java clone

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java clone

有时我们确实需要clone一个对象，最好的方法还是使用Object的clone方法，由jdk调用native方法来实现，这样效率比较高。先来个shallow clone，再来deep clone。

shallow clone

代码如下：

待clone的对象

package practisejava.others.clone;

/**
* 待clone的对象，必须实现Cloneable这个tag interface
*/
public class ObjectToClone implements Cloneable{
   private int i=0;

   public void setNum(int i){
       this.i=i;
   }

   public int getNum(){
       return this.i;
   }

   public void tellNum(){
       System.out.println("the number is "+i);
   }
   // 重写Object的clone
   public Object clone() throws CloneNotSupportedException {
        return (ObjectToClone)super.clone();
    }
}

clone测试

package practisejava.others.clone;

/**
* 测试clone是否成功
*/
public class CloneTest {
    public static void main(String[] args) {
        ObjectToClone otc = new ObjectToClone();
        otc.setNum(888);

        // 开始clone了！
        ObjectToClone cloneObject = null;
        try {
            cloneObject = (ObjectToClone) otc.clone();
        } catch (CloneNotSupportedException ex) {
            System.out.println("Sorry,Clone Not Supported!");
        }

        // 测试clone是否成功
        if(cloneObject!=null){
            System.out.println("before clone");
            System.out.println("ObjectToClone get number : " + otc.getNum()+"/n");

            System.out.println("after clone");
            System.out.println("ObjectToClone get number : " + otc.getNum());

            cloneObject.setNum(999);
            System.out.println("CloneObject get number : " + cloneObject.getNum());
        }
    }
}

执行结果

before clone
ObjectToClone get number : 888

after clone
ObjectToClone get number : 888
CloneObject get number : 999

从运行结果看，修改了clone体的值并没有影响到原来被clone体的值，哈哈，clone成功。高兴得太早了，ObjectToClone里只有简单的非引用属性，如果有引用的属性，还能成吗？答案是否定的！看下面的例子：

被clone对象引用的对象

package practisejava.others.shallowclone;

public class ReferencedObject {
    private int i = 0;
    public ReferencedObject(int i) {
        this.i = i;
    }
    public void doubleValue(){
        this.i = 2*this.i;
    }

    public int getNumber(){
        return this.i;
    }
}

待clone对象

package practisejava.others.shallowclone;

/**
* 必须实现Cloneable接口，否则调用函数的clone方法时会报CloneNotSupportedException错
*/
public class ObjectForShallowClone implements Cloneable{
    private int i = 0;
    private ReferencedObject rf = null;

    public void setNum(int i) {
        this.i = i;
    }

    public void setReferencedObject(ReferencedObject rf){
        this.rf = rf;
    }

    public int getNumber(){
        return this.i;
    }

    public ReferencedObject getReferencedObject(){
        return this.rf;
    }

    // 重写Object的clone
    public Object clone() throws CloneNotSupportedException {
         return (ObjectForShallowClone)super.clone();
     }
}

clone测试

package practisejava.others.shallowclone;

/**
* clone测试
*/
public class CloneTest {
    public static void main(String[] args) {
        ObjectForShallowClone ofsc = new ObjectForShallowClone();
        ofsc.setNum(888);
        ofsc.setReferencedObject(new ReferencedObject(1));

        // 开始clone了！
        ObjectForShallowClone deepCloneObject = null;
        try {
            deepCloneObject = (ObjectForShallowClone) ofsc.clone();
        } catch (CloneNotSupportedException ex) {
            System.out.println("Sorry,Clone Not Supported!");
        }

        // 测试clone是否成功
        if(deepCloneObject!=null){
            System.out.println("before clone");
            System.out.println("ObjectForShallowClone get number : " + ofsc.getNumber());
            System.out.println("ObjectForShallowClone ReferencedObject get number : "+ofsc.getReferencedObject().getNumber()+"/n");

            deepCloneObject.setNum(999);
            deepCloneObject.getReferencedObject().doubleValue();

            System.out.println("after clone");
            System.out.println("ObjectForShallowClone get number : " + ofsc.getNumber());
            System.out.println("ObjectForShallowClone ReferencedObject get number : " + ofsc.getReferencedObject().getNumber());

            System.out.println("CloneObject get number : " + deepCloneObject.getNumber());
            System.out.println("CloneObject ReferencedObject get number : " + deepCloneObject.getReferencedObject().getNumber());
        }
    }
}

运行结果如下：

before clone
ObjectForShallowClone get number : 888
ObjectForShallowClone ReferencedObject get number : 1

after clone
ObjectForShallowClone get number : 888
ObjectForShallowClone ReferencedObject get number : 2
CloneObject get number : 999
CloneObject ReferencedObject get number : 2

显然，修改了clone对象的引用对象的值后，被clone对象引用对象的值也随之改变。这不是我们想要的效果，改进的方法就是deep clone。

deep clone

被clone对象引用的对象（比shallow clone多加了clone的方法）

package practisejava.others.deepclone;

public class ReferencedObject implements Cloneable{
    private int i = 0;
    public ReferencedObject(int i) {
        this.i = i;
    }
    public void doubleValue(){
        this.i = 2*this.i;
    }

    public int getNumber(){
        return this.i;
    }

    // 重写clone的方法
    public Object clone() throws CloneNotSupportedException {
        return (ReferencedObject)super.clone();
    }
}

待clone对象(在clone方法里同时clone引用对象)

package practisejava.others.deepclone;

import practisejava.others.deepclone.ReferencedObject;

public class ObjectForDeepClone implements Cloneable{
private int i = 0;
private ReferencedObject rf = null;

    public void setNum(int i) {
        this.i = i;
    }

    public void setReferencedObject(ReferencedObject rf){
        this.rf = rf;
    }

    public int getNumber(){
        return this.i;
    }

    public ReferencedObject getReferencedObject(){
        return this.rf;
    }

    // 重写Object的clone
    public Object clone() throws CloneNotSupportedException {
         ObjectForDeepClone cloneObject = (ObjectForDeepClone)super.clone();
         // 调用引用对象的clone方法
         if(rf!=null){
             ReferencedObject rfClone = (ReferencedObject)rf.clone();
             cloneObject.setReferencedObject(rfClone);
         }
         return cloneObject;
     }
}

clone测试

package practisejava.others.deepclone;

public class CloneTest {
    public static void main(String[] args) {
        ObjectForDeepClone ofdc = new ObjectForDeepClone();
        ofdc.setNum(888);
        ofdc.setReferencedObject(new ReferencedObject(1));

        // 开始clone了！
        ObjectForDeepClone deepCloneObject = null;
        try {
            deepCloneObject = (ObjectForDeepClone) ofdc.clone();
        } catch (CloneNotSupportedException ex) {
            System.out.println("Sorry,Clone Not Supported!");
        }

        // 测试clone是否成功
        if(deepCloneObject!=null){
            System.out.println("before clone");
            System.out.println("ObjectForDeepClone get number : " + ofdc.getNumber());
            System.out.println("ObjectForDeepClone ReferencedObject get number : "+ofdc.getReferencedObject().getNumber()+"/n");

deepCloneObject.setNum(999);
deepCloneObject.getReferencedObject().doubleValue();

            System.out.println("after clone");
            System.out.println("ObjectForDeepClone get number : " + ofdc.getNumber());
            System.out.println("ObjectForDeepClone ReferencedObject get number : " + ofdc.getReferencedObject().getNumber());

运行结果：

before clone
ObjectForDeepClone get number : 888
ObjectForDeepClone ReferencedObject get number : 1

after clone
ObjectForDeepClone get number : 888
ObjectForDeepClone ReferencedObject get number : 1
CloneObject get number : 999
CloneObject ReferencedObject get number : 2

哇，成功！