Launch com.sun.tools.example.debug.gui.GUI on a simple program
such as HelloWorld.java.

You will see something like this:

>>> stop in HelloWorld.main
>>> load HelloWorld
Starting child.
Connected to VM
VM started
Breakpoint resolved to breakpoint HelloWorld.main (resolved).
Breakpoint hit: thread="main", HelloWorld.main(), line=3
>>> list
1 System.out.println("Hello, world!");
2 System.out.println("Hello, world!");
3 => System.out.println("Hello, world!");
4 System.out.println("Hello, world!");
5 System.out.println("Hello, world!");
6 System.out.println("Hello, world!");
7 System.out.println("Hello, world!");
8 System.out.println("Hello, world!");
9 System.out.println("Hello, world!");
10 System.out.println("Hello, world!");