Homework Assignments Week 2.3: Virtual Machines

In these assignments you explore the Java Virtual Machine.

Verify your answers with the model answers.

Java Virtual Machine

• What are the steps to call a method in Java Bytecode?
• How does the Java Virtual Machine handle a method call?
• Where are the parameters of a method stored?
• What are the steps to return from a method in Java Bytecode?
• How does the Java Virtual Machine handle a return?

Bytecode Execution

Execute the bytecode instructions of A/main()V, starting with an empty stack. The initial value of local variable 0 is 4242 4103, pointing to an object of class A. Show stacks and local variables after each instruction.

Code for A/main()V

aload_0
bipush 5
iconst_4
isub
invokevirtual A/m(I)V

Code for A/m(I)V

    goto l2
l1: iinc 1 -1
l2: iload_1
    ifne l1
return

Code for Hint: iinc 1 -1

iload_1
ldc -1
iadd
istore_1

Bytecode Execution

Execute the bytecode instructions of Main/main()V, starting with an empty stack. The initial value of local variable 0 is 4242 4303, pointing to an object of class Foo. Show stacks and local variables after each instruction. If you have to invoke a method, execute the bytecode of this method as well. Make clear when stack frames are created and destroyed, and which data is passed between frames.

Code for Main/main()V:

    aload_0
    bipush 4
    iconst_2
    isub
    invokevirtual Foo/f(I)V

Code for Foo/f(I)V

    iload_1
    goto l2
l1: iload_1
    ldc -2
    iadd
    dup
    istore_1
l2: ifne l1
    return

Bytecode Execution

Execute the bytecode instructions of Main/main()V, starting with an empty stack. If you have to invoke a method, execute the bytecode of this method as well. The initial value of local variable 0 is 4242 4303, pointing to an object of class Foo. Show stacks and local variables after each instruction.

Code for Main/main()V

aload_0
bipush 3
iconst_2
isub
invokevirtual Foo/f(I)V

Code for Foo/f(I)V

    goto l2
l1: iinc 1 -1
l2: iload_1
    ifne l1
    return

Hint: iinc 1 -1

iload_1
ldc -1
iadd
istore_1

Bytecode Execution

Execute the following bytecode instructions, starting with an empty stack. Show the stack after each instruction. If an instruction performs a jump, name the next instruction.

    bipush 21
    ldc 5
    iconst_4
    dup
    imul
    iadd
    isub
    ifeq l1
    iconst_1
l1: nop

Stack Frames

A Java class A defines a method int m(int p1, int p2).

• Which steps (on bytecode level) are needed to call m(1,2) on an object a of type A? Which steps are needed to return from a call?
• Explain how the Java Virtual Machine handles the call and the return.
• Explain single dispatch as used by the Java Virtual Machine. Compare this approach with two other kinds of dispatch.