Java Loops

Whenever you need to execute a block of code multiple times, a loop will often come in handy. In general, a loop looks something like this:

Types of Loops

Java has 4 types of loops - the while, do while, for and for each. While you can pick your favorite and use that almost everywhere, each of them has certain advantages in different applications.

You can use a debugger (a lot of IDEs have one - check out Eclipse, Netbeans, IntelliJ etc.), set up breakpoints and visualize how the following code about loops, works step-by-step.

The While loop

The while statement evaluates the expression (which must return a boolean value), and if it is true, the Statements get executed.

while (expression) 
{
  // Statements
}

In the following example, the expression is given by iter_While < 10. As we increment iter_While by 1 each time the loop is executed, the while loop will keep going till iter_While reaches 10.

int iter_While = 0;
while (iter_While < 10) 
{
    System.out.print (iter_While + " ");
    // Increment the counter
    // Iterated 10 times, iter_While 0,1,2...9
    iter_While++;
}
System.out.println ("iter_While Value: " + iter_While);

Output:

0 1 2 3 4 5 6 7 8 9
iter_While Value: 10

The Do While loop

The do while is very similar to the while loop in the way it works, but is exit controlled (unlike the for and while loops which are entry controlled), that is, the truth value of its expression is evaluated after the execution of Statements.

do 
{
    // Statements
} 
while (expression);

This kind of loop is particularly useful if you want your Statements to be executed atleast once, irrespective of what expression evaluates to. You want to do this if you are initializing a variable inside your loop and plan on using its value later.

int iter_DoWhile = 20;
do 
{
    System.out.print (iter_DoWhile + " ");

    // Increment the counter
    iter_DoWhile++;
} 
while(iter_DoWhile < 10);
System.out.println ("iter_DoWhile Value: " + iter_DoWhile);

Output:

20
iter_DoWhile Value: 21

The For loop

The for loop give you a compact way to iterate over a range of values.

for (initialization; expression; increment) 
{
    // Statements
}

• initialization - Initializes the loop and is executed just once, at the beginning.
• expression - Evaluated at the beginning of each iteration. If the expression evaluates to true, Statements will get executed.
• increment - Invoked after each iteration through the loop. You can increase/decrease the value of variables here.

for (int iter_For = 0; iter_For < 10; iter_For++) 
{
    System.out.print (iter_For + " ");
    // Iterated 10 times, iter_For 0,1,2...9
}
System.out.println("iter_For Value: " + fooFor);

Output:

0 1 2 3 4 5 6 7 8 9
iter_For Value: 10

The For Each loop

Also called the enhanced for loop, it is an extremely useful and simple way to iterate over each item in a collection, arrays as well as objects that implement the Iterable interface.

for (object : iterable)
{
    // Statements
}

The loop is read as - for each element in the iterable (could be an array, collectable etc.). The object type must match the element type of the iterable.

int[] number_list = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

for (int numbers : number_list) 
{
    System.out.print (numbers + " ");
    //Iterated 10 times, numbers 0,1,2...9
}

Output:

0 1 2 3 4 5 6 7 8 9

In the above code snippet, number_list is an array. If you don't know what this is, don't worry about it. An array is a container object that holds a fixed number of values of a single type, but more on this later.

Loop Control Statemens

break

Terminates the loop and starts the execution of the code that immediately follows the loop. If you have nested loops, the break statement will only end the loop in which it is placed.

for (int i = 0; i < 10; i++) // Loop 1
{
    for (int j = 0; j < 10; j++) // Loop 2
    {
        if (i == 5 && j == 5) 
        {
            break;
            // Will terminate Loop 2, but Loop 1 will keep going
        }
    }
}

But if you do want to break out of the outer loop too, you can use a label to exit:

loop1: // This is a label
for (int i = 0; i < 10; i++) // Loop 1
{
    for (int j = 0; j < 10; j++) // Loop 2
    {
        if (i == 5 && j == 5) 
        {
            break loop1;
            // Will break out of Loop 1, instead of just breaking out of Loop 2
        }
    }
}

break statements can be particulary useful while searching for an element in an array. Using break in the following code improves efficiency as the loop stops as soon as the element we are looking for (searchFor) is found, instead of going on till the end of arrayInts is reached.

int[] number_list = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
int searchFor = 5;

for (int i : arrayInts) 
{
    if (arayOfInts[i] == searchfor)
        break;
}

continue

The continue statement makes a loop skip all the following lines after the continue and jump ahead to the beginning of the next iteration. In a for loop, control jumps to the update statement, and in a while or do while loop, control jumps to the boolean expression/condition.

for (int j = 0; j < 10; j++) 
{
    if (j == 5) 
    {
        continue;
    }
    System.out.print (j + " ");
}

The value of j will be printed for each iteration, except when it is equal to 5. The print statement will get skipped because of the continue and the output will be:

0 1 2 3 4 6 7 8 9 

Say you want to count the number of is in a the word mississippi. Here you could use a loop with the continue statement, as follows:

String searchWord = "mississippi";

// max stores the length of the string
int max = searchWord.length();
int count_i = 0;

for (int i = 0; i < max; i++) 
{
    // We only want to count i's - skip other letters
    if (searchMe.charAt(i) != 'i')
        continue;

    // Increase count_i for each i encountered
    numPs++;
}

Infinite Loops

If you want your loop to go on infinitely, you can use the while, do while and for statement.

// Infinite For Loop
for ( ; ; ) 
{
    // your code here
}

// Infinite While Loop
while (true)
{
    // your code here
}

// Infinite Do While Loop
do
{
    // your code here
}
while (true);