A Guide to Java Loops

1. Overview

In this article, we’ll look at a core aspect of the Java language – executing a statement or a group of statements repeatedly – using loops.

2. Intro to Loops

In programming languages, looping is a feature which facilitates the execution of a set of instructions until the controlling Boolean-expression evaluates to false.

Java provides different types of loops to fit any programming need. Each loop has its own purpose and a suitable use case to serve.

Here are the types of loops that we can find in Java:

• Simple for loop
• Enhanced for-each loop
• While loop
• Do-While loop

3. Simple for Loop

A for loop is a control structure that allows us to repeat certain operations by incrementing and evaluating a loop counter.

Before the first iteration, the loop counter gets initialized, then the condition evaluation is performed followed by the step definition (usually a simple incrementation).

The syntax of the for loop is:

for (initialization; Boolean-expression; step) 
  statement;

Let’s see it in a simple example:

for (int i = 0; i < 5; i++) {
    System.out.println("Simple for loop: i = " + i);
}

The initialization, Boolean-expression and step used in for statement are optional. Here’s an example of an infinite for loop:

for ( ; ; ) {
    // Infinite for loop
}

3.1. Labeled for Loops

We can also have labeled for loops. It’s useful if we’ve got nested for loops so that we can break/continue from aspecific for loop:

aa: for (int i = 1; i <= 3; i++) {
    if (i == 1)
      continue;
    bb: for (int j = 1; j <= 3; j++) {
        if (i == 2 && j == 2) {
            break aa;
        }
        System.out.println(i + " " + j);
    }
}

4. Enhanced for Loop

Since Java 5, we have a second kind of for loop called the enhanced for which makes it easier to iterate over all elements in an array or a collection.

The syntax of the enhanced for loop is:

for(Type item : items)
  statement;

Since this loop is simplified in comparison to the standard for loop, we need to declare only two things when initializing a loop:

1. The handle for an element we’re currently iterating over
2. The source array/collection we’re iterating

Therefore, we can say that: For each element in items, assign the element to the item variable and run the body of the loop. 

Let’s have a look at the simple example:

int[] intArr = { 0,1,2,3,4 }; 
for (int num : intArr) {
    System.out.println("Enhanced for-each loop: i = " + num);
}

We can use it to iterate over various Java’s data structures:

Given a List<String> list  object – we can iterate it:

for (String item : list) {
    System.out.println(item);
}

We can similarly iterate over a Set<String> set:

for (String item : set) {
    System.out.println(item);
}

And, given a Map<String,Integer> map we can iterate over it as well:

for (Entry<String, Integer> entry : map.entrySet()) {
    System.out.println(
      "Key: " + entry.getKey() + 
      " - " + 
      "Value: " + entry.getValue());
}

4.1. Iterable.forEach()

Since Java 8, we can leverage for-each loops in a slightly different way. We now have a dedicated forEach() method in the Iterable interface that accepts a lambda expression representing an action we want to perform.

Internally, it simply delegates the job to the standard loop:

default void forEach(Consumer<? super T> action) {
    Objects.requireNonNull(action);
    for (T t : this) {
        action.accept(t);
    }
}

Let’s have a look at the example:

List<String> names = new ArrayList<>();
names.add("Larry");
names.add("Steve");
names.add("James");
names.add("Conan");
names.add("Ellen");

names.forEach(name -> System.out.println(name));

5. While Loop

The while loop is Java’s most fundamental loop statement. It repeats a statement or a block of statements while its controlling Boolean-expression is true.

The syntax of the while loop is:

while (Boolean-expression) 
    statement;

The loop’s Boolean-expression is evaluated before the first iteration of the loop – which means that if the condition is evaluated to false, the loop might not run even once.

Let’s have a look at a simple example:

int i = 0;
while (i < 5) {
    System.out.println("While loop: i = " + i);
}

6. Do-While Loop

The do-while loop works just like the while loop except for the fact that the first condition evaluation happens after the first iteration of the loop:

do {
    statement;
} while (Boolean-expression);

Let’s have a look at a simple example:

int i = 0;
do {
    System.out.println("Do-While loop: i = " + i++);
} while (i < 5);

7. Conclusion

In this quick tutorial, we showed the different types of loops that are available in the Java programming language.

We also saw how each loop serves a particular purpose given a suitable use case. We discussed the circumstances that are suitable for a given loop implementation.

As always, examples can be found over on GitHub.