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Generics (Generic Types) were introduced in Java to allow classes, interfaces, and methods to operate on different types as parameters. This ensures compile-time type safety and avoids expensive explicit casts at runtime.

Generic Classes

A generic class is defined using angle brackets <T> where T acts as a placeholder for a type that will be specified when the class is instantiated.

public class Box<T> {
    private T content;

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

    public T getContent() {
        return content;
    }
}

When creating a Box object, we specify the actual type:

Box<Integer> integerBox = new Box<>();
integerBox.setContent(123);
int value = integerBox.getContent(); // No cast needed!

Generic Methods

You can define a single generic method that can accept different type parameters. The type parameter is placed before the return type of the method.

public static <E> void printArray(E[] elements) {
    for (E element : elements) {
        System.out.println(element);
    }
}

Wildcards and Bounded Types

We can restrict the types accepted by a generic using the extends keyword (upper bound) to accept a specific type or its subclasses.

public static <T extends Number> void processNumber(T number) {
    // Accepts Integer, Double, Float, etc.
    System.out.println(number.doubleValue());
}

Try it yourself