Stream

With the Java stream() method we can create a Stream from an array or ArrayList. With lambdas we can manipulate the stream.

When using streams, we must change our approach to processing data. We call methods, one after another, to change the output. Many transformations are possible.

Arrays.stream

We begin with the Arrays.stream method. We must import the java.util.Arrays class. Stream() returns specially-typed streams.

import java.util.Arrays;
import java.util.stream.DoubleStream;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class Program {
    public static void main(String[] args) {

        // Use Arrays.stream to create an IntStream.
        int[] array = { 10, 20, 30 };
        IntStream stream = Arrays.stream(array);
        // ... Call anyMatch on the IntStream.
        boolean result = stream.anyMatch(number -> number >= 25);
        System.out.println(result);

        // Create a DoubleStream.
        double[] array2 = { 1.2, 1.3, 1.4 };
        DoubleStream stream2 = Arrays.stream(array2);
        // ... Test the DoubleStream.
        boolean result2 = stream2.anyMatch(number -> number >= 1.35);
        System.out.println(result2);

        // Create a Stream of Strings.
        String[] array3 = { "cat", "dog", "bird" };
        Stream<String> stream3 = Arrays.stream(array3);
        // ... Test the strings.
        boolean result3 = stream3.anyMatch(value -> value.length() >= 4);
        System.out.println(result3);
    }
}
true
true
true

ArrayList stream

We can also get a stream from an ArrayList. The stream() method on an ArrayList instance returns a Stream object with a matching type.

import java.util.ArrayList;
import java.util.stream.Stream;

public class Program {
    public static void main(String[] args) {

        // Create an Integer ArrayList and add three numbers to it.
        ArrayList<Integer> list = new ArrayList<Integer>();
        list.add(10);
        list.add(200);
        list.add(3000);

        // Get a Stream from the ArrayList.
        Stream<Integer> stream = list.stream();

        // Test the Stream.
        boolean result = stream.allMatch(value -> value >= 5);
        System.out.println(result);
    }
}
true

Sorted, forEach

A Stream can be sorted with the sorted() method. With strings, this sorts from first to last (from "A" to "Z"). With integers it goes also from low to high.

import java.util.ArrayList;
import java.util.stream.Stream;

public class Program {

    static void display(String value) {
        // Called on each string.
        System.out.println("Value: " + value);
    }

    public static void main(String[] args) {

        // Create list of birds.
        ArrayList<String> list = new ArrayList<>();
        list.add("parrot");
        list.add("sparrow");
        list.add("finch");
        list.add("canary");

        // Get the stream and call sorted on it.
        Stream<String> sorted = list.stream().sorted();
        // ... Call display method on each element in sorted order.
        sorted.forEach(x -> display(x));
    }
}
Value: canary
Value: finch
Value: parrot
Value: sparrow

IntStream.Range

Suppose you want a range of numbers like 1, 2, 3. With the static method IntStream.range (and rangeClosed) we can easily generate this.

Random streams

A Random class returns one random number after another. With Random.ints() we get an IntStream of Random numbers. This is an infinite data source.

Summary

With streams, we access powerful logic. We can transform, filter and reduce streams with method calls. Arrays and ArrayLists can be converted into streams.