Trail: Learning the Java Language
Lesson: Classes and Objects
Questions and Exercises
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Questions and Exercises: Objects

Questions

  1. What's wrong with the following program?

    public class SomethingIsWrong {
        public static void main(String[] args) {
            Rectangle myRect;
            myRect.width = 40;
            myRect.height = 50;
            System.out.println("myRect's area is "
                               + myRect.area());
        }
    }
    
  2. The following code creates one array and one string object. How many references to those objects exist after the code executes? Is either object eligible for garbage collection?

    ...
    String[] students = new String[10];
    String studentName = "Peter Parker";
    students[0] = studentName;
    studentName = null;
    ...
    
  3. How does a program destroy an object that it creates?

Exercises

  1. Fix the program called SomethingIsWrong shown in Question 1.

  2. Given the following class, called NumberHolder, write some code that creates an instance of the class, initializes its two member variables, and then displays the value of each member variable.

    public class NumberHolder {
        public int anInt;
        public float aFloat;
    }
    

Check your answers.


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