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Encapsulation and visibility

Hiding an object's state: private, public, protected and package visibility, accessors and mutators, an invariant enforced by the mutator, and an internal reference not to hand out, in Java.

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  • Encapsulation: what is it?

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    The principle of bundling an object's state with the methods that handle it, exposing only those methods to the outside.

    The outside no longer touches the attributes: the class stays free to change its representation without breaking its users.

  • What does the compiler answer to this test program?

    ```java
    class Compte {
    private int solde;
    private void secret() { }
    }

    public class Main {
    public static void main(String[] args) {
    Compte c = new Compte();
    c.solde = 100;
    c.secret();
    }
    }
    `

    • Two errors has private access in Compte
    • None, Main and Compte are in the same file
    • A single error, on c.solde
    • Two warnings, the program runs
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    Two errors has private access in Compte

    private restricts access to the class itself, even if another class sits in the same file. Messages: solde has private access in Compte and secret() has private access in Compte.

  • To protect an object's state, its attributes are declared … and the methods offered to the outside ….

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    To protect an object's state, its attributes are declared private and the methods offered to the outside public.

    A private attribute can only be read and changed by the code of its own class.

  • Access modifier: what is it?

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    A keyword placed before a class member, which sets which parts of the program may use it.

    Java offers four levels: private, no keyword (package visibility), protected and public.

  • Why declare attributes private rather than public and add methods?

    • A private attribute takes less memory than a public attribute
    • Only its methods change the state: the class guarantees consistency
    • The compiler refuses to compile a class that has public attributes
    • Methods run faster than direct accesses to attributes
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    Only its methods change the state: the class guarantees consistency

    Visibility affects neither memory nor speed: it decides who may modify the state. That is what makes an invariant possible.

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