Java Tutorial #5 – Loop Statements

Syntax

A while statement has the following syntax:

while (condition){
//body of the loop
}

The while keyword is followed by a condition that evaluates to a boolean value. If the boolean value is true, the while loop is entered. Once the code in the loop is completed, the condition is checked again and this continues till the condition becomes false.

Example

The following code demonstrates a basic while statement:

int i = 0;

while (i < 3) {
System.out.println("Loop Iteration: " + i);
	i++;
}
System.out.println("Done!");

Here, the while statement checks if i is less than 3. Since this is true, the loop is entered and the System.out statement is executed. i is then incremented in the while loop and the condition is checked again. This goes on till the value of i becomes 3 after which the loop is exited. So, this code produces the following output:

Loop Iteration: 0
Loop Iteration: 1
Loop Iteration: 2
Done!

Syntax

do{
// body of the loop
} while(condition);

The do keyword is followed by some code followed by a while statement. A condition that evaluates to a boolean value is specified in the while statement. The loop first executes and then checks the condition. If the condition is true, the loop is repeated. The loop is exited when the condition becomes false.

Example

The following code demonstrates a basic do-while statement:

int i = 0;

do {
System.out.println("Loop Iteration: " + i);
	i++;
} while (i < 3);
		
System.out.println("Done!");
This code is similar to the code above except that it uses a do-while loop. So, it produces the same output as before. 
However, consider the following code:
int i = 5;

do {
System.out.println("Loop Iteration: " + i);
	i++;
} while (i < 3);
		
System.out.println("Done!");

In this case, i is initialized to the value 5. Since the do-while loop checks the condition at the end, the body of the loop is executed once. After that, the condition is checked and since it is false, the loop is not entered again. So, this code produces the following output:

Loop Iteration: 5
Done!

If a while loop had been used here, the body of the loop would not have been executed even once since the condition is false.

Syntax

for(initialization;condition;iteration){
//body of the loop
}

The for keyword is followed by an initialization statement, a condition and an iteration statement. The initialization statement executes only once, at the start of the loop. The condition and iteration parts are executed each time the loop is repeated. The condition statement evaluates to a boolean value. If it is true, the body of the loop is executed. Once the body completes, the iteration part is executed and the condition is again checked. This continues till the condition evaluates to false. 

Example

The following code demonstrates a basic for statement:

for(int i = 0;i < 3;i++) {
System.out.println("Loop Iteration: " + i);
}
System.out.println("Done!");

Here i=0 is the initialization part, i < 3 is the condition and i++ is the iteration part. The for loop initializes i to 0 at the start of the loop. It then checks if i<3 is true and since this is true, it enters the loop. Once the code it in the loop completes, it increments i and again checks if the condition i<3 is true. This continues till i becomes 3 after which the loop is exited. So, this code produces the following output:

Loop Iteration: 0
Loop Iteration: 1
Loop Iteration: 2
Done!

Syntax

for(datatype var : collection) {
//body of the loop
}

The for keyword is followed by a variable and the collection over which the loop should iterate. In each iteration of the loop, it fetches the next element from the array or collection and executes the body of the loop for that element. This continues till all the elements in the collection are exhausted after which the loop is terminated.

Example

The following code demonstrates a basic for-each statement:

int[] input = { 10, 20, 30 };

for (int num : input) {
	System.out.println("Number is " + num);
}

This code first declares an integer array called input. It then uses a for-each loop to iterate through this array. For each iteration, the next value from the array input is fetched and assigned to the variable num. The body of the loop is then executed with this value. The loop is exited once all the elements in the array are exhausted. So, this code produces the following output:

Number is 10
Number is 20
Number is 30

When you want to iterate over an array or a collection a for-each loop is better than a for loop since it automatically fetches the next element and assigns to a variable