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Java中的clone()和Cloneable接口实例

作者：lylodlig　　发布时间：2022-09-27 15:31:05　

标签：Java,clone,Cloneable,接口

clone()和Cloneable接口

clone顾名思义就是克隆，即，复制一个相等的对象，但是不同的引用地址。

我们知道拿到一个对象的地址，只要提供相应的方法就可以修改这个对象，但是如果我们想要得到这个对象去修改它，又想保留这个对象原来的属性，这是就可以使用clone()，它会复制一个内容相同的对象而具有不同内存地址。

Cloneable接口，就是我们要使用clone()必须实现的接口，不然会抛出异常。

public class Bean implements Cloneable {
private String a;
public Bean(String a) {
this.a = a;
}

public String getA() {
return a;
}

@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}

@Override
public boolean equals(Object o) {
if(o instanceof Bean){
Bean bean = (Bean) o;
return bean.getA().equals(a);
}
return false;
}
}

在Cloneable 接口中并没有给我们定义任何方法

这里需要重写clone()方法

protected native Object clone() throws CloneNotSupportedException;
protected Object clone() throws CloneNotSupportedException {
if (!(this instanceof Cloneable)) {
throw new CloneNotSupportedException("Class " + getClass().getName() +
" doesn't implement Cloneable");
}
return internalClone();
}

它是Object类里面的native方法，它是protected的，根据需要可以写为public，可以看到如果不实现Cloneable接口将会抛出CloneNotSupportedException 异常。

测试一下

try {
Bean a = new Bean("lzy");
Bean b = a;
Bean c = (Bean) a.clone();

Log.i(TAG, "onCreate: " + (a == b)); //true
Log.i(TAG, "onCreate: " + (a.equals(b))); //true

Log.i(TAG, "onCreate: " + (a == c)); //false
Log.i(TAG, "onCreate: " + (a.equals(c))); //true

} catch (CloneNotSupportedException e) {
e.printStackTrace();
}

可以看到克隆出来的类的地址是不同的，而内容是相同的。

下面修改一下，在Bean加一个成员变量ChildBean

public class ChildBean implements Cloneable {
private String c;
public String getC() {
return c;
}
public ChildBean(String c) {
this.c = c;
}
@Override
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
@Override
public boolean equals(Object o) {
if (o instanceof ChildBean) {
ChildBean bean = (ChildBean) o;
return bean.getC().equals(c);
}
return false;
}
}

public class Bean implements Cloneable {
private String a;
private ChildBean childBean;
public Bean(String a, ChildBean childBean) {
this.a = a;
this.childBean = childBean;
}
public String getA() {
return a;
}

public ChildBean getChildBean() {
return childBean;
}

@Override
public Object clone() throws CloneNotSupportedException {
return super.clone();
}

@Override
public boolean equals(Object o) {
if (o instanceof Bean) {
Bean bean = (Bean) o;
return bean.getA().equals(a);
}
return false;
}
}

Bean a = new Bean("lzy", new ChildBean("child"));
Bean b = a;
Bean c = (Bean) a.clone();

Log.i(TAG, "onCreate: " + (a.getChildBean() == b.getChildBean())); //true
Log.i(TAG, "onCreate: " + (a.getChildBean().equals(b.getChildBean()))); //true

Log.i(TAG, "onCreate: " + (a.getChildBean() == c.getChildBean())); //true
Log.i(TAG, "onCreate: " + (a.getChildBean().equals(c.getChildBean()))); //true

测试发现有一个结果不是我们所预期的，这意味着并没有真正克隆ChildBean，只是克隆的它的内存地址，导致两个具有相同的内存地址，这也就是浅克隆，此时我们需要的是深克隆，需要按照下面方法修改，重写clone()方法

@Override
public Object clone() throws CloneNotSupportedException {
Bean bean = (Bean) super.clone();
bean.childBean = (ChildBean) bean.childBean.clone();
return bean;
}

但是这样做如果有很多层的类，那每一层都需要去重写，显得很麻烦。所以我们可以用下面的工具类来实现

public class BeanUtil {
public static <T> T cloneTo(T src) throws RuntimeException {
ByteArrayOutputStream memoryBuffer = new ByteArrayOutputStream();
ObjectOutputStream out = null;
ObjectInputStream in = null;
T dist = null;
try {
out = new ObjectOutputStream(memoryBuffer);
out.writeObject(src);
out.flush();
in = new ObjectInputStream(new ByteArrayInputStream(memoryBuffer.toByteArray()));
dist = (T) in.readObject();
} catch (Exception e) {
throw new RuntimeException(e);
} finally {
if (out != null)
try {
out.close();
out = null;
} catch (IOException e) {
throw new RuntimeException(e);
}
if (in != null)
try {
in.close();
in = null;
} catch (IOException e) {
throw new RuntimeException(e);
}
}
return dist;
}
}

Bean a = new Bean("lzy", new ChildBean("child"));
Bean b = BeanUtil.cloneTo(a);
Log.i(TAG, "onCreate: " + (a.getChildBean() == b.getChildBean())); //false
Log.i(TAG, "onCreate: " + (a.getChildBean().equals(b.getChildBean()))); //true

