Java stream collect to array

How to convert a Java 8 Stream to an Array?

I’d suggest you to revert the rollback as the question was more complete and showed you had tried something.

@skiwi Thanks! but i thought the attempted code does not really add more information to the question, and nobody has screamed «show us your attempt» yet =)

@skiwi: Although I usually shout at the do-my-homework-instead-of-me questions, this particular question seems to be clearer to me without any additional mess. Let’s keep it tidy.

You can find a lot of answers and guidance in the official docs of the package: docs.oracle.com/javase/8/docs/api/java/util/stream/…

9 Answers 9

The easiest method is to use the toArray(IntFunction generator) method with an array constructor reference. This is suggested in the API documentation for the method.

String[] stringArray = stringStream.toArray(String[]::new); 

What it does is find a method that takes in an integer (the size) as argument, and returns a String[] , which is exactly what (one of the overloads of) new String[] does.

You could also write your own IntFunction :

Stream stringStream = . ; String[] stringArray = stringStream.toArray(size -> new String[size]); 

The purpose of the IntFunction generator is to convert an integer, the size of the array, to a new array.

Stream stringStream = Stream.of("a", "b", "c"); String[] stringArray = stringStream.toArray(size -> new String[size]); Arrays.stream(stringArray).forEach(System.out::println); 

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Java Stream toArray()

Learn to convert a Stream to an array using Stream toArray() API. In this totorial, we will see multiple examples for collecting the Stream elements into an array.

The toArray() method returns an array containing the elements of the given stream. This is a terminal operation.

Object[] toArray() T[] toArray(IntFunction generator)

toArray() method is an overloaded method. The second method uses a generator function to allocate the returned array.

The generator function takes an integer, which is the size of the desired array and produces an array of the desired size.

Example 1: Converting Stream of String to Array of String

In the given example, we are converting a stream to an array using using toArray() API.

Stream tokenStream = Arrays.asList("A", "B", "C", "D").stream(); //stream String[] tokenArray = tokenStream.toArray(String[]::new); //array System.out.println(Arrays.toString(tokenArray));

Example 2: Converting an Infinite Stream to an Array

To convert an infinite stream into array, we must limit the stream to a finite number of elements.

Infinite stream of Integers

IntStream infiniteNumberStream = IntStream.iterate(1, i -> i+1); int[] intArray = infiniteNumberStream.limit(10) .toArray(); System.out.println(Arrays.toString(intArray));

Infinite boxed stream of Integers

IntStream infiniteNumberStream = IntStream.iterate(1, i -> i+1); Integer[] integerArray = infiniteNumberStream.limit(10) .boxed() .toArray(Integer[]::new); System.out.println(Arrays.toString(integerArray));

Example 3: Stream filter and collect to an Array

Sometimes we need to find specific items in stream and then add only those elements to array. Here, we can use Stream.filter() method to pass a predicate which will return only those elements who match the pre-condition.

import java.util.ArrayList; import java.util.Arrays; import java.util.List; public class Main < public static void main(String[] args) < ListemployeeList = new ArrayList<>(Arrays.asList( new Employee(1, "A", 100), new Employee(2, "B", 200), new Employee(3, "C", 300), new Employee(4, "D", 400), new Employee(5, "E", 500), new Employee(6, "F", 600))); Employee[] employeesArray = employeeList.stream() .filter(e -> e.getSalary() < 400) .toArray(Employee[]::new); System.out.println(Arrays.toString(employeesArray)); >>

[Employee [id=1, name=A, salary=100.0], Employee [id=2, name=B, salary=200.0], Employee [id=3, name=C, salary=300.0]]

We can use Stream toArray() function is variety of ways to collect stream elements into an array in all usescases.

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Convert Java Stream to Array

On this page we will learn how to convert Java Stream into Array . The best way to convert is using Stream.toArray(IntFunction) method. In our examples we will convert Java Stream into Array in following ways.

1. We will use Stream.toArray(IntFunction) which will return the array of the desired type.

2. Using Stream.toArray() method which will return Object[] and then we change it into required datatype.

3. For integer stream we can use IntStream.toArray() that will return int[] . In the same way we can use LongStream.toArray() to get long[] and DoubleStream.toArray() to get double[] .

4. We can convert stream into list and then list into array. To convert stream into list we need to use collect(Collectors.toList()) on the stream and to convert list into array we can use List.toArray method.

Contents

1. Using Stream.toArray(IntFunction)

The toArray(IntFunction) method returns an array containing the elements of this stream using the provided generator as IntFunction . This method is terminal operation.

