JAVA 對象拷貝

為什么需要有對象拷貝？

對象拷貝相對的自然是引用拷貝。java初學者經常會問，我這個方法要改變一個對象的屬性，可以把參數傳進去了，為什么沒有改變了？

——基本數據類型傳值，而對象傳引用或引用的拷貝。

而有時候我們要獲取到一個當前狀態的對象復制品，他們是兩個獨立對象。不再是引用或者引用拷貝（實質都是指向對象本身）。就是說a是b的拷貝，b發生變化的時候，不要影響a。

對象拷貝有淺拷貝和深度拷貝兩種。

1）淺拷貝

淺拷貝是指對象中基本數據類型得到拷貝，而引用數據類型并未拷貝。
提到拷貝自然和clone聯系起來了，所有具有clone功能的類都有一個特性，那就是它直接或間接地實現了Cloneable接口。
否則，我們在嘗試調用clone（）方法時，將會觸發CloneNotSupportedException異常。
eg:

?1 public ? class ?DOG? implements ?Cloneable
?2 {
?3 ???? public ?DOG(String?name,? int ?age)
?4 ???? {
?5 ???????? this .name? = ?name;
?6 ???????? this .age? = ?age;
?7 ????}
?8
?9 ???? public ?String?getName()
10 ???? {
11 ???????? return ? this .name;
12 ????}
13
14 ???? public ? int ?getAge()
15 ???? {
16 ???????? return ? this .age;
17 ????}
18
19 ???? public ?Object?clone()
20 ???? {
21 ???????? try
22 ???????? {
23 ???????????? return ? super .clone();
24
25 ????????} ? catch ?(CloneNotSupportedException?e)
26 ???????? {
27 ???????????? return ? null ;
28 ????????}
29 ????}
30
31 ???? public ?String?name;
32
33 ???? private ? int ?age;
34
35 ???? // test
36 ???? public ? static ? void ?main(String[]?args)
37 ???? {
38 ????????DOG?dog1? = ? new ?DOG( " xiaogou " ,? 2 );
39 ????????DOG?dog2? = ?(DOG)?dog1.clone();
40 ????????dog1.name? = ? " dagou " ;
41 ????????System.out.println(dog2.getName());
42 ????????System.out.println(dog2.getAge());
43 ????????System.out.println(dog1.getName());
44 ????????System.out.println(dog1.getAge());
45
46 ????}
47
48 }
49

運行結果：

xiaogou
2
dagou
2

2）深度拷貝

相對淺拷貝。實現對象中基本數據類型和引用數據類型的拷貝。

請先看下面代碼：

?

?1 class ?AAA
?2 {
?3 ???? public ?AAA(String?name)
?4 ???? {
?5 ???????? this .name? = ?name;
?6 ????}
?7
?8 ???? public ?String?name;
?9 }
10
11 class ?DOG? implements ?Cloneable
12 {
13 ???? public ?DOG(String?name,? int ?age,?AAA?birthday)
14 ???? {
15 ???????? this .name? = ?name;
16 ???????? this .age? = ?age;
17 ???????? this .birthday? = ?birthday;
18 ????}
19
20 ???? public ?String?getName()
21 ???? {
22 ???????? return ?name;
23 ????}
24
25 ???? public ? int ?getAge()
26 ???? {
27 ???????? return ?age;
28 ????}
29
30 ???? public ?AAA?getBirthday()
31 ???? {
32 ???????? return ?birthday;
33 ????}
34
35 ???? public ?String?getBirth(AAA?a)
36 ???? {
37 ???????? return ?a.name;
38 ????}
39
40 ???? public ?String?name;
41
42 ???? private ? int ?age;
43
44 ???? public ?AAA?birthday;
45
46 ???? public ?Object?clone()
47 ???? {
48 ???????? try
49 ???????? {
50 ???????????? super .clone();
51 ???????????? return ? super .clone();
52 ????????} ? catch ?(Exception?e)
53 ???????? {
54 ???????????? return ? null ;
55 ????????}
56 ????}
57 }
58
59 public ? class ?TestClone
60 {
61 ???? public ? static ? void ?main(String[]?args)
62 ???? {
63 ????????AAA?Day? = ? new ?AAA( " test " );
64 ????????DOG?dog1? = ? new ?DOG( " xiaogou " ,? 2 ,?Day);
65 ????????DOG?dog2? = ?(DOG)?dog1.clone();
66 ???????? // ??dog2.birthday?=?(AAA)?dog1.birthday.clone();?
67 ????????dog1.birthday.name? = ? " 333 " ;
68 ????????System.out.println(dog1.getBirth(dog1.birthday));
69 ????????System.out.println(dog2.getBirth(dog2.birthday));
70 ????}
71 }
72

