Java Generics Example in Java

Introduction

Generics let you write classes, interfaces and methods that operate on types specified by the caller. They eliminate unsafe casts and make collection code clearer.

Example Program

import java.util.*;

class Box<T> {
    private T value;

    public void set(T value) { this.value = value; }
    public T get() { return value; }

    @Override
    public String toString() {
        return "Box holds: " + value + " (" + value.getClass().getSimpleName() + ")";
    }
}

public class GenericsDemo {
    public static void main(String[] args) {
        Box<String> message = new Box<>();
        message.set("Hello Generics");
        System.out.println(message);

        Box<Integer> number = new Box<>();
        number.set(42);
        System.out.println(number);

        List<String> languages = new ArrayList<>();
        languages.add("Java");
        languages.add("Kotlin");
        // languages.add(10); // Compile error — type-safe

        for (String lang : languages) {
            System.out.println(lang.toUpperCase());
        }
    }
}

Sample Output

Box holds: Hello Generics (String)
Box holds: 42 (Integer)
JAVA
KOTLIN

How It Works

• Box<T> declares a type parameter T replaced at compile time with a concrete type.
• List<String> ensures only strings are added; the compiler rejects incompatible types.
• Autoboxing allows number.set(42) where T is Integer.

Best Practices

• Always specify generic types on collections (List<String>, not raw List).
• Use bounded wildcards (? extends Number) when writing flexible APIs.
• Prefer generic methods when the type parameter is independent of the class.

Common Mistakes

• Using raw types and casting elements manually.
• Creating arrays of generic types directly (new T[10] is not allowed).