Warm-ups:

1. Consider the following 'nonsense class'.

class A
{ public A() { n = 0; } // constructor 
  public A(int a) { n = a; } // constructor  
  public void f() { n++; } // mutates n  
  public void g() { f(); n = 2 * n; f(); } 
  // g mutates n directly and indirectly (f) 
  public int h() { return n; } // accessor of n  
  public void k() { System.out.println(n); } 
  // does not return, only prints, the value of n 
  private int n; // instance variable  
}

2. With the nonsense class from the preceding exercise, determine what the following program prints.

It prints 9 and 17 and here's why:

public static void main(String[] args) {
  A a = new A(); // a.n is 0 
  A b = new A(2); // b.n is 2 
  A c = b; // c points where b points so c.n = b.n = 2 
  A d = new A(3); // d.n is 3  
  a.f(); // a.n is 1  
  b.g(); // b.n becomes 3, 6, then 7 
  c.f(); // c.n is b.n and it becomes 8 
  d.g(); // d.n becomes 4, 8, then 9 
  d.k(); // d.n is printed on a line by itself 
  A e = new A(a.h() // a.n is 1  
            + b.h() // b.n is 8 and is equal to c.n   
            + c.h() // which is 8 
        ); // so e.n is set to 1 + 8 + 8 = 17 
  e.k(); // e.n (17) is printed on a line by itself   
}

3. Implement all the methods of the following class:

class Person {
  public Person() {
   name = "YetToBeNamed";
   birthdayYear = 1999; // my default  
  }
  public Person(String givenName, int yearOfBirth) { 
    name = givenName;
    birthdayYear = yearOfBirth; 
  }
  public String getName() { 
    return name; 
  }
  public String changeName(String name) { 
    String aux; 
    aux = this.name;
    this.name = name;
    return aux; 
  }
  public int getAgeInYears(int currentYear) { 
    return currentYear - birthdayYear;  
  }
  private String name;
  private int birthdayYear; 
  public static void main(String[] args) {
    Person a = new Person(); 
    Person b = new Person("Richard P. Feynman", 1918); 
    String name = a.changeName("The Next Richard Feynman");     
    System.out.println(
      "Physicist " + name + " makes big " + 
      "discovery, touted as " + a.getName()    
    );
    System.out.println(
       b.getName() + " was " + 
       b.getAgeInYears(1945) + 
       " in 1945, in May. "
    );
  }  
}

As shown, the test program can be the very main method of the class we wrote.

Design exercises:

4. Implement a class Address. An address has

• a house number,
• a street,
• an optional apartment number,
• a city,
• a state and a
• postal code.

• one with an apartment number
• and one without.

Supply a method compareTo that tests whether one address comes before another when the addresses are compared by postal code.

class Address {
  int    houseNumber;
  String street;
  int    apartmentNumber;
  String city;
  String state;
  String zipCode; // e.g., "47405-1234"
  Address(int houseNumber, 
          String street, 
          // no apartmentNumber  
          String city, 
          String state, 
          String zipCode) { 
    this.houseNumber = houseNumber; 
    this.street = street; 
    this.city = city; 
    this.state = state; 
    this.zipCode = zipCode; 
  } 
  Address(int houseNumber, 
          String street, 
          int apartmentNumber, 
          String city, 
          String state, 
          String zipCode) { 
    this(houseNumber, street, city, state, zipCode);
    this.apartmentNumber = apartmentNumber; 
  } 
  void print() {
    System.out.println("Street: "      + street); 
    System.out.println("City: "        + city 
                     + "State: "       + state 
                     + "Postal Code: " + zipCode); 
  } 
  int compareTo(Address a) { 
    // same conventions as for Strings
    return this.zipCode.compareTo(a.zipCode); 
  } 
}

5. Implement a class Account. An account has

• a balance,
• functions to add
• and withdraw money,
• and a function to inquire the current balance.

If no value is passed the initial balance should be set to $0.

Charge a $5 penalty if an attempt is made to withdraw more money than available in the account.

