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