University of Washington, CSE 142

Lab 8: Classes and Objects

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

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

Basic lab instructions

Today's lab

Goals for today:

Declaring a class (syntax)

public class ClassName {
    // fields
    fieldType fieldName;

    // methods
    public returnType methodName() {
        statements;
    }
}
A couple things look different than programs for past homeworks:

Exercise : Client code method call syntax practice-it

Suppose you have the following BankAccount class (on the left) and client code (on the right):

public class BankAccount {
   public BankAccount() {...}
   public double computeInterest(int rate) {...}
}
public class BankAccountClient {
   public static void main(String[] args) {
      BankAccount acct = new BankAccount();
      // your code goes here
   }
}
What would be a valid call to computeInterest given the following client code, and a desired rate of 42?

Exercise : PointCoordinates practice-it

What are the x- and y-coordinates of the Points (iconPoint.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,]+
p1:
(20, 10)
p2:
[^0-9,]+
(46, 4)
p3:
[^0-9,]+
(46, 4)

Exercise : Point class errors

Can you find all the errors in the following code?
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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 practice-it

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

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

Exercise : PointClient practice-it

Point and PointMain

Exercise : quadrant practice-it

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 practice-it

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 practice-it

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.)

BankAccount class

// 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 practice-it

Exercise : BankAccount toString practice-it

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 practice-it

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 practice-it

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.

Exercise : Circle practice-it

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 : 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 this link that contains code files from Chapter 8 of the textbook: http://www.buildingjavaprograms.com/code_files/4ed/ch08/
  2. 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.
  3. Compile and run StockMain.java in jGRASP to see that it works.

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Exercise - jGRASP Debugger

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Exercise - jGRASP Debugger

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Exercise - jGRASP Debugger

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!