Building Java Programs

Lab 8: Classes

Except where otherwise noted, the contents of this document are Copyright 2010 Stuart Reges and Marty Stepp.

lab document created by Whitaker Brand and Marty Stepp

Today's lab

Goals for today:

• write our own new classes of objects
• declare fields, methods, and constructors
• understand the difference between class code and client code
• Where you see this icon, you can click it to check the problem in Practice-It!

Recall: Declaring a class (syntax)

public class class name {
    // fields
    field type field name;

    // methods
    public return type method name() {
        statements;
    }
}

A couple things look different than programs for past homeworks:
• 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 : Class and client code

Suppose a method in the BankAccount class is defined as:

public double computeInterest(int rate)

If the client code has declared a BankAccount variable named acct, which of the following would be a valid call to the above method?

1. int result = BankAccount.computeInterest(42);
2. double result = computeInterest(acct, 42);
3. acct.computeInterest(42.0, 15);
4. double result = acct.computeInterest(42);

Exercise : PointCoordinates

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

Point p1 = new Point();
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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import java.awt.*;
public class Point {
    int x;                                   // Each Point object has
    int y;                                   // an int x and y inside.

    public static void draw(Graphics g) {    // draws this point
        g.fillOval(p1.x, p1.y, 3, 3);
        g.drawString("(" + p1.x + ", " + p1.y + ")", p1.x, p1.y);
    }

    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;
    }

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

The above Point class has 5 errors. Can you find them all?

Exercise - answer

1. line 6: method header should not have the word static
2. line 12-13: should not re-declare fields x and y (delete word int)
3. lines 17-18: should not declare Point p nor use p. in front of any fields

Exercise - solution

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

    public void draw(Graphics g) {           // draws this point
        g.fillOval(x, y, 3, 3);
        g.drawString("(" + x + ", " + y + ")", x, y);
    }

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

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

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.
• 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.)
• Solution: PointClient.java (don't peek until you are done!)

BankAccount class

• The next exercise refers to the following BankAccount class (see next slide):

// Each BankAccount object represents one user's account
// information including name and balance of money.

public class BankAccount {
    String name;
    double balance;

    public void deposit(double amount) {
        balance = balance + amount;
    }

    public void withdraw(double amount) {
        balance = balance - amount;
    }
}

Exercise : BankAccount transactionFee

• Download the following file BankAccount.java and open it with jGrasp.
• Add a double field named transactionFee that represents an amount of money to deduct every time the user withdraws money. The default value is $0.00, but the client can change the value. Deduct the transaction fee money during every withdraw call (but not from deposits).
• Make sure that the balance cannot go negative during a withdrawal. If the withdrawal (amount plus transaction fee) would cause it to become negative, don't modify the balance value at all.
• Test your solution to this problem in Practice-It.

Point and PointMain

• Download the following file PointMain.java to your machine and open it with jGrasp.
• Now 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.

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 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 the object initially represents the point (5, -3), after a call to flip, the object should represent (3, -5). If the object initially represents the point (4, 17), after a call to flip, the object should represent (-17, -4).

(Test your code by running the PointMain program. You will need to un-comment the flip testing code.)

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 punkt represents the point (5, -17), return the string:

Point[x=5,y=-17]

If the client code were to call System.out.println(punkt); , that text would be shown on the console.

(Test your code by running your PointClient or PointMain and printing a Point there.)

Exercise : BankAccount toString

Add a toString method to the BankAccount class. Your method should return a string that contains the account's name and balance separated by a comma and space. For example, if an account object named benben has the name "Benson" and a balance of 17.25, benben.toString() should return:

Benson, $17.25

If the client code were to call System.out.println(benben); , that text would be shown on the console.

(Test your code by writing a short client program that constructs and initializes an account, then prints it.)

Exercise : manhattanDistance

Add the following method to the Point class:

public int manhattanDistance(Point other)

Returns the "Manhattan distance" between the current Point object and the given other Point object. The Manhattan distance refers to how far apart two places are if the person can only travel straight horizontally or vertically, as though driving on the streets of Manhattan. In our case, the Manhattan distance is the sum of the absolute values of the differences in their coordinates; in other words, the difference in x plus the difference in y between the points.

Click on the check-mark above to try out your solution in Practice-it! (For this problem, write just the new method, not the entire Point class.)

Exercise : TimeSpan

Define a class named TimeSpan. A TimeSpan object stores a span of time in hours and minutes (for example, the time span between 8:00am and 10:30am is 2 hours, 30 minutes). The minutes should always be reported as being in the range of 0 to 59. That means that you may have to "carry" 60 minutes into a full hour.

To solve this problem, you will need to peek at the upcoming Wednesday lecture slides to learn the syntax for a constructor.

See the Practice-It link above for a full description of the class and the methods/constructors it should have. You can also test your class in Practice-It.

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 Chapter 8 and try to solve them!