1. Lesson: Language Basics
  2. Variables
    1. Naming
    2. Primitive Data Types
    3. Default Values
    4. Literals
    5. Using Underscore Characters in Numeric Literals
    6. Arrays
    7. Declaring a Variable to Refer to an Array
    8. Creating, Initializing, and Accessing an Array
    9. Copying Arrays
    10. Summary of Variables
    11. Questions and Exercises: Variables
  3. Expressions, Statements, and Blocks
    1. Expressions
    2. Statements
    3. Blocks
    4. Questions and Exercises: Expressions, Statements, and Blocks
  4. Control Flow Statements
    1. The if-then Statement
    2. The if-then-else Statement
    3. The switch Statement
    4. Using Strings in switch Statements
    5. The while and do-while Statements
    6. The for Statement
    7. The break Statement
    8. The continue Statement
    9. The return Statement
    10. Summary of Control Flow Statements
    11. Questions and Exercises: Control Flow Statements
  5. Lesson: Classes and Objects
    1. Classes
    2. Declaring Classes
    3. Declaring Member Variables
    4. Access Modifiers
    5. Types
    6. Variable Names
    7. Defining Methods
    8. Naming a Method
    9. Overloading Methods
    10. Providing Constructors for Your Classes
    11. Passing Information to a Method or a Constructor
    12. Parameter Types
    13. Arbitrary Number of Arguments
    14. Parameter Names
    15. Passing Primitive Data Type Arguments
    16. Passing Reference Data Type Arguments
    17. Objects
    18. Creating Objects
    19. Declaring a Variable to Refer to an Object
    20. Instantiating a Class
    21. Initializing an Object
    22. Using Objects
    23. Referencing an Object's Fields
    24. Calling an Object's Methods
    25. The Garbage Collector
    26. More on Classes
    27. Returning a Value from a Method
    28. Returning a Class or Interface
    29. Using the this Keyword
    30. Using this with a Field
    31. Using this with a Constructor
    32. Controlling Access to Members of a Class
    33. Understanding Instance and Class Members
    34. Class Variables
    35. Class Methods
    36. Constants
    37. The Bicycle Class
    38. Initializing Fields
    39. Static Initialization Blocks
    40. Initializing Instance Members
    41. Summary of Creating and Using Classes and Objects
    42. Questions and Exercises: Classes
    43. Questions and Exercises: Objects
  6. Nested Classes
    1. Why Use Nested Classes?
    2. Static Nested Classes
    3. Inner Classes
    4. Inner Class Example
    5. Local and Anonymous Inner Classes
    6. Modifiers
    7. Summary of Nested Classes
    8. Questions and Exercises: Nested Classes
  7. Enum Types
    1. Questions and Exercises: Enum Types
  8. Annotations
    1. Documentation
    2. Annotations Used by the Compiler
    3. Annotation Processing
    4. Questions and Exercises: Annotations
  9. Lesson: Interfaces and Inheritance
    1. Interfaces
    2. Interfaces in Java
    3. Interfaces as APIs
    4. Interfaces and Multiple Inheritance
    5. Defining an Interface
    6. The Interface Body
    7. Implementing an Interface
    8. A Sample Interface, Relatable
    9. Implementing the Relatable Interface

9.9.Implementing the Relatable Interface

Here is the Rectangle class that was presented in the Creating Objects section, rewritten to implement Relatable.

public class RectanglePlus implements Relatable {
    public int width = 0;
    public int height = 0;
    public Point origin;

    // four constructors
    public RectanglePlus() {
	origin = new Point(0, 0);
    }
    public RectanglePlus(Point p) {
	origin = p;
    }
    public RectanglePlus(int w, int h) {
	origin = new Point(0, 0);
	width = w;
	height = h;
    }
    public RectanglePlus(Point p, int w, int h) {
	origin = p;
	width = w;
	height = h;
    }

    // a method for moving the rectangle
    public void move(int x, int y) {
	origin.x = x;
	origin.y = y;
    }

    // a method for computing the area of the rectangle
    public int getArea() {
	return width * height;
    }
    
    // a method required to implement the Relatable interface
    public int isLargerThan(Relatable other) {
    	RectanglePlus otherRect = (RectanglePlus)other;
    	if (this.getArea() < otherRect.getArea()) return -1; else if (this.getArea() > otherRect.getArea())
    		return 1;
    	else
    		return 0;    		
    }
}

Because RectanglePlus implements Relatable, the size of any two RectanglePlus objects can be compared.
Note: The isLargerThan method, as defined in the Relatable interface, takes an object of type Relatable. The line of code, shown in bold in the previous example, casts other to a RectanglePlus instance. Type casting tells the compiler what the object really is. Invoking getArea directly on the other instance (other.getArea()) would fail to compile because the compiler does not understand that other is actually an instance of RectanglePlus.

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