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Trail: Learning the Java Language
Lesson: Object Basics and Simple Data Objects

Questions and Exercises: Creating and Using 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 Point (in a .java source file) object and one Rectangle (in a .java source file) object. How many references to those objects exist after the code executes? Is either object eligible for garbage collection?
    ...
    Point point = new Point(2,4);
    Rectangle rectangle = new Rectangle(point, 20, 20);
    point = 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 (in a .java source file), 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. (in the Learning the Java Language trail)

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