[Java Review] 7: Access control

The following notes are based on video series: 手把手教你用Java基础教程 - 北京尚学堂 - 马士兵

I. Access control keywords

keyword	within class	within package	son class	anywhere
private	✅
default	✅	✅
protected	✅	✅	✅
public	✅	✅	✅	✅

• For class, only public and default are used.
• default members can be accessed by a class that belongs to the same package.

II. Use extends to inherit

• A son class inherits all members of its superclass (varibales and methods).
• Java only allows single inheritage. A son can have only one father(superclass).

<modifier> class <name> [extends <superclass>]{...}

III. Overwite (override)

• A method of a son class can override the original method in its superclass.

• To realize, the followings needs to be satisfied:

  • same name.

  • same return type.

  • same parameters list.

  • can not use stricter access control.

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IV. Super

• In a son class, use super.<name> to refer superclass members.

V. Constructive method in inheritage

• Son constructive method must first use superclass constructive method.
• If no super used, the son’s constructive method first executes the superclass’s non-parametrized constructive method, namely super(); If there exists no super(), compile error.
• The alternate is to use super(p1,p2) in the first line to call specific superclass’s constructive method.