Small example of Java Functions without generics.

Function.class.java

Compiled from "Function.java"
public abstract class Function {
  public Function();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public abstract java.lang.Object apply(java.lang.Object);
}

Function.java

public abstract class Function {
    public abstract Object apply(Object a);
}

Main$1$1.class.java

Compiled from "Main.java"
// inner function of add
class Main$1$1 extends Function {
  final java.lang.Object val$a;

  final Main$1 this$0;

  Main$1$1(Main$1, java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: putfield      #1                  // Field this$0:LMain$1;
       5: aload_0
       6: aload_2
       7: putfield      #2                  // Field val$a:Ljava/lang/Object;
      10: aload_0
      11: invokespecial #3                  // Method Function."<init>":()V
      14: return

  public java.lang.Integer apply(java.lang.Object);
    Code:
       0: aload_0
       1: getfield      #2                  // Field val$a:Ljava/lang/Object;
       4: checkcast     #4                  // class java/lang/Integer
       7: invokevirtual #5                  // Method java/lang/Integer.intValue:()I
      10: aload_1
      11: checkcast     #4                  // class java/lang/Integer
      14: invokevirtual #5                  // Method java/lang/Integer.intValue:()I
      17: iadd
      18: invokestatic  #6                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      21: areturn

  public java.lang.Object apply(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: invokevirtual #7                  // Method apply:(Ljava/lang/Object;)Ljava/lang/Integer;
       5: areturn
}

Main$1.class.java

Compiled from "Main.java"
// add
final class Main$1 extends Function {
  Main$1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method Function."<init>":()V
       4: return

  public Function apply(java.lang.Object);
    Code:
       0: new           #2                  // class Main$1$1
       3: dup
       4: aload_0
       5: aload_1
       6: invokespecial #3                  // Method Main$1$1."<init>":(LMain$1;Ljava/lang/Object;)V
       9: areturn

  public java.lang.Object apply(java.lang.Object);
    Code:
       0: aload_0
       1: aload_1
       2: invokevirtual #4                  // Method apply:(Ljava/lang/Object;)LFunction;
       5: areturn
}

Main$2.class.java

Compiled from "Main.java"
// factorial
final class Main$2 extends Function {
  Main$2();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method Function."<init>":()V
       4: return

  public java.lang.Object apply(java.lang.Object);
    Code:
       0: aload_1
       1: checkcast     #2                  // class java/lang/Integer
       4: invokevirtual #3                  // Method java/lang/Integer.intValue:()I
       7: ifne          15
      10: iconst_1
      11: invokestatic  #4                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      14: areturn
      15: aload_1
      16: checkcast     #2                  // class java/lang/Integer
      19: invokevirtual #3                  // Method java/lang/Integer.intValue:()I
      22: getstatic     #5                  // Field Main.factorial:LFunction;
      25: aload_1
      26: checkcast     #2                  // class java/lang/Integer
      29: invokevirtual #3                  // Method java/lang/Integer.intValue:()I
      32: iconst_1
      33: isub
      34: invokestatic  #4                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      37: invokevirtual #6                  // Method Function.apply:(Ljava/lang/Object;)Ljava/lang/Object;
      40: checkcast     #2                  // class java/lang/Integer
      43: invokevirtual #3                  // Method java/lang/Integer.intValue:()I
      46: imul
      47: invokestatic  #4                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      50: areturn
}

Main.class.java

Compiled from "Main.java"
public class Main {
  static Function add;

  static Function factorial;

  public Main();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: getstatic     #3                  // Field factorial:LFunction;
       6: bipush        11
       8: invokestatic  #4                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      11: invokevirtual #5                  // Method Function.apply:(Ljava/lang/Object;)Ljava/lang/Object;
      14: checkcast     #6                  // class java/lang/Integer
      17: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      20: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
      23: getstatic     #8                  // Field add:LFunction;
      26: iconst_5
      27: invokestatic  #4                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      30: invokevirtual #5                  // Method Function.apply:(Ljava/lang/Object;)Ljava/lang/Object;
      33: checkcast     #9                  // class Function
      36: iconst_3
      37: invokestatic  #4                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      40: invokevirtual #5                  // Method Function.apply:(Ljava/lang/Object;)Ljava/lang/Object;
      43: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      46: return

  static {};
    Code:
       0: new           #10                 // class Main$1
       3: dup
       4: invokespecial #11                 // Method Main$1."<init>":()V
       7: putstatic     #8                  // Field add:LFunction;
      10: new           #12                 // class Main$2
      13: dup
      14: invokespecial #13                 // Method Main$2."<init>":()V
      17: putstatic     #3                  // Field factorial:LFunction;
      20: return
}

Main.java

public class Main {
    static Function add = new Function() {
        public Function apply(Object a) {
            return new Function() {
                public Integer apply(Object b) {
                    return ((Integer) a) + ((Integer) b);
                }
            };
        }
    };

    static Function factorial = new Function() {
        public Object apply(Object a) {
            if((Integer) a == 0) {
                return 1;
            } else {
                return (Integer) a * (Integer) factorial.apply((Integer) a - 1);
            }
        }
    };

    public static void main(String []args) {
        System.out.println((Integer) factorial.apply(11));
        System.out.println(((Function) add.apply(5)).apply(3));
    }
}