How to reverse Intstream in Java?

By Harish Gowda | | 2 minutes

In this blog we will learn how to reverse an IntStream which is generated in Java.

By default, IntStream generates numbers in increasing order from start to end-1.

Before getting started let’s see how IntStream generates numbers in increasing order.

In below example, it generated numbers in increasing order as shown below.

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package examples;
import java.util.stream.IntStream;
public class App {

    public static void main(String[] args) {

        IntStream.range(1, 5)
                .forEach(System.out::println); // 1 2 3 4
    }
}

To reverse IntStream you can follow one of the below methods.

IntStream.generate()

IntStream.iterate() creates an infinite stream starting from an initial value provided.

We can directly generate decreasing stream as shown below.

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package examples;
import java.util.stream.IntStream;
public class App {

    public static void main(String[] args) {
        IntStream.iterate(4,i->i-1)
                .limit(4)
                .forEach(System.out::println); // 4 3 2 1
    }
}

Use map() method

You can reverse IntStream by mapping generated numbers in decreasing order as shown below using map() method.

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package examples;
import java.util.stream.IntStream;
public class App {

    public static void main(String[] args) {
        IntStream.range(1, 5)
                .map(i->5-i) // 1-->4, 2-->3
                .forEach(System.out::println); // 4 3 2 1
    }
}

Convert to Stream,sort and back to IntStream

You can convert IntStream to Stream, sort it and then convert back to IntStream as shown in below example. In terms of performance this method is less efficient

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package examples;
import java.util.Comparator;
import java.util.stream.IntStream;
public class App {

    public static void main(String[] args) {
        IntStream.range(1, 5)
                .boxed()
                .sorted(Comparator.reverseOrder())
                .mapToInt(Integer::intValue)
                .forEach(System.out::println); // 4 3 2 1
    }
}

Conclusion

I hope this helped you to understand how to reverse an IntStream in Java.