可以看出,现在b2.unCA的改变对b1.unCA没有产生影响。此时b1.unCA与b2.unCA指向了两个不同的UnCloneA实例,而且在CloneB b2 = (CloneB)b1.clone();调用的那一刻b1和b2拥有相同的值,在这里,b1.i = b2.i = 11。要知道不是所有的类都能实现深度clone的。例如,如果把上面的CloneB类中的UnCloneA类型变量改成StringBuffer类型,看一下JDK API中关于StringBuffer的说明,StringBuffer没有重载clone()方法,更为严重的是StringBuffer还是一个final类,这也是说我们也不能用继承的办法间接实现StringBuffer的clone。如果一个类中包含有StringBuffer类型对象或和StringBuffer相似类的对象,我们有两种选择:要么只能实现影子clone,要么就在类的clone()方法中加一句(假设是SringBuffer对象,而且变量名仍是unCA): o.unCA = new StringBuffer(unCA.toString()); //原来的是:o.unCA = (UnCloneA)unCA.clone();还要知道的是除了基本数据类型能自动实现深度clone以外,String对象是一个例外,它clone后的表现好象也实现了深度clone,虽然这只是一个假象,但却大大方便了我们的编程。Clone中String和StringBuffer的区别应该说明的是,这里不是着重说明String和StringBuffer的区别,但从这个例子里也能看出String类的一些与众不同的地方。下面的例子中包括两个类,CloneC类包含一个String类型变量和一个StringBuffer类型变量,并且实现了clone()方法。在StrClone类中声明了CloneC类型变量c1,然后调用c1的clone()方法生成c1的拷贝c2,在对c2中的String和StringBuffer类型变量用相应的方法改动之后打印结果:package clone;
class CloneC implements Cloneable{
public String str;
public StringBuffer strBuff;
public Object clone(){
CloneC o = null;
try{
o = (CloneC)super.clone();
}catch(CloneNotSupportedException e){
e.printStackTrace();
}
return o;
}
}
public class StrClone {
public static void main(String[] a){
CloneC c1 = new CloneC();
c1.str = new String("initializeStr");
c1.strBuff = new StringBuffer("initializeStrBuff");
System.out.println("before clone,c1.str = "+ c1.str);
System.out.println("before clone,c1.strBuff = "+ c1.strBuff);
CloneC c2 = (CloneC)c1.clone();
c2.str = c2.str.substring(0,5);
c2.strBuff = c2.strBuff.append(" change strBuff clone");
System.out.println("=================================");
System.out.println("after clone,c1.str = "+ c1.str);
System.out.println("after clone,c1.strBuff = "+ c1.strBuff);
System.out.println("=================================");
System.out.println("after clone,c2.str = "+ c2.str);
System.out.println("after clone,c2.strBuff = "+ c2.strBuff);
}
}
/* RUN RESULT
before clone,c1.str = initializeStr
before clone,c1.strBuff = initializeStrBuff
=================================
after clone,c1.str = initializeStr
after clone,c1.strBuff = initializeStrBuff change strBuff clone
=================================
after clone,c2.str = initi
after clone,c2.strBuff = initializeStrBuff change strBuff clone
*
*/
class CloneC implements Cloneable{
public String str;
public StringBuffer strBuff;
public Object clone(){
CloneC o = null;
try{
o = (CloneC)super.clone();
}catch(CloneNotSupportedException e){
e.printStackTrace();
}
return o;
}
}
public class StrClone {
public static void main(String[] a){
CloneC c1 = new CloneC();
c1.str = new String("initializeStr");
c1.strBuff = new StringBuffer("initializeStrBuff");
System.out.println("before clone,c1.str = "+ c1.str);
System.out.println("before clone,c1.strBuff = "+ c1.strBuff);
CloneC c2 = (CloneC)c1.clone();
c2.str = c2.str.substring(0,5);
c2.strBuff = c2.strBuff.append(" change strBuff clone");
System.out.println("=================================");
System.out.println("after clone,c1.str = "+ c1.str);
System.out.println("after clone,c1.strBuff = "+ c1.strBuff);
System.out.println("=================================");
System.out.println("after clone,c2.str = "+ c2.str);
System.out.println("after clone,c2.strBuff = "+ c2.strBuff);
}
}
/* RUN RESULT
before clone,c1.str = initializeStr
before clone,c1.strBuff = initializeStrBuff
=================================
after clone,c1.str = initializeStr
after clone,c1.strBuff = initializeStrBuff change strBuff clone
=================================
after clone,c2.str = initi
after clone,c2.strBuff = initializeStrBuff change strBuff clone
*
*/
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c2.strBuff = c2.strBuff.append(" change strBuff clone");
改成下面这样:c2.str.substring(0,5);
c2.strBuff.append(" change strBuff clone");
去掉了重新赋值的过程,c2.str也就不能有变化了,我们的把戏也就露馅了。但在编程过程中只调用c2.str.substring(0,5);
语句是没有任何意义的。应该知道的是在Java中所有的基本数据类型都有一个相对应的类,象Integer类对应int类型,Double类对应double类型等等,这些类也与String类相同,都是不可以改变的类。也就是说,这些的类中的所有方法都是不能改变其自身的值的。这也让我们在编clone类的时候有了一个更多的选择。同时我们也可以把自己的类编成不可更改的类。