Enhance the Account class to compute interest on the current balance.

class Account {
  double balance;
  double add(double sum) {
    balance += sum;
    return sum; 
  }
  double withdraw(double sum) {
    if (sum > balance) { 
      balance -= 5;
      return -5; // this will come in handy in Prob. 6 
    } else { 
      balance -= sum;
      return balance; // Notice: always >= 0 (never < 0)
    }
  }
  double inquire() { return balance; } 
  Account() { balance = 0; }
  Account(double sum) { balance = sum; }
  double interest (double rate) {
    return rate * balance; 
  }
}

6. Implement a class Bank. This bank has two objects

• checking
• and savings

Implement four instance methods:

deposit(double amount, String account)

withdraw(double amount, String account)

transfer(double amount, String account) 

printBalances()

class Bank {
  Account checking;
  Account savings; 
  void deposit(double amount, String account) {
    if (account.equals("C")) checking.add(amount); 
    else // my default
      savings.add(amount); 
  }
  void withdraw(double amount, String account) {
    if (account.equals("C")) checking.withdraw(amount); 
    else // my default
      savings.withdraw(amount); 
  }
  void transfer (double amount, String account) {
    if (account.equals("C")) 
      if (checking.withdraw(amount) >= 0) 
        savings.add(amount); 
      else checking.add(5); // somewhat fault-tolerant
    else // default
      if (savings.withdraw(amount) >= 0) 
        checking.add(amount); 
      else savings.add(5);  // no penalty for transfers
  }
  void printBalances() {
    System.out.println(
      "Checking: " + checking.inquire() +
      "\nSavings: " + savings.inquire()
    ); 
  }
}

7. Define a class Country that stores the name of the country, its population, and its area.

class Country {
  String name;
  long population;
  double area;
  Country(String name, long population, double area) {
    this.name = name; 
    this.population = population; 
    this.area = area; 
  }
}

8. Write a class called Triangle that can be used to represent a triangle. It should include the following methods that return boolean values indicating if the particular property holds:

• isRight (a right triangle)
• isScalene (no two sides are the same length)
• isIsosceles (exactly two sides are the same length)
• isEquilateral (all three sides are the same length)

class Triangle {
  int sideA, sideB, sideC; 

  Triangle (int a, int b, int c) { // constructor 
    sideA = a; sideB = b; sideC = c; 
  } // negative numbers and zero lengths accepted 
    // but see how isTriangle() used below

  boolean isTriangle () {
    int max = Math.max(Math.max(sideA, sideB), sideC); 
    return sideA > 0 && sideB > 0 && sideC > 0 &&
           (max < sideA + sideB + sideC - max); 
  }
  boolean isRight() {
    int max = Math.max(Math.max(sideA, sideB), sideC);     
    return isTriangle() &&
           (max * max) == (sideA * sideA) + 
                          (sideB * sideB) + 
                          (sideC * sideC) - 
                          (max * max);
  } 
  boolean isScalene() {
    return isTriangle() &&
           ! isIsosceles(); // we get some peer help 
  }
  boolean isIsosceles() {
    return isTriangle() &&
           (sideA == sideB || 
            sideA == sideC || 
            sideB == sideC); 
  }   
  boolean isEquilateral() {
    return isTriangle() && 
           (sideA == sideB && sideA == sideC); 
  }
  public static void main(String[] args) {
    Triangle a, b, c; 
    a = new Triangle( 3,  4,  5); 
    b = new Triangle( 0,  0, -1); 
    c = new Triangle( 6,  6,  6); 
    System.out.println(
      a + " isosceles? Answer: " + a.isIsosceles()
    );     
    System.out.println(
      a + " right? Answer: "     + a.isRight()
    );     
    System.out.println(
      c + " isosceles? Answer: " + c.isIsosceles()
    );     
    System.out.println(
      c + " scalene? Answer: "   + c.isScalene()
    );     
  } 
  String toString() {
    System.out.println(
      "Triangle: (" + sideA + ", " 
                    + sideB + ", " 
                    + sideC + 
                ") ";  
    ); 
  } 
}

9. For this problem what's listed below is just to get started.

This problem has several parts:

1. Write a simple Vehicle class that has fields for (at least) current speed, current direction in degrees, and owner name.

   class Vehicle {
     int currentSpeed;
     int currentDirection; 
     String owner; 
   }

