Loops in Java

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1- Loops in Java

In computer programming, a loop is used regularly, and the purpose is to execute a program section several times. The Java supports 3 different types of loops:
  1. for loop
  2. while loop
  3. do-while loop
The for loop has 2 variants:
  1. Common for loop.
  2. for-each loop (Introduced from the version of Java 5).
Statements can use in a loop:
  • break
  • continue.

2- for Loop

Syntax:
** for **
// start_value: Start Value
// end_value: End Value
// increment_number: Added value after each iteration.
for ( start_value; end_value; increment_number ) {
   // Do something here
}
Consider an illustration:
ForLoopExample1.java
package org.o7planning.tutorial.javabasic.loop;

public class ForLoopExample1 {

	public static void main(String[] args) {

		// Declare a variable 'step', step in loop
		int step = 1;

		// Declare a variable value with the start value is 0
		// After each iteration, value will increase 3
		// And the loop will end when the value greater than or equal to 10
		for (int value = 0; value < 10; value = value + 3) {

			System.out.println("Step =" + step + "  value = " + value);

			// Increase 1
			step = step + 1;

		}

	}

}
Results of running the ForLoopExample1 class:
See another example, sum the numbers from 1 to 100:
ForLoopExample2.java
package org.o7planning.tutorial.javabasic.loop;

public class ForLoopExample2 {

	// This is an example to sum the numbers from 1 to 100,
	// and print out the results to the console.
	public static void main(String[] args) {

		// Declare a variable
		int total = 0;

		// Declare a variable value
		// Initial value is 1
		// After each iteration increases the 'value' by adding 1
		// Note: value++ equivalent to the statement: value = value + 1;
		// value-- equivalent to the statement: value = value - 1;
		for (int value = 1; value <= 100; value++) {

			// Increase 'total' by adding 'value'.
			total = total + value;
		}

		System.out.println("Total = " + total);

	}

}
Result:

3- for-each loop

The for-each loop is introduced into  Java from version 5. As a loop used to traverse an array or a collection, it will move from the first element to the last element of the array or collection respectively.
See more:
  • TODO Link!

Syntax:

// arrayOrCollection: Array or Collection
for (Type variable: arrayOrCollection)  {
      // Code ...
}
Example:
ForeachExample.java
package org.o7planning.tutorial.javabasic.loop;

public class ForeachExample {

	public static void main(String[] args) {

		// Declare an array of String.
		String[] fruits = new String[] { "Apple", "Apricot", "Banana" };

		// The for-each loop to traverse array.
		for (String fruit : fruits) {

			System.out.println(fruit);

		}

	}

}
Run the example:

4- while Loop

The while loop is used to execute a section of the program repeatedly, when a condition is still true. The while loop is usually used when iterations  are not defined before (not fixed).
This is while loop structure:
** while **
// While condition is true then do something.
while ( condition ) {
   // Do something here...
}
See illustration
WhileExample1.java
package org.o7planning.tutorial.javabasic.loop;

public class WhileExampe1 {

	public static void main(String[] args) {

		int value = 3;

		// While the 'value' is still less than 10, the loop still works.
		while (value < 10) {

			System.out.println("Value = " + value);

			// Increase 'value' by adding 2
			value = value + 2;
		}
	}
	
}

5- do-while Loop

The do-while loop is used to execute a section of program many times.The characteristics of the  do-while is that the block of statement is always executed at least once. After each iteration, the program will check the condition, if the condition is still correct, the statement block will be executed again. 
This is the structure do-while loop:
** do while **
// The do-while loop performs at least one iteration.
// And while the condition is true, it continues to work
do {
   // Do something here.
}
while (condition);
Example:
DoWhileExample1.java
package org.o7planning.tutorial.javabasic.loop;

public class DoWhileExample1 {

	public static void main(String[] args) {

		int value = 3;

		// do-while loop will execute at least once
		do {

			System.out.println("Value = " + value);

			// Increase by 3
			value = value + 3;

		} while (value < 10);

	}
}
Result:

6- The break statement in loop

The break statement can appear in a loop. It is the statement that help the program exit the loop
Example:
LoopBreakExample.java
package org.o7planning.tutorial.javabasic.loop;

public class LoopBreakExample {

	public static void main(String[] args) {
		
		System.out.println("Break example");

		int x = 2;

		while (x < 15) {

			System.out.println("----------------------\n");
			System.out.println("x = " + x);

			// If x = 5 then exit the loop.
			if (x == 5) {
				break;
			}

			x++;
			System.out.println("x after ++ = " + x);

		}

		System.out.println("Done!");

	}

}
Running the example:

7- The continue statement in a loop

The continue statement can appear in a loop. When encountering the  continue statement, the program will ignore the statement lines within the block and at the bottom of the continue and start an new iteration (If the condition is still correct).
Example:
LoopContinueExample.java
package org.o7planning.tutorial.javabasic.loop;

public class LoopContinueExample {

	public static void main(String[] args) {

		System.out.println("Continue example");

		int x = 2;

		while (x < 7) {

			System.out.println("----------------------\n");
			System.out.println("x = " + x);

			// x = x + 1;
			x++;

			// % operator is used for calculating remainder.
			// If x is even, then ignore the command line below of 'continue',
			// and start new iteration (if condition still true).
			if (x % 2 == 0) {

				continue;
			}

			System.out.println("x after ++ = " + x);

		}

		System.out.println("Done!");

	}

}
Run the example:

8- Labelled Loop

The Java allows you to stick a label to a loop. It is like you name a loop, which is useful when you use multiple nested loops in a program. 
  • You can use break labelX statement; to break a loop is attached labelX.
  • You can use continue labelX statement; to continue a loop is attached​​​​​​​ labelX.
Syntax:
** Labelled Loops **
// for loop with Label.
label1: for( ... )  {
    
}

// while loop with Label.
label2: while ( ... )  {

}

// do-while loop with Label.
label3: do  {
    
    
} while ( ... );
Example of using nested loops, labelled and labelled break statement.
LabelledLoopBreakExample.java
package org.o7planning.tutorial.javabasic.loop;

public class LabelledLoopBreakExample {

	public static void main(String[] args) {

		System.out.println("Labelled Loop Break example");

		int i = 0;

		label1: while (i < 5) {

			System.out.println("----------------------\n");
			System.out.println("i = " + i);
			i++;

			label2: for (int j = 0; j < 3; j++) {

				System.out.println("  --> " + j);
				if (j > 0) {
					// Exit the loop with label1.
					break label1;
				}
			}

		}

		System.out.println("Done!");

	}

}
Run the example:
Example of using nested labelled loops and labelled continue statement.
LabelledLoopContinueExample.java
package org.o7planning.tutorial.javabasic.loop;

public class LabelledLoopContinueExample {

    public static void main(String[] args) {

        System.out.println("Labelled Loop Continue example");

        int i = 0;

        label1: while (i < 5) {
            System.out.println("outer i= " + i);
            i++;

            label2: for (int j = 0; j < 3; j++) {
                if (j > 0) {

                    continue label2;
                }
                if (i > 1) {

                    continue label1;
                }
                System.out.println("inner i= " + i + ", j= " + j);
            }

        }
    }

}

 
Run the example:

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