多态是面向对象的三大特征之一

谈到多态,不得不提的就是,多态中的成员绑定问题.

方法绑定

将一个方法调用同一个方法主体关联起来被称为绑定.若在程序执行前进行绑定(如果有的话,由编译器和连接程序实现),叫做前期绑定.在运行时根据对象的类型进行绑定,称为后期绑定,也称为动态绑定或者运行时绑定.

Java中除了static方法和final方法(private方法属于final方法)之外,其他所有的方法都是后期绑定.

public class Shape {

public void draw() {

System.out.println("Shape draw.");

}

public void erase() {

System.out.println("Shape erase");

}

}

public class Circle extends Shape {

public void draw() {

System.out.println("Circle draw.");

}

public void erase() {

System.out.println("Circle erase.");

}

}

public class Shapes {

public static void main(String[] args) {

Shape shape = new Circle();

shape.draw();

}

}

//output

Circle draw.

缺陷:"覆盖"私有方法

public class PrivateOverride {

private void f(){

System.out.println("private f()");

}

public static void main(String[] args){

PrivateOverride privateOverride = new Derived();

privateOverride.f();

}

}

class Derived extends PrivateOverride{

public void f(){

System.out.println("public f()");

}

}

//output

private f()

我们期望的输出是public f(),但是由于private方法被自动认为是final方法,而且对导出类(子类)是屏蔽的.因此,在这种情况下,Derived类中的f()方法就是一个全新的方法,既然基类中的f()方法在子类Derived中不可见,因此甚至也不能被重载.

缺陷:域与静态方法

如果你直接访问某个域,这个访问就将在编译期进行解析,就像下面的示例所演示的:

class Super {

public int field = 0;

public int getField() {

return field;

}

}

class Sub extends Super {

public int field = 1;

public int getField() {

return field;

}

public int getSuperField() {

return super.field;

}

}

public class FieldAccess {

public static void main(String[] args) {

Super sup = new Sub();

System.out.println("sup.field = " + sup.field + ", sup.getField() = " + sup.getField());

Sub sub = new Sub();

System.out.println("sub.field = " + sub.field + ", sub.getField() = " + sub.getField());

System.out.println("sub.getSuperField() = " + sub.getSuperField());

}

}

//output

sup.field = 0, sup.getField() = 1

sub.field = 1, sub.getField() = 1

sub.getSuperField() = 0

当Sub对象转型为Super引用时,任何域访问操作都将由编译器解析,因此不是多态的.在本例中,为Super.field和Sub.field分配了不同的存储空间.这样,Sub实际上包含了两个称为field的域:它自己的和它从Super处得到的.然而,在引用Sub中的field时产生的默认域并非Super版本的field域.

如果某个方法是静态的,它的行为就不具有多态性:

class StaticSuper {

public static String staticGet() {

return "Base staticGet()";

}

public String dynamicGet() {

return "Base dynamicGet()";

}

}

class StaticSub extends StaticSuper {

public static String staticGet() {

return "Derived staticGet()";

}

public String dynamicGet() {

return "Derived dynamicGet()";

}

}

public class StaticPolymorphism {

public static void main(String[] args) {

StaticSuper sup = new StaticSub();

System.out.println(sup.staticGet());

System.out.println(sup.dynamicGet());

}

}

//outp

Base staticGet()

Derived dynamicGet()