Java Stream iterate()

In this guide, you will learn about the Stream iterate() method in Java programming and how to use it with an example.

1. Stream iterate() Method Overview

Definition:

The iterate() method in the Stream class is a static method used to create a sequential ordered Stream produced by iterative application of a function to an initial element. Each subsequent element is generated by applying the function to the previous element. There are two overloaded versions of this method: one introduced in Java 8 and another in Java 9.

Syntax:

1. static <T> Stream<T> iterate(T seed, UnaryOperator<T> f)  // Java 8

2. static <T> Stream<T> iterate(T seed, Predicate<T> hasNext, UnaryOperator<T> f) // Java 9

Parameters:

- seed: the initial element.

- hasNext: a predicate to apply to elements to determine when the iteration should terminate.

- f: a function to be applied to the previous element to produce a new element.

Key Points:

- Used to create an infinite ordered Stream (Java 8) or a finite ordered Stream (Java 9) by iteratively applying a function.

- The seed parameter represents the first element in the Stream.

- The hasNext predicate (Java 9) determines when the iteration should stop.

- The f function is applied to generate subsequent elements in the Stream.

2. Stream iterate() Method Example

import java.util.function.Predicate;
import java.util.stream.Stream;

public class IterateExample {
    public static void main(String[] args) {
        // Java 8: Creating an infinite Stream, limit is used to limit the size of the stream
        Stream<Integer> infiniteStream = Stream.iterate(1, n -> n + 1).limit(5);
        System.out.println("Infinite Stream with limit:");
        infiniteStream.forEach(System.out::println);

        // Java 9: Creating a finite Stream using a hasNext predicate
        Predicate<Integer> predicate = n -> n <= 5;
        Stream<Integer> finiteStream = Stream.iterate(1, predicate, n -> n + 1);
        System.out.println("Finite Stream:");
        finiteStream.forEach(System.out::println);
    }
}

Output:

Infinite Stream with limit:
1
2
3
4
5
Finite Stream:
1
2
3
4
5

Explanation:

In this example, we have demonstrated both overloads of the Stream::iterate() method.

1. In the first part, we used the Java 8 version of iterate(), creating an infinite Stream starting from 1 and incrementing each subsequent element by 1. We then used the limit() operation to limit the size of the Stream to 5 elements and printed these elements.

2. In the second part, we used the Java 9 version of iterate(), which takes an additional hasNext predicate. We defined a predicate that evaluates to true for values less than or equal to 5, making this a finite Stream. Then, we printed the elements of the finite Stream, which are the same as in the previous example.