2. Add a static field to your Vehicle class for the highest Vehicle Identification Number issued, and a non-static field that holds each vehicle's ID number.

   class Vehicle {
     int currentSpeed;
     int currentDirection; 
     String owner; 
     static int highestVIN; 
     int VIN;
   }

3. Write a main method for your Vehicle class that creates a few vehicles and prints out their field values. Note that you can also write a separate tester program as well.

   class Vehicle {
   
     int currentSpeed;
     int currentDirection; 
     String owner; 
     static int highestVIN; 
     int VIN;
   
     public static void main(String[] args) {
   
       Vehicle a = new Vehicle(); 
       a.VIN = highestVIN++; 
       a.owner = "Larry Bird"; 
   
       Vehicle b = new Vehicle();
       b.VIN = highestVIN++; 
       b.owner = "Mark Jackson";     
   
       System.out.println("VIN: " + a.VIN + " belongs to " + a.owner); 
   
       System.out.println("VIN: " + b.VIN + " belongs to " + b.owner); 
   
       // Note that both cars are stopped, and facing East. 
   
     } 
   }

4. Add two constructors to Vehicle. A no-arg constructor and one that takes an initial owner's name. Modify the tester program from the previous step and test your design.

   class Vehicle {
     int currentSpeed;
     int currentDirection; 
     String owner; 
     static int highestVIN; 
     int VIN;
   
     Vehicle() { VIN = highestVIN++; }
     Vehicle (String owner) { this(); this.owner = owner; } 
   
     public static void main(String[] args) {
   
       Vehicle a = new Vehicle("Larry Bird"); 
       Vehicle b = new Vehicle("Mark Jackson"); 
   
       System.out.println("VIN: " + a.VIN + " belongs to " + a.owner); 
       System.out.println("VIN: " + b.VIN + " belongs to " + b.owner); 
     }
   }

5. Make the fields in your Vehicle class private, and add accessor methods for the fields. Which fields should have methods to change them and which should not?

   class Vehicle {
     private int currentSpeed;
     int getSpeed() {
       return currentSpeed; 
     }
     void changeSpeed(int newSpeed) {
       currentSpeed = newSpeed; 
     }
     private int currentDirection; 
     int getCurrentDirection() {
       return currentDirection; 
     }
     void setDirection(int newDirection) {
       currentDirection = newDirection % 360; 
     }
     private String owner; 
     int getOwner() {
       return owner; 
     }
     void setOwner(String newOwner) {
       owner = newOwner; 
     }  
     private static int highestVIN; 
     private int VIN;
     int getVIN() {
       return VIN; 
     } 
     Vehicle() { VIN = highestVIN++; }
     Vehicle (String owner) { this(); this.owner = owner; }
     public static void main(String[] args) {
       Vehicle a = new Vehicle("Larry Bird"); 
       Vehicle b = new Vehicle("Mark Jackson");
       System.out.println("VIN: " + a.getVIN() + " belongs to " + a.getOwner()); 
       System.out.println("VIN: " + b.getVIN() + " belongs to " + b.getOwner()); 
     }
   }

6. Add a changeSpeed method that changes the current speed of the vehicle to a passed-in value, and a stop method that sets the speed to zero.

   class Vehicle {
     private int currentSpeed;
     int getSpeed() { return currentSpeed; }
     void changeSpeed(int newSpeed) { currentSpeed = newSpeed; }
     void stop() { currentSpeed = 0; }
     private int currentDirection; 
     int getCurrentDirection() { return currentDirection; }
     void setDirection(int newDirection) { currentDirection = newDirection % 360; }  
     private String owner; 
     int getOwner() { return owner; }
     void setOwner(String newOwner) { owner = newOwner; }
     private static int highestVIN; 
     private int VIN;
     Vehicle() { VIN = highestVIN++; }
     Vehicle (String owner) { this(); this.owner = owner; }
     public static void main(String[] args) {
       // ... 
     }
   }

7. Add two turn methods to Vehicle. One that takes a number of degrees to turn, and one that takes simply either a Vehicle.TURN_LEFT or a Vehicle.TURN_RIGHT constant. Define the two constants accordingly.

   class Vehicle {
     private int currentSpeed;
     int getSpeed() { return currentSpeed; }
     void changeSpeed(int newSpeed) { currentSpeed = newSpeed; }
     void stop() { currentSpeed = 0; }
     private int currentDirection; 
     int getCurrentDirection() { return currentDirection; }
     void setDirection(int newDirection) { currentDirection = newDirection % 360; }
     static final String TURN_LEFT = "Turn left"; 
     static final String TURN_LEFT = "Turn left"; 
     void turn(int degrees) {
       currentDirection = (currentDirection + degrees) % 360; 
     } 
     void turn(String direction) {
       if (direction.equals(left)) {
         currentDirection = (currentDirection + 90) % 360; 
       } else { //default
         currentDirection = (currentDirection - 90) % 360; 
       } 
     } 
     private String owner; 
     int getOwner() { return owner; }
     void setOwner(String newOwner) { owner = newOwner; }
     private static int highestVIN; 
     private int VIN;
     Vehicle() { VIN = highestVIN++; }
     Vehicle (String owner) { this(); this.owner = owner; }
     public static void main(String[] args) {
       // ... 
     }
   }

8. Add a static method to Vehicle that returns the highest identification number used so far.

   class Vehicle {
     private int currentSpeed;
     int getSpeed() { return currentSpeed; }
     void changeSpeed(int newSpeed) { currentSpeed = newSpeed; }
     void stop() { currentSpeed = 0; }
     private int currentDirection; 
     int getCurrentDirection() { return currentDirection; }
     void setDirection(int newDirection) { currentDirection = newDirection % 360; }
     static final String TURN_LEFT = "Turn left"; 
     static final String TURN_LEFT = "Turn left"; 
     void turn(int degrees) { currentDirection = (currentDirection + degrees) % 360; } 
     void turn(String direction) {
       if (direction.equals(left)) { currentDirection = (currentDirection + 90) % 360; } 
       else { currentDirection = (currentDirection - 90) % 360; } 
     }
     private String owner; 
     int getOwner() { return owner; }
     void setOwner(String newOwner) { owner = newOwner; }
     private static int highestVIN; 
     static int highestVIN() {
       return highestVIN;
     } 
     private int VIN;
     Vehicle() { VIN = highestVIN++; }
     Vehicle (String owner) { this(); this.owner = owner; }
     public static void main(String[] args) {
       // ... 
     }
   }

9. Add a toString method to Vehicle.

   class Vehicle {
     String toString() {
       return "The radar reports: VIN " +  VIN + 
              " owned by " + owner + 
              " heading " + currentDirection + 
              " (degrees) " +
              " at a speed of " + speed); 
     }
     private int currentSpeed;
     int getSpeed() { return currentSpeed; }
     void changeSpeed(int newSpeed) { currentSpeed = newSpeed; }
     void stop() { currentSpeed = 0; }
     private int currentDirection; 
     int getCurrentDirection() { return currentDirection; }
     void setDirection(int newDirection) { currentDirection = newDirection % 360; }
     static final String TURN_LEFT = "Turn left"; 
     static final String TURN_LEFT = "Turn left"; 
     void turn(int degrees) { currentDirection = (currentDirection + degrees) % 360; } 
     void turn(String direction) {
       if (direction.equals(left)) { currentDirection = (currentDirection + 90) % 360; }
       else { currentDirection = (currentDirection - 90) % 360; } 
     }
     private String owner; 
     int getOwner() { return owner; }
     void setOwner(String newOwner) { owner = newOwner; }
     private static int highestVIN; 
     static int highestVIN() { return highestVIN; } 
     private int VIN;
     Vehicle() { VIN = highestVIN++; }
     Vehicle (String owner) { this(); this.owner = owner; }
     public static void main(String[] args) {
       // ... 
     }
   }

10. Define a class of Fractions. I leave this out, as part of it was presented in the lecture. This problem used to have an extra-part (that I mention below).

For the ambitious:

Add a private method lcm to the class Fraction that takes two ints and returns their least common multiple. (For example, lcm(15, 12) is 60. Hint: use gcd). Modify minus and plus in Fraction to use lcm.

One could prove the following:

lcm(p, q) * gcd(p, q) = p * q

If you have any questions please let us know.