To Notes

CSC 212 -- 1/22/08

Review Questions

1. True or False: An instance variable cannot be defined inside of any method.

   Ans: False. Instance variables are never defined inside of a method. Only local variables are defined inside of methods.

2. Why are instance variables usually defined to be private?

   Ans: Because you don't want just anyone modifying instance variables. You want to inforce a more controlled access through the public getters and setters.

3. What are getters and setters?

   Ans: Methods for getting the value of an instance variable of for setting or changing the value of an instance variable.

4. How do you make instance variables read only?

   Ans: Only supply a getter method for that instance variable.

5. Write a line of code that (1) declares the instance variable p of type Person and (2) instanitates a Person object with instance variable values name="Judy", gender='F', age=19.

   Ans:   Person p = new Person("Judy", 'F', 19);

6. What happens if you invoke the toString method of a class object after forgetting to define the toString method in that class?

   Ans: The toString method of the Object class will be called, which only returns the name of the class and its address.

7. What is the difference between
   String s = "dog"
   String t = "dog"

   and
   String s = new String("dog");
   String t = new String("dog");

   Ans: When the new operator is not used, both s and t contain the address of the same string, which contains "dog". When the new operator is not used, two different objects are created, each containing the string "dog".

8. What is the difference between arguments and parameters for methods?

   Ans: Arguments are what you pass in to a method when you call it. Parameters are used in the definition of a method to tell the method what to do with the passed in arguments.

Types of Constructors

• A parameterized constructor accepts input values often to be used for initializing instance variables.

• Example:
  Person p = new Person("Alice", 'F', 23);

• A noarg constructor is a constructor that does not accept input arguments.

• Example:
  Person p = new Person( );

  makes these initializations in the Person3 Example:
  name="Unknown", gender='U', age=-1.

The Person3 Example

• Look at the Person3 Example: Person.java   TestPerson3.java

The Current Object

• A current object is an object whose code is currently executing.

• Example: If the Person constructor is currently executing for the statement
  Person p = new Person("Alice", 'F', 23);

  then p is the current object.

• However, the code for the Person constructor does not know that p contains the reference to the current object. We need a way for the current object to refer to itself without knowing how its constructor was called.

• Solution: use the reference variable this anywhere in the class definition to refer to the current object.

• Example:
  public Person( )
  {
      name = "Unknown";
      gender = 'F';
      age = 23;
      System.out.println(this);
  }

• this is useful for defining the equals and compareTo methods in the Person3 Example.

Project Proj3

• Look at the Project Proj3 Description.

The Coin Class

• Design a Coin class according to this UML diagram.
  1. Write Java code to define your Coin class.

  2. Write a TestCoin class to test Coin.