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The Java Interview Handbook: 300+ Interview Questions

Bridge Methods and Reification

Explore how Java compilers manage generics through bridge methods and type erasure to maintain type safety and backward compatibility. Understand reifiable versus non-reifiable types, safe generic varargs usage, and how these concepts ensure robust generic API design.

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Generics introduce significant compile-time safety, but they also require the Java compiler to perform clever adjustments behind the scenes to maintain backward compatibility. Two key concepts that arise from this process are bridge methods and reification.

Let us examine how the compiler handles generic subtyping and what happens to our types at runtime.

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What are bridge methods?

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We can see how the compiler resolves these method signature mismatches by examining a concrete implementation of a generic interface.

Java 25
public class Demonstration {
    public static void main(String[] args) {
        GenericPrinter<Integer> printer = new IntegerPrinter();
        printer.print(100);
    }
}
interface GenericPrinter<T> {
    void print(T item);
}
class IntegerPrinter implements GenericPrinter<Integer> {
    @Override
    public void print(Integer item) {
        System.out.println(item);
    }
    // Synthetic bridge method inserted by the compiler:
    // public void print(Object item) {
    //     print((Integer) item);
    // }
}
    ...