Java克隆Clone

☆ 什么是Clone

Clone就是得到一个对象x的副本，而且x.clone() != x。Clone分为两种：浅度克隆Shallow Clone和浅度克隆Deep Clone

Object类有clone()方法： protected native Object clone() throws CloneNotSupportedException; 但是Object本身没有implements Cloneable，在执行clone方法时，先检查this.getClass()是否实现了cloneable接口，如果没有实现Cloneable接口，clone()就会抛CloneNotSupportedException返回。否则就会创建一个新的this.getClass()的对象other，并将this每一个field的值赋值给other的对应field，然后返回other。

接口Cloneable起一个标志作用(和Serializable类似)，用来标志该类具有克隆功能。

☆ 实现Shallow clone

Shallow Clone只是将基本类型赋值过去，对于对象传的是引用。(String对于比较特别，[一般内容一样则引用一样，轻量级模式])

Shallow Clone
public class Test {
    public static void main(String[] args) throws Exception{
        Area area=new Area("fujian");
        System.out.println("area:"+area);
        Area areaClone=area.clone();
        System.out.println("areaClone:"+areaClone);
        System.out.println("area==areaClone:"+(area==areaClone));
        System.out.println("area.getClass()==areaClone.getClass():"+(area.getClass()==areaClone.getClass()));
        area.province="jiangxi";
        System.out.println("area:"+area);
        System.out.println("areaClone:"+areaClone);
    }
}

class Area implements Cloneable{
    String province;
    Area(String province){
        this.province=province;
    }

    @Override
    public Area clone(){
        Area area=null;
        try{
            area=(Area)super.clone();
        }catch(CloneNotSupportedException cnse){
            cnse.printStackTrace();
        }
        return area;
    }

    @Override
    public String toString(){
        return "\tprovince="+province;
    }
}

输出结果
area:    province=fujian
areaClone:    province=fujian
area==areaClone:false
area.getClass()==areaClone.getClass():true
area:    province=jiangxi
areaClone:    province=fujian

☆ Shallow Clone的问题

因为对于对象只是拷贝引用，所有就会造成this对其中对象的修改引起other中相应对象的修改，例子如下：

Shallow Clone问题
public class Test {
    public static void main(String[] args) throws Exception{
        Area area=new Area("fujian","fujian info");
        Area areaClone=area.clone();
        System.out.println("area:"+area);
        System.out.println("areaClone:"+areaClone);
        area.province="jiangxi";
        area.info.name="new info";
        System.out.println("--------After Change-------");
        System.out.println("area:"+area);
        System.out.println("areaClone:"+areaClone);
    }
}

class Area implements Cloneable{
    String province;
    Info info;
    Area(String province,String infoName){
        this.province=province;
        info=new Info(infoName);
    }

    @Override
    public Area clone(){
        Area area=null;
        try{
            area=(Area)super.clone();
        }catch(CloneNotSupportedException cnse){
            cnse.printStackTrace();
        }
        return area;
    }

    @Override
    public String toString(){
        return "\tprovince="+province+"\t info="+info.name;
    }
}

class Info{
    String name;
    Info(String name){
        this.name=name;
    }
}

输出结果
area:    province=fujian     info=fujian info
areaClone:    province=fujian     info=fujian info
--------After Change-------
area:    province=jiangxi     info=new info
areaClone:    province=fujian     info=new info

☆ Deep Clone

有些时候我们不希望this与other之间的修改相互影响，这时就应该用深度克隆。对所有涉及到的对象也实现Clone。

Deep Clone
public class Test {
    public static void main(String[] args) throws Exception{
        Area area=new Area("fujian","fujian info");
        Area areaClone=area.clone();
        System.out.println("area:"+area);
        System.out.println("areaClone:"+areaClone);
        area.province="jiangxi";
        area.info.name="new info";
        System.out.println("--------After Change-------");
        System.out.println("area:"+area);
        System.out.println("areaClone:"+areaClone);
    }
}

class Area implements Cloneable{
    String province;
    Info info;
    Area(String province,String infoName){
        this.province=province;
        info=new Info(infoName);
    }

    @Override
    public Area clone(){
        Area area=null;
        try{
            area=(Area)super.clone();
            area.info=this.info.clone();
        }catch(CloneNotSupportedException cnse){
            cnse.printStackTrace();
        }
        return area;
    }

    @Override
    public String toString(){
        return "\tprovince="+province+"\t info="+info.name;
    }
}

class Info implements Cloneable{
    String name;
    Info(String name){
        this.name=name;
    }
    @Override
    public Info clone(){
        Info info=null;
        try{
            info=(Info)super.clone();
        }catch(CloneNotSupportedException cnse){
            cnse.printStackTrace();
        }
        return info;
    }
}

      ☆ 参考：
      http://www.blogjava.net/orangelizq/archive/2007/10/17/153573.html
      http://www.blogjava.net/junky/archive/2007/05/08/115928.html

Java克隆Clone

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