Java中的深拷貝(深複制)和淺拷貝(淺複制)
深拷貝(深複制)和淺拷貝(淺複制)是兩個比較通用的概念,尤其在C++語言中,若不弄懂,則會在delete的時候出問題,但是我們在這幸好用的是Java。雖然java自動管理對象的回收,但對于深拷貝(深複制)和淺拷貝(淺複制),我們還是要給予足夠的重視,因為有時這兩個概念往往會給我們帶來不小的困惑。
淺拷貝是指拷貝對象時僅僅拷貝對象本身(包括對象中的基本變量),而不拷貝對象包含的引用指向的對象。深拷貝不僅拷貝對象本身,而且拷貝對象包含的引用指向的所有對象。舉例來說更加清楚:對象A1中包含對B1的引用,B1中包含對C1的引用。淺拷貝A1得到A2,A2 中依然包含對B1的引用,B1中依然包含對C1的引用。深拷貝則是對淺拷貝的遞歸,深拷貝A1得到A2,A2中包含對B2(B1的copy)的引用,B2 中包含對C2(C1的copy)的引用。
若不對clone()方法進行改寫,則調用此方法得到的對象即為淺拷貝,下面我們着重談一下深拷貝。
運作下面的程式,看一看淺拷貝:
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;// 常量對象。
int age;
Professor0 p;// 學生1和學生2的引用值都是一樣的。
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("學生s1的姓名:" + s1.name + "\n學生s1教授的姓名:" + s1.p.name + "," + "\n學生s1教授的年紀" + s1.p.age);// 學生1的教授
}
}
s2變了,但s1也變了,證明s1的p和s2的p指向的是同一個對象。這在我們有的實際需求中,卻不是這樣,因而我們需要深拷貝:
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);// 學生1的教授不改變。
long t2 = System.currentTimeMillis();
System.out.println(t2-t1);
}
}
當然我們還有一種深拷貝方法,就是将對象串行化:
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;// 常量對象。
int age;
Professor2 p;// 學生1和學生2的引用值都是一樣的。
Student2(String name, int age, Professor2 p) {
this.name = name;
this.age = age;
this.p = p;
}
public Object deepClone() throws IOException, OptionalDataException,
ClassNotFoundException {
// 将對象寫到流裡
ByteArrayOutputStream bo = new ByteArrayOutputStream();
ObjectOutputStream oo = new ObjectOutputStream(bo);
oo.writeObject(this);
// 從流裡讀出來
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); // 學生1的教授不改變。
long t2 = System.currentTimeMillis();
System.out.println(t2-t1);
}
}
但是串行化卻很耗時,在一些架構中,我們便可以感受到,它們往往将對象進行串行化後進行傳遞,耗時較多。
本文轉自:http://www.cnblogs.com/shuaiwhu/archive/2010/12/14/2065088.html