a=12
  2. b=13

Different package subclass:

  1. package com.accesiblitymodifiers;
  2.  
  3. public class Sample extends ProtectedDemo {
  4.  
  5. public static void main(String[] args) {
  6.  
  7.        Sample  obj= new Sample();
  8.  
  9.         obj.a=12;
  10.         obj.b=13;
  11.         obj.show();
  12.  
  13.     }
  14. }

Output:

  1. a=12
  2. b=13

3.public

  • If we declare any variable or method with public access specifier then those members will be accessible to everywhere.


  1. package com.instanceofjava;
  2.  
  3. public class PublicDemo {
  4.  
  5.     public int x;
  6.     public int y;
  7.  
  8. public void show(){
  9.  
  10.   System.out.println("x="+x);
  11.   System.out.println("y="+y);
  12.  
  13. }
  14.  
  15. public static void main(String[] args) {
  16.  
  17.        PublicDemo obj= new PublicDemo ();
  18.  
  19.         obj.x=1;
  20.         obj.y=2;
  21.         obj.show();
  22.  
  23.     }
  24. }


Output:

  1. x=1
  2. y=2

4.default

  • If we declare any member with no keyword those members are called default members.
  • default members are accessible to package level.
  • Means we can access anywhere in same package but we can not access in out side the package under any condition.
  • So default will acts as public inside package and private out side the package.

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