Collectors

Syntax

stream.collect(Collectors.methodName())

Examples

import java.util.List;
import java.util.stream.Collectors;

List<String> names = List.of("Fola", "Zain", "Jamal", "Fola");

List<String> asList = names.stream().distinct().collect(Collectors.toList());
System.out.println(asList);  // [Fola, Zain, Jamal]

var asMap = names.stream()
    .distinct()
    .collect(Collectors.toMap(name -> name, String::length));
System.out.println(asMap);  // {Fola=4, Zain=4, Jamal=5}

import java.util.List;
import java.util.stream.Collectors;

List<String> names = List.of("Fola", "Zain", "Jamal");

String joined = names.stream().collect(Collectors.joining(", "));
System.out.println(joined);  // Fola, Zain, Jamal

String withBrackets = names.stream().collect(Collectors.joining(", ", "[", "]"));
System.out.println(withBrackets);  // [Fola, Zain, Jamal]

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

record Student(String name, String grade) {}

List<Student> students = List.of(
    new Student("Fola", "A"),
    new Student("Zain", "B"),
    new Student("Jamal", "A"),
    new Student("Amir", "C")
);

Map<String, List<Student>> byGrade = students.stream()
    .collect(Collectors.groupingBy(Student::grade));

System.out.println(byGrade.get("A"));  // [Student[name=Fola, grade=A], Student[name=Jamal, grade=A]]

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

List<String> words = List.of("apple", "banana", "avocado", "blueberry", "cherry");

Map<Character, Long> countByFirstLetter = words.stream()
    .collect(Collectors.groupingBy(w -> w.charAt(0), Collectors.counting()));

System.out.println(countByFirstLetter);  // {a=2, b=2, c=1}

Best practices

• Use Collectors.toList() (or the shorter .toList() method directly) as the default when you just need a plain result list
• Use Collectors.groupingBy() instead of manually building a Map<K, List<V>> with a loop - it handles the bucketing logic for you
• Combine groupingBy() with a downstream collector (counting(), summingInt(), etc.) to compute per-group statistics in one pass
• Use Collectors.joining() with a delimiter for building comma or newline-separated output instead of a manual StringBuilder loop

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation