Variable scope

• Local variables:
  • block scope in Java
• Instance variables:
  • everywhere!
  • but outside the class definition you must prefix with an object reference
  • E.g. this is an error – think about why

    public class DogTester {
    		                    
    	public static void main(String[] args) {
    	    Dog d1 = new Dog();
    	    Dog d2 = new Dog();
    	    
    	    name = "Charles";
    	}
    }

  • But this is OK:

    public class DogTester {
    		                    
    	public static void main(String[] args) {
    	    Dog d1 = new Dog();
    	    Dog d2 = new Dog();
    	    
    	    d1.name = "Charles";
    	}
    }

Class exercise

What will d1's instance variables contain? And d2's?

class Dog {

    String name;
    int age;
    String breed = "Mongrel";

    public void setName(String s) {
        name = s;
    }

    // etc.

}

 

class DogTester {

    public static void main(String[] args) {
        Dog d1 = new Dog();
        Dog d2 = new Dog();
        d2.setBreed("Colombian Drug Lord");
        // etc.
    }

}

Constructors: why we want them

• So far, we have to create an object and then use setters to set its state to the values that we want, e.g.:

  public static void main(String[] args) {
      Dog d1 = new Dog();
      d1.setName("Charles");
      d1.setAge(2);
      d1.setBreed("Charlywawa");
      // etc.
  }

• More convenient would be to supply the values when we create the object, e.g.:

  public static void main(String[] args) {
      Dog d1 = new Dog("Charles", 2, "Charlywawa");
      // etc.
  }

Constructors: how to write them

• Constructors are used to re-initialize an object's instance variables when it is being created
• In your class definition, their signature looks similar to the signature of an instance method, but
  • constructors have the same name as the class
  • they have no return type

  public Dog(String s1, int n, String s2) {
      name = s1;
      age = n;
      breed = s2;
  }

• In effect, they are executed whenever you use new
• Now that we have a constructor that takes in 3 pieces of data, we can no longer do this:

  Dog d1 = new Dog();

Class exercise

What will d1's instance variables contain? And d2's?

class Dog {

    String name;
    int age;
    String breed;
    
    public Dog(String s1, int n, String s2) {
        name = s1;
        age = n;
        breed= s2;
    }

    public void setName(String s) {
        name = s;
    }

    // etc.

}

class DogTester {

    public static void main(String[] args) {
        Dog d1 = new Dog("Charles", 2, "Charlywawa");
        Dog d2 = new Dog("Colin", 7, "Colombian Drug Lord");
        d1.setName("Charlie");
        // etc.
    }

}

Class exercise

Define a BankAccount class

• First, work out its interface from the main method below
• Then, write the class definition itself

class BankAccountTester {

    public static void main(String[] args) {
        BankAccount ac1 = new BankAccount("Sir Hugh Jeegoh");
        BankAccount ac2 = new BankAccount("UCC");
        
        ac2.deposit(1000);
        System.out.println(ac2.getBalance());
        
        ac1.withdraw(50);
        System.out.println(ac1.getNumTransactions());
        
        System.out.println(ac1);
    }
}

public BankAccount(String s)
            
public void deposit(int amount)

public void withdraw(int amount)

public int getBalance()

public int getNumTransactions()

public String toString()

class BankAccount {

    String name;
    int balance;
    int numTransactions;
    
    public BankAccount(String s) {
        name = s;
    }
    
    public void deposit(int amount) {
        balance = balance + amount;
        numTransactions = numTransactions + 1;
    }
    
    public void withdraw(int amount) {
        balance = balance - amount;
        numTransactions = numTransactions + 1;
    }
    
    public int getBalance() {
        return balance;
    }
    
    public int getNumTransactions() {
        return numTransactions;
    }
    
    public String toString() {
        return "Account name: " + name + " Balance: " + balance + " Number of transcations: " + numTransactions;
    }

}