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