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Java 中的拷贝

由于java中只有值传递，u2 = u1，拿到的是u1的地址，u2和u1指向的是同一个内存区域，u1改变值，u2也会改变。

        UseCloneable u1 = new UseCloneable();
        UseCloneable u2 = u1;
        u1.a = 10;

那么我们怎么构造一个新的对象呢

        u2 = new UseCloneable();
        u2.a = u1.a;

但是如果有20个30个字段，这么写就比较臃肿。

clone 方法 与 Cloneable 接口

通俗理解

实现该接口并重写方法来实现拷贝。没实现该接口调用clone方法会报错。

Ojbect类中的clone方法

访问修饰符为protected ，只能由子类或者同包的类调用。

protected native Object clone() throws CloneNotSupportedException;

我们尝试调用clone方法，会提示CloneNotSupportedException

public static void main(String[] args) throws CloneNotSupportedException{ 
   
        UseCloneable u1 = new UseCloneable();
        u1.clone();
-------
Exception in thread "main" java.lang.CloneNotSupportedException: com.example.miccommon.collection.UseCloneable
	at java.lang.Object.clone(Native Method)
	at com.example.miccommon.collection.UseCloneable.main(UseCloneable.java:16)

实现接口实现拷贝过程

@Data
public class UseCloneable implements Cloneable { 
   

    private int a;
    private int b;
    private int c;
    private int d;
    private int e;
    private int f;
    private int g;

    public static void main(String[] args) throws CloneNotSupportedException { 
   

        UseCloneable vvvv = new UseCloneable();
        vvvv.setA(10);
        vvvv.setB(12);
        vvvv.setC(14);
        vvvv.setD(16);
        vvvv.setE(18);
        vvvv.setF(20);
        vvvv.setG(22);
        UseCloneable clone = (UseCloneable) vvvv.clone();
        System.out.println(clone);
    }

    @Override
    public String toString() { 
   
        return "UseCloneable{" +
                "a=" + a +
                ", b=" + b +
                ", c=" + c +
                ", d=" + d +
                ", e=" + e +
                ", f=" + f +
                ", g=" + g +
                '}';
    }
}
-------
UseCloneable{ 
   a=10, b=12, c=14, d=16, e=18, f=20, g=22}

可以看到是两个不同的对象

重写clone方法，来自定义克隆内容。

  @Override
    protected Object clone() throws CloneNotSupportedException { 
   
        UseCloneable clone = (UseCloneable) super.clone();
        clone.setA(999);
        return clone;
    }
-------
UseCloneable{ 
   a=999, b=12, c=14, d=16, e=18, f=20, g=22}

ArrayList 中相关的实现

public Object clone() { 
   
        try { 
   
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) { 
   
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

        ArrayList<Object> objects = new ArrayList<>();
        objects.add("123");
        Object o = objects.clone();
        System.out.println(o);

深拷贝

构造一个类Deep，通过clone拷贝。

@Data
public class UseCloneable implements Cloneable { 
   

    Deep deep;


    public static void main(String[] args) throws CloneNotSupportedException { 
   
        UseCloneable source = new UseCloneable();
        source.deep = new Deep();
        UseCloneable target = (UseCloneable) source.clone();
        System.out.println(target);
    }

}

class Deep { 
   

}

虽然source和target已经不是同一个对象，但是两个对象中的deep都是同一个指向。

我们稍作改造来看看有什么影响，在Deep类中加入属性A。

@Data
public class UseCloneable implements Cloneable { 
   

    Deep deep;

    public static void main(String[] args) throws CloneNotSupportedException { 
   
        UseCloneable source = new UseCloneable();
        source.deep = new Deep();
        source.deep.setA(10);
        UseCloneable target = (UseCloneable) source.clone();
        System.out.println(target);
        source.deep.setA(5);
        System.out.println(target);
    }

}

@Data
class Deep { 
   

    private int a ;

}
--------
UseCloneable(deep=Deep(a=10))
UseCloneable(deep=Deep(a=5))

对source的改动会影响到target中deep的字段，这样肯定是不合理的于是我们。

我们对代码进行改动

@Data
public class UseCloneable implements Cloneable { 
   

    Deep deep;

    public static void main(String[] args) throws CloneNotSupportedException { 
   
        UseCloneable source = new UseCloneable();
        source.deep = new Deep();
        source.deep.setA(10);
        UseCloneable target = (UseCloneable) source.clone();
        System.out.println(target);
        source.deep.setA(5);
        System.out.println(target);
    }

    @Override
    protected Object clone() throws CloneNotSupportedException { 
   
        UseCloneable target = (UseCloneable) super.clone();
        Deep deep = target.getDeep();
        if (deep != null){ 
   
            Deep targetDeep = (Deep) target.getDeep().clone();
            target.setDeep(targetDeep);
        }
        return target;
    }
}

@Data
class Deep implements Cloneable { 
   

    private int a;

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

可以看到两个对象的deep的指向已经改变。