这样就可以很轻松的得到我们预期的结果，但是记得每一个类都要去实现Serializable接口。

Cloneable和clone()的总结

1.Cloneable 的用途

Cloneable和Serializable一样都是标记型接口，它们内部都没有方法和属性，implements Cloneable表示该对象能被克隆，能使用Object.clone()方法。如果没有implements Cloneable的类调用Object.clone()方法就会抛出CloneNotSupportedException。

2.克隆的分类

(1)浅克隆（shallow clone），浅拷贝是指拷贝对象时仅仅拷贝对象本身和对象中的基本变量，而不拷贝对象包含的引用指向的对象。

(2)深克隆（deep clone），深拷贝不仅拷贝对象本身，而且拷贝对象包含的引用指向的所有对象。

举例区别一下：对象A1中包含对B1的引用，B1中包含对C1的引用。浅拷贝A1得到A2，A2中依然包含对B1的引用，B1中依然包含对C1的引用。深拷贝则是对浅拷贝的递归，深拷贝A1得到A2，A2中包含对B2（B1的copy）的引用，B2中包含对C2（C1的copy）的引用。

3.克隆的举例

要让一个对象进行克隆，其实就是两个步骤：

1.让该类实现java.lang.Cloneable接口；

2.重写（override）Object类的clone()方法。

public class Wife implements Cloneable {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Wife(int id,String name) {
this.id = id;
this.name = name;
}
@Override
public int hashCode() {//myeclipse自动生成的
final int prime = 31;
int result = 1;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {//myeclipse自动生成的
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Wife other = (Wife) obj;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
@Override
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
/**
* @param args
* @throws CloneNotSupportedException
*/
public static void main(String[] args) throws CloneNotSupportedException {
Wife wife = new Wife(1,"wang");
Wife wife2 = null;
wife2 = (Wife) wife.clone();
System.out.println("class same="+(wife.getClass()==wife2.getClass()));//true
System.out.println("object same="+(wife==wife2));//false
System.out.println("object equals="+(wife.equals(wife2)));//true
}
}

4.浅克隆的举例

public class Husband implements Cloneable {
private int id;
private Wife wife;
public Wife getWife() {
return wife;
}
public void setWife(Wife wife) {
this.wife = wife;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public Husband(int id) {
this.id = id;
}
@Override
public int hashCode() {//myeclipse自动生成的
final int prime = 31;
int result = 1;
result = prime * result + id;
return result;
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
@Override
public boolean equals(Object obj) {//myeclipse自动生成的
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Husband other = (Husband) obj;
if (id != other.id)
return false;
return true;
}
/**
* @param args
* @throws CloneNotSupportedException
*/
public static void main(String[] args) throws CloneNotSupportedException {
Wife wife = new Wife(1,"jin");
Husband husband = new Husband(1);
Husband husband2 = null;
husband.setWife(wife);
husband2 = (Husband) husband.clone();
System.out.println("husband class same="+(husband.getClass()==husband2.getClass()));//true
System.out.println("husband object same="+(husband==husband2));//false
System.out.println("husband object equals="+(husband.equals(husband)));//true
System.out.println("wife class same="+(husband.getWife().getClass()==husband2.getWife().getClass()));//true
System.out.println("wife object same="+(husband.getWife()==husband2.getWife()));//true
System.out.println("wife object equals="+(husband.getWife().equals(husband.getWife())));//true
}
}

5.深克隆的举例

如果要深克隆，需要重写（override）Object类的clone()方法，并且在方法内部调用持有对象的clone()方法；注意如下代码的clone()方法

public class Husband implements Cloneable {
private int id;
private Wife wife;
public Wife getWife() {
return wife;
}
public void setWife(Wife wife) {
this.wife = wife;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public Husband(int id) {
this.id = id;
}
@Override
public int hashCode() {//myeclipse自动生成的
final int prime = 31;
int result = 1;
result = prime * result + id;
return result;
}
@Override
protected Object clone() throws CloneNotSupportedException {
Husband husband = (Husband) super.clone();
husband.wife = (Wife) husband.getWife().clone();
return husband;
}
@Override
public boolean equals(Object obj) {//myeclipse自动生成的
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Husband other = (Husband) obj;
if (id != other.id)
return false;
return true;
}
/**
* @param args
* @throws CloneNotSupportedException
*/
public static void main(String[] args) throws CloneNotSupportedException {
Wife wife = new Wife(1,"jin");
Husband husband = new Husband(1);
Husband husband2 = null;
husband.setWife(wife);
husband2 = (Husband) husband.clone();
System.out.println("husband class same="+(husband.getClass()==husband2.getClass()));//true
System.out.println("husband object same="+(husband==husband2));//false
System.out.println("husband object equals="+(husband.equals(husband)));//true
System.out.println("wife class same="+(husband.getWife().getClass()==husband2.getWife().getClass()));//true
System.out.println("wife object same="+(husband.getWife()==husband2.getWife()));//false
System.out.println("wife object equals="+(husband.getWife().equals(husband.getWife())));//true
}
}

但是也有不足之处，如果Husband内有N个对象属性，突然改变了类的结构，还要重新修改clone()方法。

解决办法：可以使用Serializable运用反序列化手段，调用java.io.ObjectInputStream对象的 readObject()方法。

来源：https://blog.csdn.net/lylodyf/article/details/52763720

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