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Java中的深拷贝与浅拷贝 Java中的深拷贝(深复制)和浅拷贝(浅复制)介绍

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深拷贝(深复制)和浅拷贝(浅复制)是两个比较通用的概念,尤其在C++语言中,若不弄懂,则会在delete的时候出问题,但是我们在这幸好用的是Java。虽然java自动管理对象的回收,但对于深拷贝(深复制)和浅拷贝(浅复制),我们还是要给予足够的重视,因为有时这两个概念往往会给我们带来不小的困惑。

浅拷贝是指拷贝对象时仅仅拷贝对象本身(包括对象中的基本变量),而不拷贝对象包含的引用指向的对象。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。举例来说更加清楚: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。

若不对clone()方法进行改写,则调用此方法得到的对象即为浅拷贝,下面我们着重谈一下深拷贝。

运行下面的程序,看一看浅拷贝:

class Professor0 implements Cloneable { 
  String name; 
  int age; 
 
  Professor0(String name, int age) { 
    this.name = name; 
    this.age = age; 
  } 
 
  public Object clone() throws CloneNotSupportedException { 
    return super.clone(); 
  } 
} 
 
class Student0 implements Cloneable { 
  String name;// 常量对象。 
  int age; 
  Professor0 p;// 学生1和学生2的引用值都是一样的。 
 
  Student0(String name, int age, Professor0 p) { 
    this.name = name; 
    this.age = age; 
    this.p = p; 
  } 
 
  public Object clone() { 
    Student0 o = null; 
    try { 
      o = (Student0) super.clone(); 
    } catch (CloneNotSupportedException e) { 
      System.out.println(e.toString()); 
    } 
 
    return o; 
  } 
} 
 
public class ShallowCopy { 
  public static void main(String[] args) { 
    Professor0 p = new Professor0("wangwu", 50); 
    Student0 s1 = new Student0("zhangsan", 18, p); 
    Student0 s2 = (Student0) s1.clone(); 
    s2.p.name = "lisi"; 
    s2.p.age = 30; 
    s2.name = "z"; 
    s2.age = 45; 
    System.out.println("学生s1的姓名:" + s1.name + "\n学生s1教授的姓名:" + s1.p.name + "," + "\n学生s1教授的年纪" + s1.p.age);// 学生1的教授 
  } 
}

s2变了,但s1也变了,证明s1的p和s2的p指向的是同一个对象。这在我们有的实际需求中,却不是这样,因而我们需要深拷贝:

class Professor implements Cloneable { 
  String name; 
  int age; 
 
  Professor(String name, int age) { 
    this.name = name; 
    this.age = age; 
  } 
 
  public Object clone() { 
    Object o = null; 
    try { 
      o = super.clone(); 
    } catch (CloneNotSupportedException e) { 
      System.out.println(e.toString()); 
    } 
    return o; 
  } 
} 
 
class Student implements Cloneable { 
  String name; 
  int age; 
  Professor p; 
 
  Student(String name, int age, Professor p) { 
    this.name = name; 
    this.age = age; 
    this.p = p; 
  } 
 
  public Object clone() { 
    Student o = null; 
    try { 
      o = (Student) super.clone(); 
    } catch (CloneNotSupportedException e) { 
      System.out.println(e.toString()); 
    } 
    o.p = (Professor) p.clone(); 
    return o; 
  } 
} 
 
public class DeepCopy { 
  public static void main(String args[]) { 
    long t1 = System.currentTimeMillis(); 
    Professor p = new Professor("wangwu", 50); 
    Student s1 = new Student("zhangsan", 18, p); 
    Student s2 = (Student) s1.clone(); 
    s2.p.name = "lisi"; 
    s2.p.age = 30; 
    System.out.println("name=" + s1.p.name + "," + "age=" + s1.p.age);// 学生1的教授不改变。 
    long t2 = System.currentTimeMillis(); 
    System.out.println(t2-t1); 
  } 
}

当然我们还有一种深拷贝方法,就是将对象串行化:

import java.io.*; 
//Serialization is time-consuming 
class Professor2 implements Serializable { 
  /** 
   * 
   */
  private static final long serialVersionUID = 1L; 
  String name; 
  int age; 
 
  Professor2(String name, int age) { 
    this.name = name; 
    this.age = age; 
  } 
} 
 
class Student2 implements Serializable { 
  /** 
   * 
   */
  private static final long serialVersionUID = 1L; 
  String name;// 常量对象。 
  int age; 
  Professor2 p;// 学生1和学生2的引用值都是一样的。 
 
  Student2(String name, int age, Professor2 p) { 
    this.name = name; 
    this.age = age; 
    this.p = p; 
  } 
 
  public Object deepClone() throws IOException, OptionalDataException, 
      ClassNotFoundException { 
    // 将对象写到流里 
    ByteArrayOutputStream bo = new ByteArrayOutputStream(); 
    ObjectOutputStream oo = new ObjectOutputStream(bo); 
    oo.writeObject(this); 
    // 从流里读出来 
    ByteArrayInputStream bi = new ByteArrayInputStream(bo.toByteArray()); 
    ObjectInputStream oi = new ObjectInputStream(bi); 
    return (oi.readObject()); 
  } 
 
} 
 
public class DeepCopy2 { 
 
  /** 
   * @param args 
   */
  public static void main(String[] args) throws OptionalDataException, 
      IOException, ClassNotFoundException { 
    long t1 = System.currentTimeMillis(); 
    Professor2 p = new Professor2("wangwu", 50); 
    Student2 s1 = new Student2("zhangsan", 18, p); 
    Student2 s2 = (Student2) s1.deepClone(); 
    s2.p.name = "lisi"; 
    s2.p.age = 30; 
    System.out.println("name=" + s1.p.name + "," + "age=" + s1.p.age); // 学生1的教授不改变。 
    long t2 = System.currentTimeMillis(); 
    System.out.println(t2-t1); 
  } 
 
}

但是串行化却很耗时,在一些框架中,我们便可以感受到,它们往往将对象进行串行化后进行传递,耗时较多。

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