In Java, bridge methods are a lesser-known but critical mechanism generated by the compiler to ensure type safety and method overriding compatibility when generics and inheritance interact. They are especially relevant when type erasure comes into play, which removes generic type information at runtime.
1. What Are Bridge Methods?
A bridge method is a synthetic method automatically created by the Java compiler. It acts as a “bridge” between a type-erased method and the overridden method with generic parameters, allowing polymorphism to work correctly when generics are involved.
Bridge methods are not written by the programmer and do not appear in source code, but they are part of the compiled .class files and visible through reflection or decompilers.
2. Why Are Bridge Methods Needed?
Java uses type erasure to implement generics. This means that the generic type information is removed at runtime, and replaced with raw types. Due to this, method signatures that look different at the source level may appear identical or incompatible at the bytecode level.
To preserve polymorphic behavior and ensure that overriding works correctly, the compiler generates bridge methods that:
-
Have the erased signature of the method in the superclass or interface.
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Internally call the actual overridden method with the correct parameter and return types.
Program
// Generic superclass
class GenericBox<T> {
public void setValue(T value) {
System.out.println("GenericBox: " + value);
}
}
// Subclass with a specific type
class StringBox extends GenericBox<String> {
@Override
public void setValue(String value) {
System.out.println("StringBox: " + value.toUpperCase());
}
}
// Main class to test
public class BridgeMethodDemo {
public static void main(String[] args) {
GenericBox<String> box = new StringBx();
box.setValue("lotusJavaPrince");
}
}
/*
StringBox: LOTUSJAVAPRINCE
*/What Happens Behind the Scenes
Due to type erasure, GenericBox<T> becomes GenericBox<Object> at runtime, and its method:
public void setValue(T value)
becomes...
public void setValue(Object value)
Now in StringBox, you have....
public void setValue(String value)
// Bridge method created by the compiler in StringBox
public void setValue(Object value) {
setValue((String) value); // delegates to the real method
}Code language: JavaScript (javascript)Bridge methods are compiler-generated, synthetic methods that play a vital role in preserving type safety, runtime polymorphism, and method overriding in Java’s generic type system. Because Java implements generics through type erasure, generic type information is lost at runtime. This can cause method signatures to appear incompatible between base classes and subclasses.