運行結果是：
333
333
而真正要實現拷貝還的加點代碼，如下請對比上面和下面代碼的異同之處：

?1 class ?AAA? implements ?Cloneable
?2 {
?3 ???? public ?AAA(String?name)
?4 ???? {
?5 ???????? this .name? = ?name;
?6 ????}
?7
?8 ???? public ?Object?clone()
?9 ???? {
10 ???????? try
11 ???????? {
12 ???????????? super .clone();
13 ???????????? return ? super .clone();
14 ????????} ? catch ?(Exception?e)
15 ???????? {
16 ???????????? return ? null ;
17 ????????}
18 ????}
19
20 ???? public ?String?name;
21 }
22
23 class ?DOG? implements ?Cloneable
24 {
25 ???? public ?DOG(String?name,? int ?age,?AAA?birthday)
26 ???? {
27 ???????? this .name? = ?name;
28 ???????? this .age? = ?age;
29 ???????? this .birthday? = ?birthday;
30 ????}
31
32 ???? public ?String?getName()
33 ???? {
34 ???????? return ?name;
35 ????}
36
37 ???? public ? int ?getAge()
38 ???? {
39 ???????? return ?age;
40 ????}
41
42 ???? public ?AAA?getBirthday()
43 ???? {
44 ???????? return ?birthday;
45 ????}
46
47 ???? public ?String?getBirth(AAA?a)
48 ???? {
49 ???????? return ?a.name;
50 ????}
51
52 ???? public ?String?name;
53
54 ???? private ? int ?age;
55
56 ???? public ?AAA?birthday;
57
58 ???? public ?Object?clone()
59 ???? {
60 ???????? try
61 ???????? {
62 ???????????? super .clone();
63 ???????????? return ? super .clone();
64 ????????} ? catch ?(Exception?e)
65 ???????? {
66 ???????????? return ? null ;
67 ????????}
68 ????}
69 }
70
71 public ? class ?TestClone
72 {
73 ???? public ? static ? void ?main(String[]?args)
74 ???? {
75 ????????AAA?Day? = ? new ?AAA( " test " );
76 ????????DOG?dog1? = ? new ?DOG( " xiaogou " ,? 2 ,?Day);
77 ????????DOG?dog2? = ?(DOG)?dog1.clone();
78 ????????dog2.birthday? = ?(AAA)?dog1.birthday.clone(); // 特別注意這里
79 ????????dog1.birthday.name? = ? " 333 " ;
80 ????????System.out.println(dog1.getBirth(dog1.birthday));
81 ????????System.out.println(dog2.getBirth(dog2.birthday));
82 ????}
83 }
84

運行結果：
333
test
這樣基本就達到了我們當初的母的。

但是明顯的這種方法還是有許多不足，人們總是希望一個clone就是對象直接克隆。而上面還要對對象中的對象遞歸使用clone。下面提供一種更高級點的做法：

?

?1 import ?java.io. * ;
?2
?3 class ?AAA? implements ?Serializable
?4 {
?5 ???? public ?AAA(String?name)
?6 ???? {
?7 ???????? this .name? = ?name;
?8 ????}
?9
10 ???? public ?String?name;
11 }
12
13 class ?DOG? extends ?SerialCloneable
14 {
15 ???? public ?DOG(String?name,? int ?age,?AAA?birthday)
16 ???? {
17 ???????? this .name? = ?name;
18 ???????? this .age? = ?age;
19 ???????? this .birthday? = ?birthday;
20 ????}
21
22 ???? public ?String?getName()
23 ???? {
24 ???????? return ?name;
25 ????}
26
27 ???? public ? int ?getAge()
28 ???? {
29 ???????? return ?age;
30 ????}
31
32 ???? public ?AAA?getBirthday()
33 ???? {
34 ???????? return ?birthday;
35 ????}
36
37 ???? public ?String?getBirth(AAA?a)
38 ???? {
39 ???????? return ?a.name;
40 ????}
41
42 ???? public ?String?name;
43
44 ???? private ? int ?age;
45
46 ???? public ?AAA?birthday;
47
48 ???? public ?Object?clone()
49 ???? {
50 ???????? try
51 ???????? {
52 ???????????? super .clone();
53 ???????????? return ? super .clone();
54 ????????} ? catch ?(Exception?e)
55 ???????? {
56 ???????????? return ? null ;
57 ????????}
58 ????}
59 }
60
61 public ? class ?TestClone
62 {
63 ???? public ? static ? void ?main(String[]?args)
64 ???? {
65 ????????AAA?Day? = ? new ?AAA( " test " );
66 ????????DOG?dog1? = ? new ?DOG( " xiaogou " ,? 2 ,?Day);
67 ????????DOG?dog2? = ?(DOG)?dog1.clone();
68 ???????? // dog2.birthday?=?(AAA)?dog1.birthday.clone();
69 ????????dog1.birthday.name? = ? " 333 " ;
70 ????????System.out.println(dog1.getBirth(dog1.birthday));
71 ????????System.out.println(dog2.getBirth(dog2.birthday));
72 ????}
73 }
74
75 class ?SerialCloneable? implements ?Cloneable,?Serializable
76 {
77 ???? public ?Object?clone()
78 ???? {
79 ???????? try
80 ???????? {
81 ????????????ByteArrayOutputStream?bout? = ? new ?ByteArrayOutputStream();
82 ????????????ObjectOutputStream?out? = ? new ?ObjectOutputStream(bout);
83 ????????????out.writeObject( this );
84 ????????????out.close();
85 ????????????ByteArrayInputStream?bin? = ? new ?ByteArrayInputStream(bout
86 ????????????????????.toByteArray());
87 ????????????ObjectInputStream?in? = ? new ?ObjectInputStream(bin);
88 ????????????Object?ret? = ?in.readObject();
89 ????????????in.close();
90 ???????????? return ?ret;
91 ????????} ? catch ?(Exception?e)
92 ???????? {
93 ???????????? return ? null ;
94 ????????}
95 ????}
96 }
97

輸出：
333
test

上面的代碼用序列化與反序列化實現了對象拷貝。比較通用。但是得注意的是其中的類得implements Serializable。

?

3）后記

我們如果利用強大的反射機制+序列化與反序列化，能做出更加靈活的對象拷貝。有興趣的朋友可以自行去研究。
我在javaeye上看到一篇短文：http://www.javaeye.com/post/367014 主要講的就是反射在對象拷貝中的應用。

?