Type Erasure (The Java(tm) Tutorials

To implement generics, the Java compiler applies type erasure to:

• Replace all type parameters in generic types with their bounds or Object if the type parameters are unbounded. The produced bytecode, therefore, contains only ordinary classes, interfaces, and methods.
• Insert type casts if necessary to preserve type safety.
• Generate bridge methods to preserve polymorphism in extended generic types.

Bridge Methods

After type erasure, the method signatures do not not match. The Node method becomes setData(Object) and the MyNode method becomes setData(Integer). Therefore, the MyNodesetData method does not override the Node setData method.To solve this problem and preserve the polymorphism of generic types after type erasure, a Java compiler generates a bridge method to ensure that subtyping works as expected. 

public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }

Read full article from Type Erasure (The Java™ Tutorials > Learning the Java Language > Generics (Updated))