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("Operator '+' invokes toString() too: " + c6) // '+' invokes toString() too (c6) // println() calls toString() implicitly, same as above (c5.toString()) // explicit call toString() is called implicitly when an instance is passed to println() method, for example, Try calling toString() method explicitly, just like any other method: ** Return a self-descriptive string of this instance in the form of Circle */ For example, include the following toString() methods to the Circle class: If an instance is passed to the println( anInstance) method, the toString() method of that instance will be invoked implicitly. The toString() method can be called explicitly (via instanceName.toString()) just like any other method or implicitly through println(). Method toString(): Every well-designed Java class should contain a public method called toString() that returns a description of the instance (in the return type of String).Modify ALL the constructors and setters in the Circle class to use the keyword " this". "radius" refers to the method's argument

** Sets the radius to the given value */ "radius" refers to the method's parameter This.radius = radius // "this.radius" refers to the instance variable ** Constructs a Circle instance with the given radius and default color */

Keyword " this": Instead of using variable names such as r (for radius) and c (for color) in the methods' arguments, it is better to use variable names radius (for radius) and color (for color) and use the special keyword " this" to resolve the conflict between instance variables and methods' arguments.

("color is: " + c4.getColor()) // Print color via getter // You cannot do the following because setRadius() returns void, which cannot be printed ("radius is: " + c4.getRadius()) // Print radius via getter Modify the TestCircle to test these methods, e.g.,Ĭircle c4 = new Circle() // construct an instance of Circle * The Circle class models a circle with a radius and color. The source codes for Circle.java is as follows: /**

Two public methods: getRadius() and getArea(), which return the radius and area of this instance, respectively.

Two overloaded constructors - a default constructor with no argument, and a constructor which takes a double argument for radius.

Two private instance variables: radius (of the type double) and color (of the type String), with default value of 1.0 and " red", respectively.

This first exercise shall lead you through all the basic concepts in OOP.Ī class called circle is designed as shown in the following class diagram.

Exercises on Classes An Introduction to Classes and Instances by Example - The Circle Class