How does Karel run without “main” method?

Question

I was wondering in the program like Karel the Robot runs because it does not used main() method. Instead it used run() method:

import stanford.karel.Karel;

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Answer 1

The key to this is quite simple: extends Karel

The Karel class is one that implements Runnable, and has quite a few methods within it.

Without trying to go into detail about what all it does, it looks something like this:

public class Karel implements java.lang.Runnable {
    private static final int NORTH = 0;
    private static final int EAST = 1;
    private static final int SOUTH = 2;
    private static final int WEST = 3;
    private static final int INFINITE = 99999999;
    private stanford.karel.KarelWorld world;
    private int x;
    private int y;
    private int dir;
    private int beepers;

    public Karel() { /* compiled code */ }
    public void run() { /* compiled code */ }
    public void move() { /* compiled code */ }
    public void turnLeft() { /* compiled code */ }
    public boolean beepersPresent() { /* compiled code */ }
    public boolean noBeepersPresent() { /* compiled code */ }
    public boolean beepersInBag() { /* compiled code */ }
    public boolean noBeepersInBag() { /* compiled code */ }
    public boolean facingNorth() { /* compiled code */ }
    public boolean facingEast() { /* compiled code */ }
    public boolean facingSouth() { /* compiled code */ }
    public boolean facingWest() { /* compiled code */ }

    public static void main(java.lang.String[] args) { /* compiled code */ }
    protected void start() { /* compiled code */ }
    protected void start(java.lang.String[] args) { /* compiled code */ }

    ...
}

I took out quite a few methods from there because its a very long list of things it defines.

But ore that last one that I left in there... protected void start(java.lang.String[] args) (or at least, that's how my library inspector interpreted it). ^{_{Digging into this code, it appears that this invokes main() in KarelProgram, but for the most part, thats neither here nor there.}}

And thus you have inheritance. You are inheriting a number of other methods that the code is using. That you are using move(); in that code but not defining it should be no more surprising than having a main(java.lang.String[] args) defined.

All it takes is opening up the class in the jar in your IDE.