collect and the Basic Collectors

Key points

• collect is the general terminal op: it pours the stream into a container you pick.
• Stream.toList() (Java 16+) is the short path, but the list it hands back is unmodifiable.
• Need a mutable result? Use Collectors.toCollection(ArrayList::new).
• Collectors.joining(sep, prefix, suffix) builds strings without a StringBuilder.
• Collectors.toMap throws on duplicate keys. Pass a merge function unless the keys are truly unique.

Example

import java.util.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        List<String> langs = List.of("Java", "Kotlin", "Scala", "Java");

        List<String> immutable = langs.stream().distinct().toList();
        List<String> mutable = langs.stream().collect(Collectors.toCollection(ArrayList::new));
        Set<String> unique = langs.stream().collect(Collectors.toSet());
        String csv = langs.stream().distinct().collect(Collectors.joining(", ", "[", "]"));
        Map<String, Integer> lengths = langs.stream().distinct()
                .collect(Collectors.toMap(s -> s, String::length));

        System.out.println("immutable : " + immutable);
        System.out.println("mutable   : " + mutable);
        System.out.println("set size  : " + unique.size());
        System.out.println("joined    : " + csv);
        System.out.println("toMap     : " + new TreeMap<>(lengths));

        // Duplicate keys throw unless you supply a merge function.
        Map<Integer, String> byLength = langs.stream()
                .collect(Collectors.toMap(String::length, s -> s, (a, b) -> a + "|" + b));
        System.out.println("merged    : " + new TreeMap<>(byLength));

        try {
            immutable.add("Groovy");
        } catch (UnsupportedOperationException e) {
            System.out.println("toList() result is unmodifiable");
        }
    }
}

toList() for a quick read-only list; collect() when you need control over the container.