浅析java的浅拷贝和深拷贝

Posted 2020-07-19 沧海一滴

      Java中任何实现了Cloneable接口的类都可以通过调用clone()方法来复制一份自身然后传给调用者。一般而言，clone()方法满足： 
      (1) 对任何的对象x，都有x.clone() !=x，即克隆对象与原对象不是同一个对象。 
      (2) 对任何的对象x，都有x.clone().getClass()==x.getClass()，即克隆对象与原对象的类型一样。 
      (3) 如果对象x的equals()方法定义恰当，那么x.clone().equals(x)应该成立。

   

 /**
     * Creates and returns a copy of this object.  The precise meaning
     * of "copy" may depend on the class of the object. The general
     * intent is that, for any object {@code x}, the expression:
     * <blockquote>
     * <pre>
     * x.clone() != x</pre></blockquote>
     * will be true, and that the expression:
     * <blockquote>
     * <pre>
     * x.clone().getClass() == x.getClass()</pre></blockquote>
     * will be {@code true}, but these are not absolute requirements.
     * While it is typically the case that:
     * <blockquote>
     * <pre>
     * x.clone().equals(x)</pre></blockquote>
     * will be {@code true}, this is not an absolute requirement.
     * <p>
     * By convention, the returned object should be obtained by calling
     * {@code super.clone}.  If a class and all of its superclasses (except
     * {@code Object}) obey this convention, it will be the case that
     * {@code x.clone().getClass() == x.getClass()}.
     * <p>
     * By convention, the object returned by this method should be independent
     * of this object (which is being cloned).  To achieve this independence,
     * it may be necessary to modify one or more fields of the object returned
     * by {@code super.clone} before returning it.  Typically, this means
     * copying any mutable objects that comprise the internal "deep structure"
     * of the object being cloned and replacing the references to these
     * objects with references to the copies.  If a class contains only
     * primitive fields or references to immutable objects, then it is usually
     * the case that no fields in the object returned by {@code super.clone}
     * need to be modified.
     * <p>
     * The method {@code clone} for class {@code Object} performs a
     * specific cloning operation. First, if the class of this object does
     * not implement the interface {@code Cloneable}, then a
     * {@code CloneNotSupportedException} is thrown. Note that all arrays
     * are considered to implement the interface {@code Cloneable} and that
     * the return type of the {@code clone} method of an array type {@code T[]}
     * is {@code T[]} where T is any reference or primitive type.
     * Otherwise, this method creates a new instance of the class of this
     * object and initializes all its fields with exactly the contents of
     * the corresponding fields of this object, as if by assignment; the
     * contents of the fields are not themselves cloned. Thus, this method
     * performs a "shallow copy" of this object, not a "deep copy" operation.
     * <p>
     * The class {@code Object} does not itself implement the interface
     * {@code Cloneable}, so calling the {@code clone} method on an object
     * whose class is {@code Object} will result in throwing an
     * exception at run time.
     *
     * @return     a clone of this instance.
     * @throws  CloneNotSupportedException  if the object\'s class does not
     *               support the {@code Cloneable} interface. Subclasses
     *               that override the {@code clone} method can also
     *               throw this exception to indicate that an instance cannot
     *               be cloned.
     * @see java.lang.Cloneable
     */
    protected native Object clone() throws CloneNotSupportedException;

 

 

 

  首先来看看浅拷贝和深拷贝的定义：

      浅拷贝：使用一个已知实例对新创建实例的成员变量逐个赋值，这个方式被称为浅拷贝。

      深拷贝：当一个类的拷贝构造方法，不仅要复制对象的所有非引用成员变量值，还要为引用类型的成员变量创建新的实例，并且初始化为形式参数实例值。这个方式称为深拷贝

      也就是说浅拷贝只复制一个对象，传递引用，不能复制实例。而深拷贝对对象内部的引用均复制，它是创建一个新的实例，并且复制实例。

      对于浅拷贝当对象的成员变量是基本数据类型时，两个对象的成员变量已有存储空间，赋值运算传递值，所以浅拷贝能够复制实例。但是当对象的成员变量是引用数据类型时，就不能实现对象的复制了。

      存在一个对象Person，代码如下：

public class Person {
    private String name;
    private String sex;
    private int age;
    
    public Person(String name,String sex,int age){
        this.name = name;
        this.sex = sex;
        this.age = age;
    }
    
    public Person(Person p){                   //拷贝构造方法，复制对象
        this.name = p.name;
        this.sex = p.sex;
        this.age = p.age;
    }
}

      上面的对象Person有三个成员变量。name、sex、age。两个构造方法。第二个的参数为该对象，它称为拷贝构造方法，它将创建的新对象初始化为形式参数的实例值，通过它可以实现对象复制功能。

      又有一个对象Asian,如下：

public class Asian {
    private String skin;
    Person person;
    
    public Asian(String skin,Person person){
        this.skin = skin;
        this.person = person;                    //引用赋值
    }

    public Asian(Asian asian){                 //拷贝构造方法，复制对象
        this(asian.skin,asian.person);           
    }
}

      上面对象也存在着两个成员变量，skin 和Person对象

      对于person对象有如下：

Person p1 = new Person("李四","mam",23);

 Person p2 = new Person(P1);

      当调用上面的语句时。P2对象将会对P1进行复制。执行情况如下如下图：

      对于Asian对象有：

 

Asian a1 = new Asian("yellow",new Person("李四","mam",23));
Asian a2 = new Asian(a1);

      New Asian(a1)执行Asian类的拷贝构造方法，由于对象赋值是引用赋值。使得a1和a2引用同一个对象

 

      如下图：

      当a1执行某条可以改变该值的语句时，那么a1将会通过这个语句也可以改变a2对象的成员变量

      如果执行以下语句：a2.name = new Person(a1.name)

      这时将会创建一个新的Person对象

      如下图：

 http://www.cnblogs.com/chenssy/p/3308489.html