簡單通俗易懂的Java物件克隆

克隆，和容易理解，複製一個一模一樣的物件，克隆羊，克隆牛，克隆Java物件都是一個意思。

在Java中要克隆一個物件，只需實現Cloneable介面，重寫clone方法，看似簡單，但實際有坑。

讀者可以嘗試構建如下物件並進行clone

public class User implements Cloneable{private String name; private Integer id; public User(String name, Integer id) { this.name = name; this.id = id;}//省略get set。。。@Overrideprotected User clone() throws CloneNotSupportedException {return (User) super.clone();}@Overridepublic String toString() {return "User{" +"name='" + name + ''' +", id=" + id +'}';}}

public class UserClone {public static void main(String[] args) throws CloneNotSupportedException {User user1 = new User("aaa",1);System.out.println(user1.toString());User user2 = user1.clone();user2.setName("bbb");System.out.println(user2.toString()); System.out.println(user1.toString());}}

從上述結果看clone並沒有任何問題，繼續，我們嘗試構建如下物件並克隆：

public class User implements Cloneable{private String name; private Integer id; private Sex sex; public User(String name, Integer id,Sex sex) {this.name = name; this.id = id; this.sex = sex;}//省略get [email protected] User clone() throws CloneNotSupportedException {return (User) super.clone();}@Overridepublic String toString() {return "User{" +"name='" + name + ''' +", id=" + id +", sex=" + sex +'}';}}

public class Sex {private Integer sex; public Sex(Integer sex) {this.sex = sex;}@Overridepublic String toString() {return "Sex{" +"sex=" + sex +'}';}}

public class UserClone {public static void main(String[] args) throws CloneNotSupportedException {Sex sex0 = new Sex(0);User user1 = new User("aaa",1,sex0);System.out.println(user1.toString());User user2 = user1.clone();user2.getSex().setSex(1);System.out.println(user2.toString());System.out.println(user1.toString());}}

此時clone就會有問題了，修改了user2的sex屬性後，user1的sex也被一起修改。

講到這裏引入本文最重要的兩個概念：淺克隆和深克隆，分別對應前後兩個例子

Java中存在基本型別和引用型別，本文中第二例clone失敗的物件中，就存在引用型別，當我們在clone包含引用型別的物件時需要將引用型別物件(對應本例中的Sex)也實現Cloneable介面,同時在被克隆物件(User)的clone方法中呼叫引用型別物件(Sex)的clone，具體如下:

@Overrideprotected User clone() throws CloneNotSupportedException {User user = (User) super.clone();user.sex = user.sex.clone(); return user;}

這種clone就是深克隆