JAVA基础之重载，覆盖/重写，多态

Posted 2020-08-15 代号菜鸟

重载发生在同一个类中

特点是两同一不同，类相同，方法名相同，参数不同。其他项与重载无关。

在调用方法的时候，会根据方法名的不同判断具体调用哪个方法。

创建一个Driver类，类中实现了对driver方法的重载操作。

public class Driver {

    //实现了重载操作
    public void drive(){
        System.out.println("No Parameter");
    }

    public void drive(String driver){
        System.out.println("One Parameter "+driver);
    }
}

测试重载操作

public class Test {

    public static void main(String[] args){

        Driver driver = new Driver();
        driver.drive();
        driver.drive("Jason");
    }

}

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重写/覆盖

重写和覆盖发生在父类和子类之间，特点是“两同两小一大”，即方法名相同，参数相同，子类返回值类型小或相等，子类抛出异常小或相等，子类方法的访问权限大或相等。

违反这一原则的话，编译会报错。

public class Person {
    public int a = 10;
    public void A(){
        System.out.println("Person A");
    }
}

public class Teacher extends Person{
    public int a = 20;
    public void A(){
        System.out.println("Teacher A");
    }
}

public class Test {

    public static void main(String[] args){
        Person person = new Person();
        Teacher teacher = new Teacher();

        System.out.println("Person A "+person.a);
        System.out.println("Teacher A "+teacher.a);
        person.A();
        teacher.A();
    }

}

Person中的变量a和方法A都被其子类覆盖

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多态

产生于基类和子类之间

有编译类型和运行类型之分

只有方法有多态，属性没有多态

public class Person {
    public int a = 10;
    public void A(){
        System.out.println("Person A");
    }
}

public class Teacher extends Person{
    public int a = 20;
    public void A(){
        System.out.println("Teacher A");
    }
}

public class Test {

    public static void main(String[] args){
        Person person = new Person();
        Teacher teacher = new Teacher();
        Person p = new Teacher();

        System.out.println("Person A "+person.a);
        System.out.println("Teacher A "+teacher.a);
        person.A();
        teacher.A();
        p.A();
        System.out.println("Person Teacher A "+p.a);
    }

}

运行结果：

Person A 10
Teacher A 20
Person A
Teacher A
Teacher A
Person Teacher A 10