https://openjdk.java.net/ 
public interface java.util.function.DoubleFunction<R>
  minor version: 0
  major version: 59
  flags: flags: (0x0601) ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT
  this_class: java.util.function.DoubleFunction
  super_class: java.lang.Object
{
  public abstract R apply();
    descriptor: (D)Ljava/lang/Object;
    flags: (0x0401) ACC_PUBLIC, ACC_ABSTRACT
    Signature: (D)TR;
    MethodParameters:
       Name  Flags
      value  
}
Signature: <R:Ljava/lang/Object;>Ljava/lang/Object;
SourceFile: "DoubleFunction.java"
    RuntimeVisibleAnnotations: 
      java.lang.FunctionalInterface()