Questions

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• Quiz?
• Homework?

Concept - The Object class

• Every class extends Object, either directly or indirectly
• Object defines some general methods
  • toString
  • equals
  • hashCode (covered in chapter 15)
  • clone (covered in chapter 10)
• Classes you write should override these in order to use them

Concept - toString

• Every class extends Object class
• Every class inherits the toString method defined in Object
• Object's toString method is not helpful, as is
  • It prints the class name and something like @6f92322e
• You need to override it in your classes if you want to use it
• Have it return a String composed of the class name and the values of instance variables
• Sample format (For a Student class with instance variables name and gpa)

      getClass().getName() + "[name=" + name  + ",gpa=" + gpa + "]"
  

• Separate the variables with commas. Do not use any spaces

Activity - toString

Record your participation in Piazza clicker question Lesson17 Q1

Download overriding_project.zip. Modify the Product class to implement toString. You can use the example from the previous slide as a model. Run the tester. Verify your output looks like the Expected.

1. Got it
2. Got it, but it was hard
3. Could not do it in the time allowed
4. Did not try

Concept - toString in a subclass

In the substring method of a subclass

• Do not repeat the code to get the instance variables of the parent
• Call super.substring
• Then add the additonal instance variables in brackets
• Sample format (For a CollegeStudent class that includes an instance variable major)

  return super.toString() + "[major=" + major + "]";

• Notice you will have two sets of braces

Activity - toString in a subclass

Record your participation in Piazza clicker question Lesson17 Q2

Now modify Painting to implement toString. Call super.toString() and then add the artist instance variable. You can use the example from the previous slide as a model. Uncomment the next two statements and run the tester again. Verify your output looks like the Expected.

1. Got it
2. Got it, but it was hard
3. Could not do it in the time allowed
4. Did not try

Activity - Printing toString

Record your participation in Piazza clicker question Lesson17 Q3

What happens when you execute this statement? Try it and see.

System.out.println(System.out);

Why?

Activity - Reference of type Object

Record your participation in Piazza clicker question Lesson17 Q4

What will happen when you try to compile and run this code

Object college = "SJSU";
System.out.println(college.length());

1. It compiles and runs and prints 4
2. It gets a syntax error in the first statement
3. It gets a systax error in the second statement
4. It compiles but throws an exception at run time

Concept - The equals method

• Every class inherits the equals method defined in Object
• Object's equals method is not helpful, as is
  • It uses == to determine if the two references are the same
  • If they both point to the same object.
• You need to override it in your classes if you want to use it
• It should determine if two objects have the same content.

Concept - Overriding equals

• Object's equal method has this method header

  public boolean equals(Object otherObject)

• When overriding, you have to use the exact header
• You can not change the parameter type
• If you do, you have overloaded the equals method - not overridden it
• Now you need to check that the contents are the same

Concept - Overriding equals: the problem

If we are providing an equals method for a Student class with a name and gpa,

• We want to know is the gpas and the name's are the same
• But we only have an Object reference even though the underlying object is a Student
  • Object does not have a gpa or a name
• We need to cast otherObject to Student

   Student other = (Student)otherObject;
  

• Then we can test if the instance variables are equal
• Sample equals method for Student class

• public boolean equals(Object otherObject)
  {
     Student other = (Student)otherObject;
     return (this.name.equals(other.name))
        && (this.gpa == other.gpa);
  }
  

Activity - An equals method for Product

Record your participation in Piazza clicker question Lesson17 Q5

• Override the equals method for Product. You can use the example from the previous slide as a model.
• Uncommemt "Testing equals" and run it

1. Got it
2. Got it, but it was hard
3. Could not do it in the time allowed
4. Did not try

Activity - Problems with this equals

Record your participation in Piazza clicker question Lesson17 Q6

• There are some problems with this implementation of the equals method
• Uncomment the code for "equals problem1" in the tester
  • Is there a syntax error?
  • What happens when you execute?
  • equals should have returned false in this case
• Comment it out again for right now and uncomment "equals problem2"
  • Is there a syntax error?
  • What happens when you execute?
  • equals should have returned false in this case. The two objects are not equal
• Uncomment the code for "equals problem3"
  • the definition of equals says the method should return false if otherObject is null

Activity - Solution to the problems

Record your participation in Piazza clicker question Lesson17 Q7

• Add this code to the equals before the cast

  if (otherObject == null) {return false;} //solves problem 3
  if (this.getClass() != otherObject.getClass()) {return false;}//solves problems 1 and 2
    

• Uncomment equals problem, 2, and 3
• Run the tester again
• All three problems should be solved

Concept - Implementing equals in a subclass

• Do not repeat the work done in the super class equals method
• You should call super.equals(otherObject) to test if the superclass instance variables are equal
• if it returns false, the objects are not equal and you should also return false
• Now cast to get a reference to a subclass variable
• Then test to see if the new instance variables are the same
• Sample using Student with a subclass of CollegeStudent which has an instance variable major

   public boolean equals (Object otherObject)
   {
       if (!super.equals(otherObject)) 
       {
           return false; //the objects are not equal
       }
       
       CollegeStudent other = (CollegeStudent)otherObject;
       return this.major.equals(other.major);
   }
      

Activity - Implementing equals in a subclass

Record your participation in Piazza clicker question Lesson17 Q8

• Implement the equals method in Painting. You can use the example from the previous slide as a model.
• Uncomment "subclass equals" test cases