Differences between BeanInfo:methodDescriptors and class:declaredMethods : multiple methods with same name and method masking

Question

When trying to get JPA annotations at runtime for some properties, I encountered this problem. I can't explain why.

PS: after a debugging session with Spring, I found the explanation of this problem: bridged methods that are introduced at compile time by the compiler. Please see my own answer to this question..

Here is a sample source code replicating the issue (simplified version of real code).

import java.beans.BeanInfo; import java.beans.IntrospectionException; import java.beans.Introspector; import java.beans.MethodDescriptor; import java.io.Serializable; import java.lang.reflect.Method;

public class MethodMasking {

public interface HasId<ID extends Serializable>  {
    void setId(ID id);
    ID getId();
}

public interface Storeable extends HasId<Long> {}

class Item implements Storeable {Long id; String code;
    Item(Long id, String code) { this.id = id; this.code = code; }
    public Long getId() { return id; }
    public void setId(Long id) {this.id = id;}
}

public static void main(String[] args) throws IntrospectionException {
    final BeanInfo beanInfo = Introspector.getBeanInfo(Item.class);

    java.lang.System.out.println("BeanInfo:methodDescriptors:");
    final MethodDescriptor[] methodDescriptors = beanInfo.getMethodDescriptors();
    for (MethodDescriptor methodDescriptor : methodDescriptors) {
        java.lang.System.out.println("\t"+methodDescriptor.getMethod().getName());
    }

    java.lang.System.out.println("class:declaredMethods:");
    final Method[] declaredMethods = Item.class.getDeclaredMethods();
    for (Method declaredMethod : declaredMethods) {
        java.lang.System.out.println("\t"+declaredMethod.getName());
    }
}

} Output of the program:

BeanInfo:methodDescriptors:
    hashCode
    wait
    getId
    notifyAll
    equals
    wait
    wait
    toString
    setId
    notify
    setId
    getClass
class:declaredMethods:
    getId
    getId
    setId
    setId

Now I'm confused:
why in beanInfo there are 2 methods descriptors for setId but only one for getId ?
why in declared method there are 2 methods for getId and 2 methods for setId ?

While debugging I have these methods signatures when using the getDeclaredMethods :

[0] = {java.lang.reflect.Method@139}"public java.lang.Long MethodMasking$Item.getId()"
[1] = {java.lang.reflect.Method@446}"public java.io.Serializable MethodMasking$Item.getId()"
[2] = {java.lang.reflect.Method@447}"public void MethodMasking$Item.setId(java.lang.Long)"
[3] = {java.lang.reflect.Method@448}"public void MethodMasking$Item.setId(java.io.Serializable)"

Edit: After some tests I have found that the cause of the problem is the usage of generics in the HasId interface...

Declared this way, the problem disapear: no more duplicate methods.

public interface HasId  {
        void setId(Long id);
        Long getId();
    }

    public interface Storeable extends HasId {} 

Answer 1

It is because the compiler is introducing bridge methods when Generics are used:
some explanation here