Java中的深拷贝(深复制)和浅拷贝(浅复制)
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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们也会感到困惑与傍徨,但每个程序员的心中都有一个比尔盖茨或是乔布斯的梦想“用智慧开创属于自己的事业”。我想说的是,其
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