Exercise : LabTA Version 4

To allow your LabTA critters to change colors on alternate moves, they need to keep track of where they are in the cycle. For example, you could introduce a boolean field that keeps track of whether a LabTA should be displaying itself as magenta or as white. You can then switch the boolean value each time the LabTA makes a move.

• Add a field to your class called isWhite to keep track of whether the LabTA should be white, or not (in which case it should be magenta). Remember that fields should be declared to be private. Also recall that the default value of a boolean is false.
• Modify the getMove method to flip this boolean value each time getMove() is called.
  Important: Can the boolean switch be flipped in a different method? Why/why not?
• Rewrite the getColor method using the isWhite boolean to change colors as expected.
• Test your new version by recompiling the LabTA class and running CritterMain.

Expression : Solution

import java.awt.*;
public class LabTA extends Critter {
    
    // default value = false
    private boolean isWhite;
    
    public Direction getMove() {
        isWhite = !isWhite;
        return Direction.NORTH;
    }
    
    public Color getColor() {
        if (isWhite) {
            return Color.WHITE;
        } else {
            return Color.MAGENTA;
        }
    }
    
    // methods implemented earlier ommitted
}

Exercise : Important: another solution

import java.awt.*;
public class LabTA extends Critter {
    
    // default value = 0
    private int moves;
    
    public Direction getMove() {
        moves++;
        return Direction.NORTH;
    }
    
    public Color getColor() {
        if (moves % 2 == 1) {
            return Color.WHITE;
        } else {
            return Color.MAGENTA;
        }
    }
    
    // methods implemented earlier ommitted
}

Exercise : LabTA Version 4

Now we're going to work on the moves that the LabTA critters make.

• Open the FlyTrap class to see how it is written. Notice its getMove method:

      public Action getMove(CritterInfo info) {
          if (info.getFront() == Neighbor.OTHER) {
              return Action.INFECT;
          } else {
              return Action.LEFT;
          }
      }
  

• Copy and paste this code to replace the current version of getMove in the LabTA class. Then modify it so that it always infects when it can and otherwise it hops if it can and otherwise it turns left. This involves adding a new branch in the if/else. Remember that the constant Neighbor.EMPTY can be used to see if a position is empty and that you move forward with the Action.HOP move.
• Test your new version by running CritterMain and observing the LabTA critters.

Exercise : LabTA Version 5

Now we're going to include a bit of randomness. The current version of the LabTA class always moves NORTH. We are going to change it so that it instead randomly chooses between going EAST and WEST.

To solve this problem, we will need access to a Random object. We don't want to construct one every time we call getMove, so we will instead store it as a field. Remember to import java.util.*; to use Random.

• Declare a field called rand of type Random. Do not initialize the field where it is declared.
• Constructors exist to set the initial state of our Objects (initializing fields!) So far, we've been using an implicit constructor that takes no parameters and does nothing when it constructs LabTAs. Now add a constructor that takes no parameters, and in the constructor, initialize the Random object.
• Modify the getMove method so that it uses the Random object to randomly pick between going WEST and going EAST.
• Test your new version by recompiling LabTA, and running CritterMain again.

Exercise : Solution

import java.awt.*;
import java.util.*;
public class LabTA extends Critter {

    private boolean isWhite; 
    private Random rand;
    
    public LabTA() {
        rand = new Random();
        isWhite = false;      // this line is not necessary, boolean default value is false
    }
    
    public Direction getMove() {
        isWhite = !isWhite;
        boolean goEast = rand.nextBoolean();
        if (goEast) {
            return Direction.EAST;
        } else {
            return Direction.WEST;
        }
    }    
    // methods implemented earlier ommitted
}

Declaring a class (syntax)

public class ClassName {
    // fields
    fieldType fieldName;

    // methods
    public returnType methodName() {
        statements;
    }
}

• no main method. It won't be run like a client program.
• methods don't have the static keyword in the header.
• variables declared outside of any method (fields) are visible in every method.

