CSC8303 - Conditionals and Inheritance

It's not enough to be able to loop, with need code that sometimes does one thing and sometimes does another. Hence the if statement.

Write a class with a body like this:

public class Bounce{

    public void animateBouncingCircle(){

        Circle c = new Circle();
        c.makeVisible();

        // the canvas is 300 in size, and the circle is 30 in diameter. We
        // could hard-code this as 270 but this explains where the value comes
        // from.
        int maxSize = 300 - 30;

        for (int i = 0; i < getNumberOfSteps(); i++) {
            c.moveHorizontal(getHorizontalStep());
            c.moveVertical( getVerticalStep());
            animateStep();

        }
        eraseObject( c );
    }


    private int numberOfSteps;

    public int getNumberOfSteps() {
        return numberOfSteps;
    }

    public void setNumberOfSteps(int numberOfSteps) {
        this.numberOfSteps = numberOfSteps;
    }


    private int verticalStep;

    public int getVerticalStep() {
        return verticalStep;
    }

    public void setVerticalStep(int verticalStep) {
        this.verticalStep = verticalStep;
    }


    private int horizontalStep;
    public int getHorizontalStep() {
        return horizontalStep;
    }

    public void setHorizontalStep(int horizontalStep) {
        this.horizontalStep = horizontalStep;
    }
}

and another like this.

public class BounceTest {

    public static void main(String[] args) throws InterruptedException {

        Bounce b = new Bounce();
        b.setHorizontalStep( 1 );
        b.setVerticalStep( 2 );
        b.setNumberOfSteps( 2000 );

        b.animateBouncingCircle();

        Thread.currentThread().sleep( 2000 );
        System.exit( 0 );

    }
}

This is fine, but the Circle falls of the edge of the Canvas which is not what we want. What would be nice is to change the direction of the Circle when it hits the edge. For this, we need two things. First, we need to know where the Circle object is and, second, we need a conditional statement.

Unfortunately, although the Circle objects know what their position is, there is no way to access this. So, first we need to change this class.
Add to Circle.java the following code:

    public int getXPosition(){
        return xPosition;
    }

    public int getYPosition(){
        return yPosition;
    }

This introduces two new methods to the Circle class; these are called accessor methods. They allow you to get the location information that was previously hidden.
Now, you need to introduce a if statement into this for loop.

     for (int i = 0; i < getNumberOfSteps(); i++) {
            c.moveHorizontal(getHorizontalStep());
            // check here if getXPosition has got too large.

            c.moveVertical( getVerticalStep());
            // check here if getYPosition has got too large.

            animateStep();

     }

At the points highlighted, you should introduce an if statement which checks to see if getXPosition() or getYPosition() have returned greater than maxSize. If you, they should reverse their direction.
Run BounceTest.java again. Your circle should now bounce of the edge of the Canvas. However, when it reaches the left edge, it will disappear again.
Put another conditional check in, so that when getXPosition() or getYPosition() go below 0, they reverse direction.
It is possible to do this with another two if statements, or using only the existing ones. See if you can work out how.

Inheritance

Our code now works, so it's worth creating a copy of it. Next thing is to change it!

One problem is that some the code appears in both Bounce.java and Grow.java. This is a pity; if we want to change the code, then we need to change it in two places. So we are going to use inheritance to get around this. You can read more about inheritance if you choose.

You should have three pieces of code in both places. We are going to move them to a new class called Animate.java.

Create a new java class called Animate.java.
Add in all of this code which you should have earlier copied from Bounce.java ¹.

  private int animateStep;

    /**
     * Get the value of animateStep
     *
     * @return the value of animateStep
     */
    public int getAnimateStep() {
        return animateStep;
    }

    /**
     * Set the value of animateStep
     *
     * @param animateStep new value of animateStep
     */
    public void setAnimateStep(int animateStep) {
        this.animateStep = animateStep;
    }


    protected void animateStep() {
        Canvas.getCanvas().wait( 50 );
    }

    protected void eraseObject(Object erased) {
        Canvas.getCanvas().erase(erased);
    }

Delete this code from both Bounce.java and Grow.java. Grow.java should now be pretty small!
Now we have only got the code once, but we cannot compile; we've moved code away from these two classes, but not told them were it has gone.
Change the first line of Bounce.java to this.

public class Bounce extends Animate

This essential tells Java "if you can't find something in Bounce.java look in Animate.java instead.
Change Grow.java also.

1. There are two small changes to the original code which you should be able to find. Don't worry about these too much.

CSC8303 -- Bioinformatics Programming in Java

Conditionals and arrays

If statements

Inheritance