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How to iterate over a list in Java

Anjana Shankar

Iterating over a list is a common task in most programs. This shot will focus on the different ways to iterate over a List object in Java.

There are a number of different ways to iterate over a list:

  • Basic for loop
  • Enhanced for loop
  • Iterators
  • forEach iterables

Basic for loop

You can iterate over a list with a simple for loop. A for loop has three parts to it: the initializing statement, the loop condition check statement, and the increment statement, as shown below:

widget

The code below shows how you can use a basic for loop to iterate over a list in Java.

import java.util.List;
import java.util.ArrayList;

class HelloWorld {
    public static void main( String args[] ) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);
        numbers.add(6);
        numbers.add(7);
        numbers.add(8);

        for(int i=0; i<numbers.size(); i++) {
            System.out.print(numbers.get(i)+ " ");
        }
    }
}

Enhanced for loop

The enhanced for loop is a more compact form of the basic for loop that can be used to iterate over a list, as shown below:

import java.util.List;
import java.util.ArrayList;

class HelloWorld {
    public static void main( String args[] ) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);
        numbers.add(6);
        numbers.add(7);
        numbers.add(8);

        for(int i: numbers) {
            System.out.print(i+ " ");
        }
    }
}

Modifying the list while iterating on it with the above constructs leads to the ConcurrentModificationException.

Iterators

Iterators can also be used to iterate over a list. An iterator is especially useful when you need to modify the list that is being iterated on.

import java.util.List;
import java.util.ArrayList;
import java.util.Iterator;

class HelloWorld {
    public static void main( String args[] ) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);
        numbers.add(6);
        numbers.add(7);
        numbers.add(8);

        Iterator<Integer> itr = numbers.iterator();
        while(itr.hasNext()) {
            int i = itr.next();
            System.out.print(i+ " ");
            if(i==3) {
                itr.remove();
            }
        }
    }
}

The two important methods for an iterator are hasNext() and next(). The above snippet shows how these two can be used to iterate over the list.

Java also provides the listIterator class, which can be used to traverse a list in both directions, as shown below:

import java.util.List;
import java.util.ArrayList;
import java.util.ListIterator;

class HelloWorld {
    public static void main( String args[] ) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);
        numbers.add(6);
        numbers.add(7);
        numbers.add(8);

        ListIterator<Integer> litr = numbers.listIterator();
        while(litr.hasNext()) {
            int i = litr.next();
            System.out.print(i+ " ");
        }
        System.out.print("\n");
        while (litr.hasPrevious()) {
            int i= litr.previous();
            System.out.print(i+ " ");
        }
    }
}

forEach iterable

The introduction of lambdas in Java led to the addition of the forEach function, which can be used to iterate over any collection easily.

The code below shows how the forEach method can be used to iterate over a list:

import java.util.List;
import java.util.ArrayList;

class HelloWorld {
    public static void main( String args[] ) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);
        numbers.add(6);
        numbers.add(7);
        numbers.add(8);

        numbers.forEach(System.out::println);
    }
}

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