多態(tài)是面向?qū)ο蟮娜筇卣髦?/strong>
談到多態(tài),不得不提的就是,多態(tài)中的成員綁定問(wèn)題.
方法綁定
將一個(gè)方法調(diào)用同一個(gè)方法主體關(guān)聯(lián)起來(lái)被稱(chēng)為綁定.若在程序執(zhí)行前進(jìn)行綁定(如果有的話(huà),由編譯器和連接程序?qū)崿F(xiàn)),叫做前期綁定.在運(yùn)行時(shí)根據(jù)對(duì)象的類(lèi)型進(jìn)行綁定,稱(chēng)為后期綁定,也稱(chēng)為動(dòng)態(tài)綁定或者運(yùn)行時(shí)綁定.
Java中除了static方法和final方法(private方法屬于final方法)之外,其他所有的方法都是后期綁定.
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:"覆蓋"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()
我們期望的輸出是public f(),但是由于private方法被自動(dòng)認(rèn)為是final方法,而且對(duì)導(dǎo)出類(lèi)(子類(lèi))是屏蔽的.因此,在這種情況下,Derived類(lèi)中的f()方法就是一個(gè)全新的方法,既然基類(lèi)中的f()方法在子類(lèi)Derived中不可見(jiàn),因此甚至也不能被重載.
缺陷:域與靜態(tài)方法
如果你直接訪(fǎng)問(wèn)某個(gè)域,這個(gè)訪(fǎng)問(wèn)就將在編譯期進(jìn)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
當(dāng)Sub對(duì)象轉(zhuǎn)型為Super引用時(shí),任何域訪(fǎng)問(wèn)操作都將由編譯器解析,因此不是多態(tài)的.在本例中,為Super.field和Sub.field分配了不同的存儲(chǔ)空間.這樣,Sub實(shí)際上包含了兩個(gè)稱(chēng)為field的域:它自己的和它從Super處得到的.然而,在引用Sub中的field時(shí)產(chǎn)生的默認(rèn)域并非Super版本的field域.
如果某個(gè)方法是靜態(tài)的,它的行為就不具有多態(tài)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()