A[] toArray(IntFunction generator)

Parameters: Pass a generator as IntFunction which produces a new array of the desired type and the provided length.
Returns: The method returns an array consisting the elements of stream.
Throws: The method throws ArrayStoreException if the runtime type of any element of this stream is not assignable to the runtime component type of the generated array.

Example-1:
In this example we will convert stream of string into array of string.

List list = Arrays.asList(«A», «B», «C», «D»); String[] strArray = list.stream().toArray(String[]::new); for(String s : strArray)

The output will be A B C D. In the above example we have instantiated IntFunction as generator in toArray method using method reference.
Now find the example with lambda expression.

String[] strArray = list.stream().toArray(size -> new String[size]);

package com.concretepage; import java.util.Arrays; import java.util.List; public class StreamToStringArray < public static void main(String[] args) < Listlist = Arrays.asList("Krishna", "Mahesh", "Kush"); String[] strArray = list.stream() .filter(e -> e.startsWith("K")) .toArray(size -> new String[size]); for(String s : strArray) < System.out.println(s); >> >

Example-2:
In this example we will convert stream of integer into array of integer.
StreamToIntegerArray.java

package com.concretepage; import java.util.Arrays; import java.util.List; public class StreamToIntegerArray < public static void main(String[] args) < Listlist = Arrays.asList(10, 20, 30, 40); Integer[] intArray = list.stream() .map(e -> e * 2) .toArray(Integer[]::new); for(Integer i : intArray) < System.out.println(i); >> >

The output will be 20 40 60 80.
In the above example we have used method reference. Now find the code with lambda expression.

List list = Arrays.asList(10, 20, 30, 40); Integer[] intArray = list.stream() .map(e -> e * 2) .toArray(size -> new Integer[size]);

2. Using Stream.toArray()

This method is terminal operation.

Example-1: In this example we will convert a stream of string into array of string. We know that toArray() returns Object[] , so to convert it in our required datatype, we can use Arrays.copyOf method.

Object[] objArray = Stream.of(«AA», «BB», «CC»).toArray(); String[] stArray = Arrays.copyOf(objArray, objArray.length, String[].class); for(String s : stArray)

The output will be AA BB CC.

Example-2: In this example we will convert stream of integer into array of integer.

Object[] objArray = Stream.of(10, 20, 30, 40).toArray(); Integer[] intArray = Arrays.copyOf(objArray, objArray.length, Integer[].class); for(Integer i : intArray)

3. Using IntStream.toArray()

The IntStream is the stream of int-valued elements. The IntStream.toArray() method converts stream of int values into the int array.

IntStream intStream = IntStream.of(1,2,3,4,5);

IntStream intStream = IntStream.rangeClosed(1, 5);

IntStream intStream = Stream.of(4,5,6,7,8).mapToInt(i -> i);

Now let us discuss some examples to use IntStream.toArray() method.

int[] intArray = IntStream.of(10, 20, 30, 40).toArray(); for(Integer i : intArray)
int[] intArray = IntStream.rangeClosed(10, 15).toArray(); for(Integer i : intArray)
int[] intArray = Stream.of(3,4,5,6).mapToInt(i -> i * 2).toArray(); for(Integer i : intArray)

4. Using Collectors.toList()

We can convert stream into list and then convert list into array. To convert stream into list we need to use collect(Collectors.toList()) on the stream. To convert list into array we can use List.toArray method.
StreamToListToArray.java

package com.concretepage; import java.util.stream.Collectors; import java.util.stream.Stream; public class StreamToListToArray < public static void main(String[] args) < System.out.println("--- For String ---"); String[] ar = Stream.of("Java", "Angular", "Spring") .collect(Collectors.toList()) .toArray(new String[0]); for(String e : ar) < System.out.println(e); >System.out.println("--- For Integer ---"); Integer[] intArray = Stream.of(15, 20, 30) .map(e -> e * 2) .collect(Collectors.toList()) .toArray(new Integer[0]); for(Integer i : intArray) < System.out.println(i); >> >

--- For String --- Java Angular Spring --- For Integer --- 30 40 60

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Java Stream collect to array

In this post, we will see how to collect any Java 8 Stream to array.

There are many ways to do it but I am going to show easiest way.

You can simply use toArray(IntFunction generator).This generator function takes size as input and creates new array of that size.

Convert Steam to Array

Let’s understand with the help of simple example.

When you run above program, you will get below output:

It is recommended to simply use an array constructor reference as below.
Change line no. 15-16 as below.

Array constructor reference is just another way of writing aboveLambda expresssion.

Collect Stream to Integer array

If you want to convert to Integer array, you can use IntStream to do it. You can call toArray() on IntStream and it will simply convert it to int[]. You don’t need to pass any argument in this scenario.

That’s all about converting a Java stream to array in java.

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