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Java Dictionary

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stream.collect() / toList()

stream.collect() / toList()

A terminal operation on streams that collects stream elements into a collection (Java 8+). You can pass a method from the Collectors class to collect() to convert the stream into a list, set, or map. Since Java 16, you can call toList() directly on a stream.

Syntax

// Collects the stream into a list.
stream.collect(Collectors.toList());

// Collects the stream into a set.
stream.collect(Collectors.toSet());

// Collects the stream into a map.
stream.collect(Collectors.toMap(keyMapper, valueMapper));

// Collects the stream into an unmodifiable list (Java 16+).
stream.toList();

Method List

Method	Description
collect(Collector)	A terminal operation that collects stream elements into a collection of the specified type.
Collectors.toList()	Collects stream elements into an `ArrayList` (mutable).
Collectors.toSet()	Collects stream elements into a `HashSet` (duplicates are removed).
Collectors.toMap(k, v)	Collects stream elements into a `HashMap` using the specified key and value mapper functions.
toList()	Collects the stream into an unmodifiable list (Java 16+). Equivalent to `collect(Collectors.toUnmodifiableList())`.

Sample Code

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

List<String> fruits = Arrays.asList("apple", "banana", "cherry", "avocado");

// Collects elements starting with 'a' into a list.
List<String> aFruits = fruits.stream()
    .filter(f -> f.startsWith("a"))
    .collect(Collectors.toList());
System.out.println(aFruits); // Prints [apple, avocado].

// Collects all elements into a set (duplicates are removed).
List<Integer> nums = Arrays.asList(1, 2, 2, 3, 3, 3);
Set<Integer> uniqueNums = nums.stream()
    .collect(Collectors.toSet());
System.out.println(uniqueNums.size()); // Prints 3.

// Collects into a map with the string as the key and its length as the value.
Map<String, Integer> lengthMap = fruits.stream()
    .collect(Collectors.toMap(f -> f, String::length));
System.out.println(lengthMap.get("apple")); // Prints 5.

// Uses toList() from Java 16+ (unmodifiable list).
List<String> upperList = fruits.stream()
    .map(String::toUpperCase)
    .toList(); // Java 16+
System.out.println(upperList); // Prints [APPLE, BANANA, CHERRY, AVOCADO].

Overview

collect() is the most versatile terminal operation on streams. By combining it with the collectors provided by the Collectors class, you can convert a stream into many different formats.

toList() introduced in Java 16 offers a concise syntax, but the returned list is unmodifiable — calling methods like add() will throw an UnsupportedOperationException. Use Collectors.toList() when you need a mutable list.

For creating streams, filtering, and mapping, see stream() / filter() / map().

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