Java | Generics

Generics move type mistakes from runtime to compile time. Parameterized collections, reusable methods and wildcard bounds let an API express what types it accepts, produces and safely transforms without raw casts.

The Box class is reusable for any type but each instance preserves its chosen type. No cast is required when getting a String from Box.

Code

public class Main {
    static class Box<T> {
        private final T value;

        Box(T value) {
            this.value = value;
        }

        T get() {
            return value;
        }
    }

    public static void main(String[] args) {
        Box<String> message = new Box<>("typed value");
        String result = message.get();
        System.out.println(result.toUpperCase());
    }
}

TYPED VALUE

The method-level type parameter applies to one list operation. It works for a list of strings, integers or any reference type because the temporary variable keeps the same T as both list positions.

Code

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

public class Main {
    static <T> void swap(List<T> values, int first, int second) {
        T temporary = values.get(first);
        values.set(first, values.get(second));
        values.set(second, temporary);
    }

    public static void main(String[] args) {
        List<String> planets = new ArrayList<>(List.of("Mars", "Venus", "Earth"));
        swap(planets, 0, 2);
        System.out.println(planets);
    }
}

[Earth, Venus, Mars]

The extends wildcard accepts List, List and other Number subtypes. The method only reads values, so it never needs to insert a value whose exact subtype is unknown.

Code

import java.util.List;

public class Main {
    static double sum(List<? extends Number> values) {
        double total = 0;
        for (Number value : values) {
            total += value.doubleValue();
        }
        return total;
    }

    public static void main(String[] args) {
        System.out.println(sum(List.of(2, 3, 5)));
        System.out.println(sum(List.of(1.5, 2.5)));
    }
}

10.0
4.0

The super wildcard permits List, List and List