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Overview of polymorphism

The concept of polymorphism: the so-called polymorphism means that the specific type pointed to by the reference variable defined in the program and the method calls made through the reference variable are not determined during programming, but are determined during the run of the program. that is, a reference variable will point to the instance object of which class, and the method call issued by the reference variable must be decided during the run of the program. Because the specific class is determined when the program is running, the reference variable can be bound to a variety of different class implementations without modifying the source code, which leads to a change in the specific method called by the reference variable. that is, without modifying the program code, you can change the specific code bound by the program, so that the program can choose multiple running states, which is polymorphism.

Three necessary conditions for the existence of Polymorphism

1. There must be an inheritance relationship (the implementation interface is also an inheritance relationship)

two。 There should be a method of rewriting.

3. There should be a parent class reference pointing to a subclass object

Note:

1. The running method depends on the running type, and the attribute depends on the compiled type.

two。 Call the property in the method, and no special case is the property of this class

3. If there is no method called in the type, follow the upward query

1. Upward transformation

1. The upward transformation of an object is actually polymorphic writing:

Format: parent class name object name = new subclass name ()

Meaning: a subclass object is created on the right and treated as a parent class.

Person person=new student ()

Create a student object and treat it as a human being, no problem.

At this time, the compilation type is person and the run type is student.

Note: the upward transition must be safe. From a small range to a large range, from a small range of cats to a larger range of animals.

two。 Downward transformation

two。 The downward transformation of the object is actually a restore action:

Format: subclass name object name = (subclass name) parent class object

Meaning: restore the subclass to the original subclass object

Cat cat = (Cat) animal;// was originally a cat, but it was transformed upwards into an animal, and then restored to become the original cat note:

a. You must ensure that the object is created as a cat before it can be transformed into a cat.

b. If the object is not a cat when it is created, and now it has to be transformed into a cat, it will report an error.

Polymorphic array

Create a polyArr package

References to Arrtest.javapackage com.polyArr;public class Arrtest {public static void main (String [] args) {/ / parent classes can point to objects of subclasses Person [] p=new Person [5]; p [0] = new Person ("aaa", 15); p [1] = new Student ("asas", 16,55); p [2] = new Student ("ccc", 17,66) P [3] = new Teacher ("nnn", 18, 2000); p [4] = new Teacher ("mmm", 19, 6666); for (int iBond I < p.compiling I +) {/ / dynamic binding mechanism / / person [I] compilation type is Person, run type is System.out.println (p[ I] .say ()) } Person.javapackage com.polyArr;public class Person {private String name; private int age; public Person (String name, int age) {this.name = name; this.age = age;} public String getName () {return name;} public void setName (String name) {this.name = name;} public int getAge () {return age } public void setAge (int age) {this.age = age;} public String say () {return "name=" + name+ "age=" + age;}} Student.javapackage com.polyArr;public class Student extends Person {private double score; public Student (String name, int age, double score) {super (name, age); this.score = score;} public double getScore () {return score } public void setScore (double score) {this.score = score;} public String say () {return super.say () + "score=" + score;}} Teacher.javapackage com.polyArr;public class Teacher extends Person {private int salary; public Teacher (String name, int age, int salary) {super (name, age); this.salary = salary;} public int getSalary () {return salary } public void setSalary (int salary) {this.salary = salary;} public String say () {return super.say () + "salary=" + salary;}}

Running result

Polymorphic array + downward transition instanceof keyword

Operator is used at run time to indicate whether the object is an instance of a particular class. Instanceof returns a Boolean value to indicate whether the object is an instance of this particular class or its subclass.

Usage: result = object instanceof class

Result is of type boolean

Parameters:

Result: Boolean type.

Object: required. Any object expression.

Class: required. Any defined object class.

Description:

If object is an instance of class, the instanceof operator returns true. If object

Is not an instance of the specified class, or object is null, then false is returned.

But there is a difference between the compiled state and the running state of instanceof in Java.

Note:

In the compiled state, class can be a parent class, its own class, or a subclass of an object object. In these three cases, Java compiles without error.

In the running transition state, class can be the parent class of the object object, its own class, and not a subclass. In the first two cases, the result of result is true, and the last one is false. However, compiling does not report an error when class is a subclass. The running result is false.

Create a polyArr package

References to Arrtest.javapackage com.polyArr;public class Arrtest {public static void main (String [] args) {/ / parent classes can point to objects of subclasses Person [] p=new Person [5]; p [0] = new Person ("aaa", 15); p [1] = new Student ("asas", 16,55); p [2] = new Student ("ccc", 17,66) P [3] = new Teacher ("nnn", 18, 2000); p [4] = new Teacher ("mmm", 19, 6666); for (int iBond I < p.compiling I +) {/ / dynamic binding mechanism / / person [I] compilation type is Person, run type is System.out.println (p[ I] .say ()) If (p [I] instanceof Student) {Student T1 = (Student) p [I]; System.out.println (t1.learn ());} else if (p [I] instanceof Teacher) {Teacher T1 = (Teacher) p [I]; System.out.println (t1.teach ()) } Person.javapackage com.polyArr;public class Person {private String name; private int age; public Person (String name, int age) {this.name = name; this.age = age;} public String getName () {return name;} public void setName (String name) {this.name = name } public int getAge () {return age;} public void setAge (int age) {this.age = age;} public String say () {return "name=" + name+ "age=" + age;}} Student.javapackage com.polyArr;public class Student extends Person {private double score; public Student (String name, int age, double score) {super (name, age); this.score = score } public double getScore () {return score;} public void setScore (double score) {this.score = score;} public String say () {return super.say () + "score=" + score;} public String learn () {return getName () + "listening";}} Teacher.javapackage com.polyArr;public class Teacher extends Person {private int salary Public Teacher (String name, int age, int salary) {super (name, age); this.salary = salary;} public int getSalary () {return salary;} public void setSalary (int salary) {this.salary = salary;} public String say () {return super.say () + "salary=" + salary } public String teach () {return getName () + "in class";}}

Running result

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