ClsBuilder b = new ClsBuilder(Opcodes.V1_6, Opcodes.ACC_PUBLIC, "test/TestGenCls", null, "java/lang/Object", null, ClassWriter.COMPUTE_FRAMES);

MethodBuilder c = b.newMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null, 0, 0);
c.aload_0();
c.invokespecial("java/lang/Object", "<init>", "()V");
c.return_();

MethodBuilder main = b.newMethod(Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC, "main", "([Ljava/lang/String;)V", null, null, 0, 0);
main.getstatic("java/lang/System", "out", "Ljava/io/PrintStream;");
main.ldc("Hello World!");
main.invokevirtual("java/io/PrintStream", "println", "(Ljava/lang/String;)V");
main.return_();

byte[] bytes = b.toByteArray();
Class<?> cls = ClassLoadingUtil.loadClsInstantly("test.TestGenCls", bytes);
Method m = cls.getMethod("main", String[].class);
m.invoke(null, new Object[] { new String[0] });
assertTrue(true);// this program should print out 'Hello world!'

$ javap -c TestGenCls -verbose

public class test.TestGenCls extends java.lang.Object
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Asciz    test/TestGenCls;
const #2 = class    #1; //  test/TestGenCls
const #3 = Asciz    java/lang/Object;
const #4 = class    #3; //  java/lang/Object
const #5 = Asciz    <init>;
const #6 = Asciz    ()V;
const #7 = NameAndType  #5:#6;//  "<init>":()V
const #8 = Method   #4.#7;  //  java/lang/Object."<init>":()V
const #9 = Asciz    main;
const #10 = Asciz   ([Ljava/lang/String;)V;
const #11 = Asciz   java/lang/System;
const #12 = class   #11;    //  java/lang/System
const #13 = Asciz   out;
const #14 = Asciz   Ljava/io/PrintStream;;
const #15 = NameAndType #13:#14;//  out:Ljava/io/PrintStream;
const #16 = Field   #12.#15;    //  java/lang/System.out:Ljava/io/PrintStream;
const #17 = Asciz   Hello World!;
const #18 = String  #17;    //  Hello World!
const #19 = Asciz   java/io/PrintStream;
const #20 = class   #19;    //  java/io/PrintStream
const #21 = Asciz   println;
const #22 = Asciz   (Ljava/lang/String;)V;
const #23 = NameAndType #21:#22;//  println:(Ljava/lang/String;)V
const #24 = Method  #20.#23;    //  java/io/PrintStream.println:(Ljava/lang/String;)V
const #25 = Asciz   Code;

{
public test.TestGenCls();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   getstatic   #16; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc #18; //String Hello World!
   5:   invokevirtual   #24; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return

}