polymorphic
1. Overview of polymorphism
1.1 What is polymorphism
Objects of the same type perform the same behavior and exhibit different behavioral characteristics;
1.2 Common forms of polymorphism
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Parent type object name= new subclass constructor;
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Interface object name= new implementation class constructor;
Member Access Characteristics in 1.3 Polymorphism
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Method calls: compile to the left and run to the right (polymorphism focuses on behavior polymorphism)
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Compile to see if there is a corresponding method in the parent class
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Run, execute subclass method results
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Variable calls: compile to the left, run to the left;
1.4 Polymorphic premise
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Has inheritance/implementation relationship;
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There are parent references to subclass objects;
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Method Rewrite
public abstract class Animal { public String name = "Animal Prototype"; public abstract void run(); } public class Dog extends Animal{ public String name = "Dog prototype"; @Override public void run() { System.out.println("Dog is run"); } } public class Test { public static void main(String[] args) { Animal dog = new Dog(); //Compile left, run right //Dog is run dog.run(); System.out.println(dog.name); Animal cat = new Cat(); cat.run(); //Animal Prototype, Variable with Parent System.out.println(cat.name); } }
2. Polymorphic Advantages and Problems
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In polymorphic form, the right object is decoupled to facilitate extended maintenance.
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Subclass objects on the right can be swapped without affecting other code
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Definition method, using parent object, strong compatibility, receiving all subclass objects, reflecting polymorphic extensibility;
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There are problems:
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Give priority to parent, use unique method of subclass, error
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public class Test { public static void main(String[] args) { Animal dog = new Dog(); go(dog); Cat cat = new Cat(); go(cat); //dog and cat work equally well in go() } /** * Require that all animals be available * @param animal */ public static void go(Animal animal){ System.out.println("Start Up"); animal.run(); System.out.println("Time Out"); } }
3. Data conversion of reference data types under polymorphism
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Automatic type conversion (from child to parent, small to large):
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Variable conversion whose subclass object is assigned to the parent type;
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Force type conversion (from parent to child):
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Subclass object variable = (subclass) parent type variable;
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Function: Solve the disadvantage of polymorphism, and realize the function of calling subclasses
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//Automatic Type Conversion Animal a = new Dog(); a.run(); //Mandatory Type Conversion Animal b = new Cat(); b.run(); //b Cast from parent type to subtype Cat cat = (Cat) b; //To subtype, you can use a unique method cat.eatFish();
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Post-transition type and object real type are not the same, type exception ClassCastException
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//Force type conversion, compile phase error-free (inheritance or implementation relationships can be enforced), runtime error may occur; Type Exception ClassCastException
//Dog dog = (Dog) b;
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Java recommends using instanceof judgment before casting
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/** * Require that all animals be available * The type of animal passed in is unknown * @param animal */ public static void go(Animal animal){ if (animal instanceof Dog){ Dog dog = (Dog) animal; dog.lookDoor(); }else if (animal instanceof Cat){ Cat cat = (Cat) animal; cat.eatFish(); } }
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4. Polymorphic synthesis
public interface USB { void connect(); void unConnect(); } public class KeyBoard implements USB{ private String name; public KeyBoard(String name) { this.name = name; } @Override public void connect() { System.out.println(name+"Successful Connection"); } /** * Unique function */ public void keyDown(){ System.out.println(name+"Type characters"); } @Override public void unConnect() { System.out.println(name+"Successfully pulled out"); } } public class Mouse implements USB{ Other as above /** * Unique function */ public void dbClick(){ System.out.println(name+"click the icon"); } } public class Computer { Other as above /** * Provide USB device installation interface */ public void installUSB(USB usb){ usb.connect(); //Judging Subclass Type and Implementing Function Strongly if (usb instanceof KeyBoard){ KeyBoard keyBoard = (KeyBoard) usb; keyBoard.keyDown(); }else if (usb instanceof Mouse){ Mouse mouse = (Mouse) usb; mouse.dbClick(); } usb.unConnect(); } } public class Test { public static void main(String[] args) { Computer computer = new Computer("Machinist T58"); computer.start(); KeyBoard keyBoard = new KeyBoard("PHILIPS"); //Insert Keyboard computer.installUSB(keyBoard); Mouse mouse = new Mouse("Logitech"); computer.installUSB(mouse); } }