How do generics work in Java and what is type erasure?

A type parameter is checked by the compiler and then replaced by its bound, usually Object, in the generated bytecode. The JVM does not retain the type argument for ordinary generic classes. This is type erasure, and it exists mainly for backward compatibility with pre-generics code.

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

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

    public T get() {
        return value;
    }

    public static void copy(List<? extends Number> source,
                            List<? super Number> target) {
        for (Number number : source) {
            target.add(number);
        }
    }
}

What erasure implies

• You cannot write new T() without passing a Class token or factory
• You cannot use instanceof List<String> or create a T array directly
• Overloads that differ only by type argument clash after erasure
• Generic types are invariant: List<Integer> is not a List<Number>
• Wildcards restore flexibility for producers and consumers

Generic arrays and varargs can cause heap pollution warnings. Prefer List<T> over T arrays, and annotate a genuinely safe varargs method with @SafeVarargs. Never suppress unchecked warnings without understanding the concrete failure mode.