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Trail: Learning the Java Language
Lesson: Classes and Objects
Classes and Objects
Classes
Declaring Classes
Declaring Member Variables
Defining Methods
Providing Constructors for Your Classes
Passing Information to a Method or a Constructor
Objects
Creating Objects
Using Objects
More on Classes
Returning a Value from a Method
Using the this Keyword
Controlling Access to Members of a Class
Understanding Instance and Class Members
Initializing Fields
Summary of Creating and Using Classes and Objects
Questions and Exercises
Questions and Exercises
Nested Classes
Inner Class Example
Summary of Nested Classes
Questions and Exercises
Enum Types
Questions and Exercises
Annotations
Questions and Exercises
Home Page > Learning the Java Language > Classes and Objects
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 `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;
>    >     ...

> 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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