https://openjdk.java.net/ 
public interface java.util.concurrent.Callable<V>
  minor version: 0
  major version: 59
  flags: flags: (0x0601) ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT
  this_class: java.util.concurrent.Callable
  super_class: java.lang.Object
{
  public abstract V call();
    descriptor: ()Ljava/lang/Object;
    flags: (0x0401) ACC_PUBLIC, ACC_ABSTRACT
    Exceptions:
      throws java.lang.Exception
    Signature: ()TV;
}
Signature: <V:Ljava/lang/Object;>Ljava/lang/Object;
SourceFile: "Callable.java"
    RuntimeVisibleAnnotations: 
      java.lang.FunctionalInterface()