Building Java Programs

Lab 8: Ch. 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 and Stuart Reges

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);

[^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: 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

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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. (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

Exercise : Point class errors 2

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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;
    }
}

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

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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;
    }
}

Point class

Suppose you are given a class named Point with the following contents:

// A Point represents a 2-D (x, y) position with integer coordinates.
public class Point {
    private int x;
    private int y;
    
    public Point()                          // (0, 0)
    public Point(int x, int y)
    
    public int getX()                       // return field values
    public int getY()
    public double distanceFromOrigin()      // distance from (0, 0)
    public double distance(Point other)     // distance from another point
    public void setLocation(int x, int y)   // set x/y to the given values
    public void translate(int dx, int dy)   // shift x/y by the given amounts
    public String toString()                // string e.g. "(4, -7)"

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 : 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.)

BankAccount class

Suppose you are given a class named BankAccount with the following contents:

// A BankAccount keeps track of a user's money balance and ID,
// and counts how many transactions (deposits/withdrawals) are made.
public class BankAccount {
    private String id;
    private double balance;
    private int transactions;
    
    // Constructs an object with the given id/balance and 0 transactions.
    public BankAccount(String id, double balance)
    
    public double getBalance()
    public String getID()
    public String getTransactions()
    
    // Adds amount to balance if between 0-500. Also counts as 1 transaction.
    public void deposit(double amount)
    
    // Subtracts amount from balance if user has enough money. Counts as 1 transaction.
    public void withdraw(double amount)
    
    ...
}

Exercise : BankAccount transactionFee

• Write a method transactionFee to go in the BankAccount class. It takes a fee (real number) parameter and applies the fee to all past transactions. The fee is applied once for the first transaction, 2x for the second, 3x for the third, etc. These fees are subtracted from the account's balance.
• If the balance is large enough to afford all fees with > $0.00 remaining, the method returns true. If not, the balance is 0.0 and it returns false. For example:

  BankAccount savings = new BankAccount("Jimmy", 0.00);
  savings.deposit(10.00);
  savings.deposit(50.00);
  savings.deposit(10.00);
  savings.deposit(70.00);         // balance = $140, with 4 transactions
  
  savings.transactionFee(5.00);   // deducts  $5+10+15+20; balance = $90; returns true
  savings.transactionFee(10.00);  // deducts $10+20+30+40; balance =  $0; returns false
  

Exercise : BankAccount transfer

• Write an instance method transfer that moves money from this bank account to another account. The method takes two parameters: a BankAccount to accept money, and a real number for the amount to transfer. There is a $5.00 fee for transferring, to be deducted from the current account's balance.
• The method should modify the two objects such that "this" has its balance decreased by the given amount plus $5 fee, and the other's balance is increased by the given amount. A transfer also counts as a transaction on both accounts. If this account object does not have enough money to make the full transfer, transfer whatever money is left after the $5 fee is deducted. If this account has under $5 or the amount is 0 or less, no transfer should occur and neither account's state should be modified.
• For example, the following calls would produce the following results:

  BankAccount ben = new BankAccount("Benson", 90.00);
  BankAccount mar = new BankAccount("Marty",  25.00);
  ben.transfer(mar, 20.00);     // ben $65, mar $45   (ben loses $25, mar gains $20)
  ben.transfer(mar, 10.00);     // ben $50, mar $55   (ben loses $15, mar gains $10)
  ben.transfer(mar, -1);        // ben $50, mar $55   (no effect; negative amount)
  mar.transfer(ben, 39.00);     // ben $89, mar $11   (mar loses $44, ben gains $39)
  mar.transfer(ben, 50.00);     // ben $95, mar $ 0   (mar loses $11, ben gains $ 6)
  mar.transfer(ben,  1.00);     // ben $95, mar $ 0   (no effect; no money in account)
  ben.transfer(mar, 88.00);     // ben $ 2, mar $88   (ben loses $93, mar gains $88)
  ben.transfer(mar,  1.00);     // ben $ 2, mar $88   (no effect; can't afford fee)
  

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.)

