java 的动态绑定机制
看一个例子：
DynamicBinding.java：
package com.hspedu.poly_.dynamic_;public class DynamicBinding {public static void main(String[] args) {// a 的编译类型是 A, 运行类型是 BA a = new B();//向上转型System.out.println(a.sum());//40System.out.println(a.sum1());//30}
}
class A {public int i= 10;public int sum() {return getl()+ 10;}public int sum1(){return i + 10;}public int getl() {return i;}
}class B extends A {public int i = 20;public int sum() {return i + 20;}public int getl() {return i;}public int sum1() {return i + 10;}
}
- 当调用对象方法的时候，该方法会和该对象的内存地址 / 运行类型绑定。
- 当调用对象属性时，没有动态绑定机制，哪里声明，哪里使用。
例子：
package com.hspedu.poly_.dynamic_;public class DynamicBinding {public static void main(String[] args) {// a 的编译类型是 A, 运行类型是 BA a = new B();//向上转型System.out.println(a.sum());//30System.out.println(a.sum1());//20}
}
class A {public int i= 10;//动态绑定机制：public int sum() {return getI()+ 10;}public int sum1(){return i + 10;}public int getI() {return i;}
}class B extends A {public int i = 20;public int getI() {return i;}
}
多态的应用
多态数组
数组的定义类型为父类类型，里面保存的实际元素类型为子类类型。
应用实例： 现有一个继承结构如下:要求创建 1 个 Person 对象、2个 Student 对象和 2 个 Teacher 对象，统一放在数组中，并调用每个对象 say 方法。
PolyArray.java：
package com.hspedu.poly_.polyarr_;public class PolyArray {public static void main(String[] args) {Person[] persons = new Person[5];persons[0] = new Person("Jack", 20);persons[1] = new Student("Jack", 18, 100);persons[2] = new Student("Smith", 19, 88);persons[3] = new Teacher("Scott", 30, 30000);persons[4] = new Teacher("Scott", 25, 25000);for (int i = 0; i < persons.length; i++) {//person[i]编译类型是 Person,运行类型是是根据实际情况有JVM来判断System.out.println(persons[i].say());//动态绑定机制}}
}
Person：
package com.hspedu.poly_.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(){// 返回名字和年龄return name + "\t" + age;}}
Student.java：
package com.hspedu.poly_.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;}//重写父类say@Overridepublic String say() {return "学生 " + super.say() + " score=" + score;}
}
Teacher.java：
package com.hspedu.poly_.polyarr_;public class Teacher extends Person {private double salary;public Teacher(String name, int age, double salary) {super(name, age);this.salary = salary;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}//重写父类的say方法@Overridepublic String say() {return "老师 " + super.say() + " salary=" + salary;}
}
应用实例升级：
如何调用子类特有的方法，比如 Teacher 有一个 teach，Student 有一个 study 怎么调用？
PolyArray.java：
package com.hspedu.poly_.polyarr_;public class PolyArray {public static void main(String[] args) {Person[] persons = new Person[5];persons[0] = new Person("Tom", 20);persons[1] = new Student("Jack", 18, 100);persons[2] = new Student("Smith", 19, 88);persons[3] = new Teacher("Scott", 30, 30000);persons[4] = new Teacher("King", 25, 25000);for (int i = 0; i < persons.length; i++) {//person[i]编译类型是 Person,运行类型是是根据实际情况有JVM来判断System.out.println(persons[i].say());//动态绑定机制//可以向下转型if(persons[i] instanceof Student) {Student student = (Student)persons[i];student.study();// 或者 ((Student)person[i]).study();}else if(persons[i] instanceof Teacher){Teacher teacher = (Teacher)persons[i];teacher.teach();}else if(persons[i] instanceof Person){}else{System.out.println("你的类型有误，请自己检查...");}}}
}
Person：
package com.hspedu.poly_.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(){// 返回名字和年龄return name + "\t" + age;}}
Teacher.java：
package com.hspedu.poly_.polyarr_;public class Teacher extends Person {private double salary;public Teacher(String name, int age, double salary) {super(name, age);this.salary = salary;}public double getSalary() {return salary;}public void setSalary(double salary) {this.salary = salary;}//重写父类的say方法@Overridepublic String say() {return "老师 " + super.say() + " salary=" + salary;}//特有方法public void teach(){System.out.println("老师 " + getName() +" 正在讲 java 课程...");}
}
Student.java：
package com.hspedu.poly_.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;}//重写父类say@Overridepublic String say() {return "学生 " + super.say() + " score=" + score;}public void study(){System.out.println("学生 " + getName() + " 正在学习java...");}
}
)
