Questions and Exercises: Creating and Using Objects - Java Tutorial 5.0 英文版

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

Questions and Exercises: Creating and Using Objects

Questions

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());
    }
}
The following code creates one Point object and one Rectangle 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;
...
How does a program destroy an object that it creates?

Exercises

Fix the program called SomethingIsWrong shown in Question 1.
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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