https://openjdk.java.net/ 
public interface java.util.function.ToDoubleBiFunction<T, U>
  minor version: 0
  major version: 59
  flags: flags: (0x0601) ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT
  this_class: java.util.function.ToDoubleBiFunction
  super_class: java.lang.Object
{
  public abstract double applyAsDouble(T, );
    descriptor: (Ljava/lang/Object;Ljava/lang/Object;)D
    flags: (0x0401) ACC_PUBLIC, ACC_ABSTRACT
    Signature: (TT;TU;)D
    MethodParameters:
      Name  Flags
      t     
      u     
}
Signature: <T:Ljava/lang/Object;U:Ljava/lang/Object;>Ljava/lang/Object;
SourceFile: "ToDoubleBiFunction.java"
    RuntimeVisibleAnnotations: 
      java.lang.FunctionalInterface()