Exercise : PointCoordinates

What are the x- and y-coordinates of the Points (Point.java) p1, p2, and p3 after all of the following code executes? Give your answer as an x-y pair such as (0, 0). (Recall that Point use reference semantics).

public class PointClient {
   public static void main(String[] args) {
      Point p1 = new Point();
      // changes the value of the x field of p1
      p1.x = 17;
      p1.y = 9;
      Point p2 = new Point();
      p2.x = 4;
      p2.y = -1;
      Point p3 = p2;

      p1.translate(3, 1);
      p2.x = 50;
      p3.translate(-4, 5);
   }
}

[^0-9,]+

(20, 10)

[^0-9,]+

(46, 4)

[^0-9,]+

(46, 4)

Exercise : Point class errors

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public class Point {
    int x;                                       // Each Point object has
    int y;                                       // an int x and y inside.

    public void Point(int initX, int initY) {    // Constructor
        initX = x;
        initY = y;
    }

    public static double distanceFromOrigin() {  // Returns this point's
        int x;                                   // distance from (0, 0).
        int y;
        double dist = Math.sqrt(x*x + y*y);
        return dist;
    }

    public void translate(int dx, int dy) {      // Shifts this point's x/y
        int x = x + dx;                          // by the given amounts.
        int y = y + dy;
    }
}

Exercise - solution

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public class Point {
    private int x;                               // Each Point object has
    private int y;                               // an int x and y inside.
    // a constructor does not have a return type
    public void Point(int initX, int initY) {    // Constructor
        // we want to be changing the values of the fields!
        x = initX;
        y = initY;
    }
   
    public static double distanceFromOrigin() {  // Returns this point's
        int x;                                   // distance from (0, 0).
        int y;
        double dist = Math.sqrt(x*x + y*y);
        return dist;
    }

    public void translate(int dx, int dy) {
	// we want to change the values of fields, not create new local variables
        int x = x + dx;
        int y = y + dy;
    }
}

Exercise : Printing objects

Point p1 = new Point();
...
System.out.println(p1);

The above println statement (the entire line) is equivalent to what?

1. System.out.println(p1.string());
2. p1.toString();
3. System.out.println(p1.toString());
4. System.out.println(Point.toString());
5. System.out.println(toString(p1));

Exercise : PointClient

• Download the following files Point.java and PointClient.java to your machine and open them with jGrasp. (or solve this problem in Practice-It!)
• The PointClient program is supposed to construct two Point objects, translate each, and then print their coordinates. Finish the program so that it runs properly. (You don't need to modify Point.java.)

Point and PointMain

• Download the following file PointMain.java to your machine and open it with jGrasp.
• Now download and edit Point.java and add two methods to the Point class as described on the next two slides. The PointMain program calls these methods; you can use it to test your code. Or you can test your code in Practice-It!

Exercise : quadrant

Add the following method to the Point class:

public int quadrant()

Returns which quadrant of the x/y plane this Point object falls in. Quadrant 1 contains all points whose x and y values are both positive. Quadrant 2 contains all points with negative x but positive y. Quadrant 3 contains all points with negative x and y values. Quadrant 4 contains all points with positive x but negative y. If the point lies directly on the x and/or y axis, return 0.

(Test your code in Practice-It! or by running the PointMain program.)

Exercise : flip

Add the following method to the Point class:

public void flip()

Negates and swaps the x/y coordinates of the Point object. For example, if an object pt initially represents the point (5, -3), after a call of pt.flip(); , the object should represent (3, -5). If the same object initially represents the point (4, 17), after a call to pt.flip();, the object should represent (-17, -4).

Test your code in Practice-It! or by running the PointMain program.

Exercise : Point toString

Modify the toString method in the Point class. Make it return a string in the following format. For example, if a Point object stored in a variable pt represents the point (5, -17), return the string:

Point[x=5,y=-17]

If the client code were to call System.out.println(pt); , that text would be shown on the console. Note that this format is slightly different in the Practice-it problem.

(Test your code in Practice-It, or by running your PointClient or PointMain and printing a Point there.)

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!