Crazy Java Learning Notes Object-oriented (vii)-SUPER keyword

Super has the following two major functions:

1. Qualifying : force access to members of the parent class (Field, method)

2. Call function : Specify/Display a constructor that invokes the parent class

Super Call rule:

1, the subclass constructor will always call the parent class constructor once , by default, the class constructor automatically calls the parent class without parameters of the constructor ;

2, if you want to explicitly specify which constructor the subclass constructor calls the parent class , you can use super (parameter, parameter,....), which parent constructor is called depending on the list of parameters passed in after Super ;

3, this () Call and super () call must appear in the first row of the constructor, so they cannot both be present at the time of the call.

Conventional:

1, if there is no super () call, nor the This () call, the default subclass constructor will automatically call the parent class parameterless constructor ;

2, if there is a super () call, according to Super () call the incoming parameter list to decide which parent class constructor to call ;

3. If there is this () call, the subclass constructor finds the constructor that is overloaded in the corresponding subclass of the This () call.

Example 1: Using Super in a method. Limiting function

1 classfruit{2     PrivateString color;3     Private Doubleweight;4     5      PublicFruit () {}6      PublicFruit (String color,Doubleweight) {7          This. color =color;8          This. Weight =weight;9     }Ten  One      Public voidinfo () { ASystem.out.println ("This is a fruit Oh!")); -SYSTEM.OUT.PRINTLN ("Fruit color is:" + color + ", Weight:" +weight); -     } the } -  -  -  Public classAppleextendsfruit{ +      Public voidinfo () { -System.out.println ("This is an apple!")); +     } A  at      Public voidTest () { -System.out.println ("~~apple test Method"); -         //This.info ();-through this. Call info by default always calls the info () method defined in this class -         Super. info ();//-Enforce calls to the info () method in the parent class by Super. -     } -  in      Public Static voidMain (string[] args) { -Apple A =NewApple (); to a.test (); +     } -}

Example 2: Use Super in field. As well as a limiting function

1 classdemo{2     intCount = 2;3 }4 5  Public classTestdemoextendsdemo{6     intCount = 10;7      Public voidTest () {8         //The default count (that is, count does not add this./super.) , the This is omitted by default in front of the9         //If you must use the Count field of the parent class, precede it with super.TenSYSTEM.OUT.PRINTLN ("The test method accesses the count of the current class:" + This. Count); OneSYSTEM.OUT.PRINTLN ("The test method accesses the count of the parent class:" +Super. Count); A     } -  -      Public Static voidMain (string[] args) { theTestdemo T =NewTestdemo (); - t.test (); -     } -}

Example 3: Calling the parent class constructor through super

1 classperson{2     protectedString name;3     protected intAge ;4     protectedString sex;5 6     //The programmer defines a parametric constructor, so the system no longer generates a parameterless constructor for that class7      PublicPerson (String name,intAge , String Sex) {8          This. Name =name;9          This. Age =Age ;Ten          This. Sex =sex; One     } A  -     //If you do not define a method, a call to the Name/age/sex parameter in the main method compiles with an error "the Staic method cannot reference a non-static variable" -      Public voidinfo () { theSystem.out.println ("The teacher's name is called:" + name + ", Age:" + ages + ", Gender:" +sex); -     } - } -  +  Public classTeacherextendsperson{ -     PrivateString sex; +  A     //If you do not specify a constructor, the default constructor for the parent class is called, but the parent class does not have a parameterless constructor to make an error . at      PublicTeacher (String name,intAge , String Sex) { -         Super(name,age,sex); -     } -      -      -      Public Static voidMain (string[] args) { inTeacher TT =NewTeacher ("haha", "male"); - tt.info (); to     } +}

Crazy Java Learning Notes Object-oriented (vii)-SUPER keyword