Java Notes 5__ construction block, static block/Singleton design pattern/inheritance/final keyword/super keyword

 Public classMain {{//building blocks (called when constructing an object, prior to construction method execution)System.out.println ("I am a building block!") "); }    Static{//static blocks (executed at class load time and executed only once "execute before building block" eg: Database connection)System.out.println ("I am a static block! "); }     PublicMain () {System.out.println ("I am the construction Method!" "); }     Public Static voidMain (string[] args) {main M1=NewMain (); Main m2=NewMain (); }  }

/*** Singleton design mode: guarantees that a class has only one instance and provides a global access point to access it * 1. Construction method Privatization * 2. Declare an object of this class * 3. Provides a static method for externally obtaining an object instance * Two implementations: A hungry man type, lazy type * Application: Tool Class (Example database) */ Public classMain { Public Static voidMain (string[] args) {Singleton1 S1=singleton1.getinstance ();        S1.print (); Singleton1 S2=singleton1.getinstance (); Singleton1 S3=singleton1.getinstance (); System.out.println (S1==S2);//The output TRUE,S1,S2,S3 all point to the same objectSingleton2 T1=singleton2.getinstance ();        T1.print (); Singleton2 T2=singleton2.getinstance (); SYSTEM.OUT.PRINTLN (T1==T2);//output true,t1,t2 point to the same object    }}/*** Design the class into a single case design mode * A hungry man type*/classsingleton1{Private StaticSingleton1 s =NewSingleton1 ();//a hungry man, start with a new object    PrivateSingleton1 () {} Public voidprint () {System.out.println ("A Hungry man single-case design mode! "); }     Public StaticSingleton1 getinstance () {returns; }}/*** Design the class into a single case design pattern * lazy "thread-safe during multi-threaded access"*/classsingleton2{Private StaticSingleton2 s =NULL;//lazy, first not new, call again new    PrivateSingleton2 () {} Public voidprint () {System.out.println ("Lazy single-case design mode!" "); }     Public StaticSingleton2 getinstance () {if(s==NULL) {s=NewSingleton2 (); }        returns; }}

/*** Inheritance * 1.Java can only achieve single inheritance, that is, a class can have only one direct parent class * 2. Inheritance can inherit only non-private properties and methods (Public,protected,default) * 3. Construction methods cannot be inherited*/ Public classMain { Public Static voidMain (string[] args) {Homedog D2=NewHomedog ("Hels");//The constructor method of the parent class executes first (and only the default constructor method of the parent class is "no parameter" unless explicitly called)D2.desc (); }  }classdog{String name;  PublicDog () {System.out.println ("A dog born."); }     PublicDog (String name) {System.out.println ("A Dog:" +name+ "is born."); }     Public voiddesc () {System.out.println ("I ' m a dog."); }}classHomedogextendsdog{ PublicHomedog (String name) {Super(name);//represents a reference to the parent class "must be put in the first sentence"System.out.println ("A Homedog born."); }    /*** Method Override * 1. The two method return values, method names, and parameter lists that occur in the method override must be exactly the same (subclasses override the method of the parent class) * 2. The exception thrown by a subclass cannot exceed the exception thrown by the corresponding method of the parent class * 3. The access level of the subclass method cannot be lower than the parent class Access level of the method (example: Parent protected child public)*/     Public voiddesc () {System.out.println ("I am the way to rewrite it!" "); }    }classWilddogextendsdog{ PublicWilddog () {System.out.println ("A Wilddog born."); }}

/*** Super keyword can be done by: * 1. Call a property in the parent class * 2. Call a method in the parent class * 3. Calling a constructor method in a parent class*/ Public classMain { Public Static voidMain (string[] args) {blackduck D1=NewBlackduck ();    D1.desc (); }  }classduck{protectedString name; protected intPrice = 55;  Public voidcount () {System.out.println ("Duck Price" +Price ); }}classBlackduckextendsduck{ Public voiddesc () {System.out.println ("I ' m a blackduck,my price" +Super. Name); Super. Count (); }}

/*** What the final keyword can do: * 1. Declare a class (The final class, cannot be inherited) * 2. Declares a method (this method cannot be overridden by the quilt Class) * 3. Declares a constant (1. Direct Assignment 2. Assigning values in the constructor method)*/ Public classMain { Public Static voidMain (string[] args) {Girl G1=NewGirl (30);    G1.desc (); }  }Final classgirl{FinalString name = "Fish7"; Final intAge ;  PublicGirl (intAge ) {         This. Age =Age ; }     Public Final voiddesc () {System.out.println ("I ' m" +name+ ", My age is" +Age ); }}

Java Notes 5__ construction block, static block/Singleton design pattern/inheritance/final keyword/super keyword