Java 天地·Core Java·Lecture Notes·Unit 1- Object Concepts & Java

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Content Page # 26

Controlling state change

The state of an object is often carefully controlled by only allowing certain variables of an object to be accessed by the object's own ‘get’ and ‘set’ methods. This prevents some other object changing an object's attribute to some invalid or meaningless value. For example:

as a part of a picture of a snowman a drawing application might have a Circle object, and wish to make the radius of the circle smaller each time some button is clicked. If the application were to simply subtract a value (say 50 pixels) from the value of the object's radius attribute each time the button were clicked, after enough clicks the Circle object would have a negative radius – clearly not something a robust program should allow to happen.

This can be avoided by making the radius attribute only changeable by the object itself, and requiring other objects (e.g. the application object) to request a change of the radius via a set message. A Circle object can then the setRadius() operation defined carefully so that it will never set the radius to less than 0 (or some reasonable value for the system).

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