Collections Utility Class

Syntax

Collections.methodName(collection);

Examples

import java.util.ArrayList;
import java.util.Collections;

ArrayList<Integer> numbers = new ArrayList<>(java.util.List.of(5, 2, 8, 1, 9));

Collections.sort(numbers);
System.out.println(numbers);  // [1, 2, 5, 8, 9]

Collections.reverse(numbers);
System.out.println(numbers);  // [9, 8, 5, 2, 1]

import java.util.ArrayList;
import java.util.Collections;

ArrayList<Integer> scores = new ArrayList<>(java.util.List.of(85, 92, 78, 92, 90));

System.out.println(Collections.max(scores));               // 92
System.out.println(Collections.min(scores));                // 78
System.out.println(Collections.frequency(scores, 92));       // 2

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

ArrayList<String> mutable = new ArrayList<>(java.util.List.of("a", "b", "c"));
List<String> readOnly = Collections.unmodifiableList(mutable);

System.out.println(readOnly);  // [a, b, c]

try {
    readOnly.add("d");  // throws UnsupportedOperationException
} catch (UnsupportedOperationException e) {
    System.out.println("Cannot modify - it's read-only");
}

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

ArrayList<Integer> deck = new ArrayList<>(java.util.List.of(1, 2, 3, 4, 5));
Collections.shuffle(deck);
System.out.println(deck);  // random order each run

List<String> empty = Collections.emptyList();
System.out.println(empty.size());  // 0

Best practices

• Use Collections.unmodifiableList()/Set()/Map() to safely expose internal collections from a class without allowing external code to modify them
• Use Collections.sort() with a custom Comparator for sorting by criteria other than natural ordering
• Use Collections.emptyList()/emptySet()/emptyMap() instead of creating a new empty ArrayList/HashSet/HashMap when you just need an immutable, empty placeholder
• Don't confuse Collections (the utility class, plural) with Collection (the root interface, singular) - they serve very different purposes

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