ClockTime class

Suppose you are given a class named ClockTime with the following contents:

// A ClockTime object represents an hour:minute time during
// the day or night, such as 10:45 AM or 6:27 PM.
public class ClockTime {
    private int hour;
    private int minute;
    private String amPm;

    // Constructs a new time for the given hour/minute
    public ClockTime(int h, int m, String ap)
    
    // returns the field values
    public int getHour()
    public int getMinute()
    public String getAmPm()

    // returns a String for this time; for example, "6:27 PM"
    public String toString()

    ...
}

Exercise : ClockTime advance

• Write an instance method advance that will be placed inside the ClockTime class. The method accepts a number of minutes as its parameter and moves your object forward in time by that amount of minutes. The minutes passed could be any non-negative number, even a large number such as 500 or 1000000. If necessary, your object might wrap into the next hour or day, or it might wrap from the morning ("AM") to the evening ("PM") or vice versa. (A ClockTime object doesn't care about what day it is; if you advance by 1 minute from 11:59 PM, it becomes 12:00 AM.)
• For example, the following calls would produce the following results:

  ClockTime time = new ClockTime(6, 27, "PM");
  time.advance(1);       //  6:28 PM
  time.advance(30);      //  6:58 PM
  time.advance(5);       //  7:03 PM
  time.advance(60);      //  8:03 PM
  time.advance(128);     // 10:11 PM
  time.advance(180);     //  1:11 AM
  time.advance(1440);    //  1:11 AM  (1 day later)
  time.advance(21075);   //  4:26 PM  (2 weeks later)
  

• Test your solution to this problem in Practice-It.

Exercise : ClockTime isWorkTime

• Write an instance method isWorkTime that will be placed inside the ClockTime class. The method returns true if the object is a time during the "work day" from 9:00 AM - 5:00 PM inclusive; otherwise false.
• The following objects would return the following results if the method were called:

  ClockTime t0 = new ClockTime(12, 45, "AM");   // false
  ClockTime t1 = new ClockTime( 6, 02, "AM");   // false
  ClockTime t2 = new ClockTime( 8, 59, "AM");   // false
  ClockTime t3 = new ClockTime( 9, 00, "AM");   // true
  ClockTime t4 = new ClockTime(11, 38, "AM");   // true
  ClockTime t5 = new ClockTime(12, 53, "PM");   // true
  ClockTime t6 = new ClockTime( 3, 15, "PM");   // true
  ClockTime t7 = new ClockTime( 4, 59, "PM");   // true
  ClockTime t8 = new ClockTime( 5, 00, "PM");   // true
  ClockTime t9 = new ClockTime( 5, 01, "PM");   // false
  ClockTime ta = new ClockTime( 8, 30, "PM");   // false
  ClockTime tb = new ClockTime(11, 59, "PM");   // false
  

• Test your solution to this problem in Practice-It.

Rectangle class

Suppose you are given a class named Rectangle with the following contents:

// A Rectangle stores an (x, y) coordinate of its top/left corner, a width and height.
public class Rectangle {
    private int x;
    private int y;
    private int width;
    private int height;

    // constructs a new Rectangle with the given x,y, width, and height
    public Rectangle(int x, int y, int w, int h)

    // returns the fields' values
    public int getX()
    public int getY()
    public int getWidth()
    public int getHeight()

    // returns a string such as {(5,12), 4x8}
    public String toString()

    ...
}

Exercise : Rectangle union

• Write an instance method union that will be placed inside the Rectangle class. The method accepts another rectangle r as a parameter and turns the current rectangle into the union of itself and r; that is, modifies the fields so that they represent the smallest rectangular region that completely contains both this rectangle and r.
• For example, the following calls would produce the following rectangles:

  Rectangle rect1 = new Rectangle(5, 12, 4, 6);
  Rectangle rect2 = new Rectangle(6,  8, 5, 7);
  Rectangle rect3 = new Rectangle(14, 9, 3, 3);
  Rectangle rect4 = new Rectangle(10, 3, 5, 8);
  
  rect1.union(rect2);   // {(5, 8), 6x10}
  rect4.union(rect3);   // {(10, 3), 7x9}
  

• Test your solution to this problem in Practice-It.

Exercise : Rectangle contains

• Write an instance method contains that will be placed inside the Rectangle class. The method accepts integers for x and y as parameters and turns true if that x/y coordinate lies within this rectangle. The edges are included; for example, a rectangle with x=2, y=5, width=8, height=10 will return true for any point from (2, 5) through (10, 15) inclusive.
• Test your solution to this problem in Practice-It.

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.

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!