Question

I have been making a Tetris-like game in Java recently, I have a interface IBlocks which goes over ConsBlock and EmptyBlock (ConsBlock is a list with a block at the beginnning and a IBlock list at the end, for some reason that’s the way my teacher wanted it).

I have the getfirst() function in the interface:

public interface IBlocks{
//returns the first block in a list of blocks
public Resting getfirst();

(A Resting is one block in the Game)

Then, in the ConsBlock class, I have:

public class ConsBlock implements IBlocks{
//returns the first block in a list of blocks
public Resting getfirst(){
    return this.first;

But in the EmptyBlock class, I want it to return something that would be similar to saying there isn’t one. I tried returning null, but that gave me a null pointer exception due to that fact that the function in the interface tells it to return a Resting. What would be the best way to represent an empty one without returning a Resting with random numbers?

Answer 1

public interface IResting;
public class Resting implements IResting;

Then

public class EmptyResting implements IResting {
    @Override
    public Object sampleMethod() {
        // return what an empty block ought to return
    }
}

Further reading: http://en.wikipedia.org/wiki/Null_Object_pattern

Response to your comment: you still need to do:

public class EmptyBlock implements IBlocks {
    @Override
    public IResting getFirst() {
        return new EmptyResting();
    }
}

Note that your IBlocks and ConsBlock classes need to return an IResting not a Resting

Make sure to read that wiki link!