Java Tutorial - Java For

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Java for loop statement provides a powerful way of writing loop statement.

The simplest form of the for loop is shown here:

   for(initialization; condition; iteration) 
     statement; 

Java for loop statement has three parts:

• initialization sets a loop control variable to an initial value.
• condition is a Boolean expression that tests the loop control variable. If condition is true, the for loop continues to iterate. If condition is false, the loop terminates.
• The iteration determines how the loop control variable is changed each time the loop iterates.

Here is a short program that illustrates the for loop. i is the loop control variable and i is initialized to zero in the initialization. At the start of each iteration, the conditional test x < 10 is performed. If the outcome of this test is true, the println() statement is executed, and then the iteration portion of the loop is executed. This process continues until the conditional test is false.

public class Main {
  public static void main(String args[]) {
    int i;

    for (i = 0; i < 10; i = i + 1)
      System.out.println("This is i: " + i);
  }
}

This program generates the following output:

Example

The following code writes the code logic from above again but loops reversively:

 
public class Main {
  public static void main(String args[]) {
    for (int n = 10; n > 0; n--)
      System.out.println("n:" + n);
  }
}  ]]>

The output:

Example 2

Here is a program that tests for prime numbers using for loop statement.

 
public class Main {
  public static void main(String args[]) {
    int num;/* w  w  w.java2 s  . com*/
    boolean isPrime = true;
    num = 50;
    for (int i = 2; i <= num / 2; i++) {
      if ((num % i) == 0) {
        isPrime = false;
        break;
      }
    }
    if (isPrime)
      System.out.println("Prime");
    else
      System.out.println("Not Prime");

  }
}

The output:

Example 3

Java allows two or more variables to control a for loop. And you can include multiple statements in both the initialization and iteration portions of the for loop. Each statement is separated from the next by a comma. Here is an example:

 
public class Main {
  public static void main(String args[]) {
//w w w . j  a  v  a2 s  . co m
    for (int a = 1, b = 4; a < b; a++, b--) {
      System.out.println("a = " + a);
      System.out.println("b = " + b);

    }
  }
}

The program generates the following output:

Example 4

The three sections of the for can be used for any purpose and parts of the for loop can be empty.

 
public class Main {
  public static void main(String args[]) {
    int i = 0;/*from  w w w. j  a va2  s  .c o m*/
    boolean done = false;
    for (; !done;) {
      System.out.println("i is " + i);
      if (i == 10)
        done = true;
      i++;

    }
  }
}

The output:

Example 5

for loop can be nested to produce powerful logic, for example, we can use nested for loop to iterate a two-dimensional array. For example, here is a program that nests for loops:

 
public class Main {
  public static void main(String args[]) {
    for (int i = 0; i < 10; i++) {
      for (int j = i; j < 10; j++)
        System.out.print(".");
      System.out.println();
    }
  }
}

The output produced by this program is shown here:

Java for each loop

The for each loop iterates elements in a sequence without using the loop counter.

The syntax of for each loop is:

for (type variable_name:array){
       
}

The type must be compatible with the array type.

The following code shows how to use for each loop.

public class Main {
  public static void main(String args[]) {
    String[] arr = new String[]{"java2s.com","a","b","c"};
    for(String s:arr){
      System.out.println(s);
    }
  }
}

The output:

Example 6

The following code uses the for-each style loop to iterate a two-dimensional array.

public class Main {
  public static void main(String args[]) {
    int sum = 0;/* w  w w.  ja  va2  s  . c o  m*/
    int nums[][] = new int[3][5];
    for (int i = 0; i < 3; i++){
      for (int j = 0; j < 5; j++){
        nums[i][j] = (i + 1) * (j + 1);
      }  
    }
    // use for-each for to display and sum the values
    for (int x[] : nums) {
      for (int y : x) {
        System.out.println("Value is: " + y);
        sum += y;
      }
    }
    System.out.println("Summation: " + sum);
  }
}

The output from this program is shown here:

Example 7

for-each style loop is useful when searching an element in an array.

public class Main {
  public static void main(String args[]) {
    int nums[] = { 6, 8, 3, 7, 5, 6, 1, 4 };
    int val = 5;//from   ww w.j a  v a 2s . c  om
    boolean found = false;
    // use for-each style for to search nums for val
    for (int x : nums) {
      if (x == val) {
        found = true;
        break;
      }
    }
    if (found)
      System.out.println("Value found!");
  }
}

The code above generates the following result.

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