Java bridge methods: preserve overrides after generic erasure

Last updated: 5 Oct 20265 min read

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A compiler may generate a synthetic bridge method when erasure would otherwise break an overriding method's dispatch signature.

Operational contract

The base source declares a generic fee method and the concrete source returns Long. At the source level, calling through FeeSource<Long> invokes the concrete override. The compiler can emit an Object-returning bridge alongside the Long-returning method so erased callers still reach that override. Reflection may show both; method scanners should inspect isBridge and isSynthetic before treating every declared method as a separate business API. Do not call the synthetic method directly. An unchecked raw-type call can still move a type error to runtime.

Failure case

An audit tool lists methods on a fee provider and reports two rate methods. Only one was written by the developer. The second is a compiler bridge that preserves polymorphism; registering both as separate handlers would double-count one operation.

Java code

import java.lang.reflect.Method;

public class FeeBridgeInspection {
    static abstract class FeeSource<T> { abstract T rate(); }
    static final class FixedFee extends FeeSource<Long> {
        @Override Long rate() { return 4700L; }
    }

    public static long rateThroughBase() {
        FeeSource<Long> source = new FixedFee();
        return source.rate();
    }

    public static int bridgeCount() {
        int bridges = 0;
        for (Method method : FixedFee.class.getDeclaredMethods()) {
            if (method.isBridge()) bridges++;
        }
        return bridges;
    }
}

Performance and ownership cost

The virtual call is ordinary dispatch after compilation. Reflecting over M declared methods costs O(M) time and O(1) application space here, aside from reflection's method objects. Bridge generation is a compatibility detail, not a reason to add runtime casts to correctly typed code.

Common Mistakes

Do not expose compiler bridge methods as separate service endpoints.
Do not assume generic parameters survive as distinct runtime classes.
Do not remove an override because reflection shows an extra method.

Connected lessons

java

generics at api boundaries

generic-bridge-method-dispatch