University of Washington, CSE 142

Lab 8: Classes and Objects

Except where otherwise noted, the contents of this document are Copyright 2012 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

• Talk to your classmates for help. You can even work on the lab with a partner if you like.
• You may want to bring your textbook to future labs to look up syntax and examples.
• Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
• Complete as many problems as you can within the allotted time. You don't need to keep working on these exercises after you leave the lab.
• Feel free to complete problems in any order.
• Before you leave today, make sure to check in with one of the TAs in the lab to get credit for your work.

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!

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 : Client code method call syntax

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);

(20, 10)

[^0-9,]+

(46, 4)

(46, 4)

Exercise : Point class errors

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21

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: should not re-declare field x (delete word int)
3. line 13: should not re-declare field y (delete word int)
4. line 17: should not declare Point p
5. line 18: should not use p. in front of the fields

Exercise - solution

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20

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. (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.)

Exercise : jGRASP Debugger

The debugger can help you learn how classes and objects work. In this exercise we will debug the Ch. 8 "Stock" Case Study example. This program tracks purchases of two stock investments. To download the example:

1. Go to the class web page and click the "Textbook" link.
2. Find the section labeled "Code Files" and click the "code files" link. Then click "ch08".
3. Download and save the files Stock.java and StockMain.java. Right-click each file name and Save the Link in the same folder you use for lab work.
4. Compile and run StockMain.java in jGRASP to see that it works.

continued on the next slide...

Exercise - jGRASP Debugger

• Set a stop on the System.out.print on line 53, and debug the program.
• Type the user input to recreate the following partial log:

  First stock's symbol: AMZN
  How many purchases did you make? 2
  1: How many shares, at what price per share? 50 35.06
  2: How many shares, at what price per share? 25 38.52
  

• At this point, what are the values of the fields in the currentStock object?
  (Ask a TA if you need help.)

  totalShares		75
  totalCost		2716.0

continued on the next slide...

Exercise - jGRASP Debugger

• Now let's debug inside one of the Stock object's methods. Your program should still be stopped. Set a new stop at the return on line 29 of Stock.java.
• Resume your program . It will prompt for today's price per share. Enter 37.29 . If you did it properly, the program should now call getProfit on the Stock and hit your stop point.

  What is today's price per share? 37.29
  

• What are the values of the object (this) 's fields, and the variable marketValue ?

  symbol		"AMZN"
  totalShares		75
  totalCost		2716.0

  ---

  marketValue

  2796.75

continued on the next slide...

Exercise - jGRASP Debugger

• Now let's watch the total shares and cost of a stock change with each purchase. Remove your existing stop points and set a new stop somewhere in the for loop on lines 42-50 of StockMain, and Resume .
• Type the user input to continue recreating the following partial log:

  Second stock's symbol: INTC
  How many purchases did you make? 3
  1: How many shares, at what price per share? 15 16.55
  2: How many shares, at what price per share? 10 18.09
  3: How many shares, at what price per share? 20 17.05
  What is today's price per share? 17.82
  

• After each call of purchase, what are the field values of currentStock?

  field	after 1st purchase	after 2nd	after 3rd
  totalShares	15	25	45
  totalCost	248.25	429.15	770.15

BankAccount class

• The next exercise refers to the following BankAccount class (download it and open it in jGRASP).

// 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

• Add a double field to the BankAccount class 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. 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.

(Test your code in Practice-It, or 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! (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 may want to refer to the lecture slides about the syntax for constructors.

See the Practice-It link above for a full description of the class and the methods/constructors it should have.

Exercise : Circle

Define a class named Circle. A Circle object stores a center point and a radius.

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.

Exercise : Point class errors 2

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21

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;
    }
}

The above Point class has 8 errors. Can you find them all?

Exercise - answer

1. line 5: constructor header should not have the word void
2. line 6-7: the assignment statements are backwards; reverse left/right sides of each
3. line 10: method header should not have the word static
4. line 11-12: should not re-declare fields x and y
5. lines 18-19: should not re-declare fields x and y (remove word int)

Exercise - solution

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19

public class Point {
    int x;
    int y;

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

    public double distanceFromOrigin() {
        double dist = Math.sqrt(x*x + y*y);
        return dist;
    }

    public void translate(int dx, int dy) {
        x = x + dx;
        y = y + dy;
    }
}

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!