Java中的clone()和Cloneable接口实例

目录

• clone()和Cloneable接口
• Cloneable和clone()的总结
• 1.Cloneable 的用途
• 2.克隆的分类
• 3.克隆的举例
• 4.浅克隆的举例
• 5.深克隆的举例

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。

以上为个人经验，希望能给大家一个参考，也希望大家多多支持我们。