Generics

Syntax

class ClassName<T> {\n  // T can be used as a type\n}

Examples

public class Box<T> {
    private T content;

    public void set(T content) {
        this.content = content;
    }

    public T get() {
        return content;
    }
}

Box<String> stringBox = new Box<>();
stringBox.set("Hello");
System.out.println(stringBox.get());  // Hello

Box<Integer> intBox = new Box<>();
intBox.set(42);
System.out.println(intBox.get());  // 42

public class Utils {
    public static <T> void printArray(T[] array) {
        for (T item : array) {
            System.out.println(item);
        }
    }

    public static void main(String[] args) {
        Integer[] numbers = {1, 2, 3};
        String[] words = {"a", "b", "c"};

        printArray(numbers);
        printArray(words);
    }
}

public class Pair<K, V> {
    private K key;
    private V value;

    public Pair(K key, V value) {
        this.key = key;
        this.value = value;
    }

    public K getKey() { return key; }
    public V getValue() { return value; }

    @Override
    public String toString() {
        return key + " -> " + value;
    }
}

Pair<String, Integer> pair = new Pair<>("age", 21);
System.out.println(pair);  // age -> 21

public class NumberBox<T extends Number> {  // T must be Number or a subtype
    private T value;

    public NumberBox(T value) {
        this.value = value;
    }

    public double doubled() {
        return value.doubleValue() * 2;  // Number guarantees doubleValue() exists
    }
}

NumberBox<Integer> box = new NumberBox<>(10);
System.out.println(box.doubled());  // 20.0

// NumberBox<String> invalid = new NumberBox<>("text");  // Error - String isn't a Number

Best practices

• Use generics whenever a class or method's logic is the same regardless of type, to avoid duplicating code for each specific type
• Follow Java naming conventions for type parameters: T for a general type, E for an element, K/V for a key/value pair
• Use bounded type parameters (<T extends Number>) when your generic code needs to call methods specific to a certain family of types
• Prefer generics over using Object and manual casting - generics catch type errors at compile time instead of causing a ClassCastException at runtime

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