Java的clone()：深複製與淺複製
阿新 • • 發佈：2018-12-19
Java中要想自定義類的物件可以被複制，自定義類就必須實現Cloneable中的clone()方法，如下：
public class Student implements Cloneable { private String name; private int age; private Professor professor; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public Professor getProfessor() { return professor; } public void setProfessor(Professor professor) { this.professor = professor; } @Override public String toString() { return "Student [name=" + name + ", age=" + age + ", professor=" + professor + "]"; } public Object clone() throws CloneNotSupportedException{ return super.clone(); } }
其中，Professor類同樣為自定義類：
public class Professor { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "Professor [name=" + name + ", age=" + age + "]"; } }
然而，當自定義類的欄位的型別不是基本資料型別時，上面實現了clone()方法會導致問題，不信看下面的程式碼：
【程式例項1】
public class ShadowCopy { public static void main(String[] args) { Professor p1 = new Professor(); p1.setName("Professor Zhang"); p1.setAge(30); Student s1 = new Student(); s1.setName("xiao ming"); s1.setAge(18); s1.setProfessor(p1); System.out.println(s1); try { Student s2 = (Student) s1.clone(); Professor p2 = s2.getProfessor(); p2.setName("Professor Li"); p2.setAge(45); s2.setProfessor(p2); System.out.println("複製後的：s1 = " + s1); System.out.println("複製後的：s2 = " + s2); } catch (CloneNotSupportedException e) { e.printStackTrace(); } } }
【執行結果1】
Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]
複製後的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]
複製後的：s2 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]【結果分析】
學生s1的導師為30歲的Professor Zhang，恰好學生s2與學生s1同名同歲，但是s2的導師為45歲的Professor Li，於是我們順理成章地複製複製s1並複製給s2，再修改下s2的導師的資訊。可是，問題出現了，當我們修改了s2的導師後，s2的資訊是對了，但是s1的導師資訊也跟著修改了，這可不是我們期望的。
【問題分析】
程式例項1中的問題出在哪兒呢？我們已經對Student類實現了clone()方法，怎麼還是出問題了呢？我們在看下面的程式碼：
【程式例項2】
public class ShadowCopy {
public static void main(String[] args) {
Professor p1 = new Professor();
p1.setName("Professor Zhang");
p1.setAge(30);
Student s1 = new Student();
s1.setName("xiao ming");
s1.setAge(18);
s1.setProfessor(p1);
System.out.println(s1);
try {
Student s2 = (Student) s1.clone();
s2.setName("xiao hong");
s2.setAge(17);
Professor p2 = s2.getProfessor();
p2.setName("Professor Li");
p2.setAge(45);
s2.setProfessor(p2);
System.out.println("複製後的：s1 = " + s1);
System.out.println("複製後的：s2 = " + s2);
} catch (CloneNotSupportedException e) {
e.printStackTrace();
}
}
}
【執行結果】
Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]
複製後的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]
複製後的：s2 = Student [name=xiao hong, age=17, professor=Professor [name=Professor Li, age=45]]【結果分析】
這次，我們在clone後，又修改了s2的name和age，從結果可以看出，s1的name和age並沒有因為s2的修改而改變。
結合程式例項1和程式例項2，我們發現Student的欄位如果不是一個引用時，修改clone()得到物件的該欄位(name, age)時並不會影響原來的物件，但是當欄位為一個引用時，修改clone()得到物件的該欄位(professor)時並會影響原來的物件。上面實現的clone()方法為淺複製（shadow copy）。
如果想要clone()得到的新物件的修改不會影響被複制的物件的欄位時，我們就需要實現深複製（deep copy），程式碼修改如下：
public class Professor implements Cloneable {
private String name;
private int age;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Professor [name=" + name + ", age=" + age + "]";
}
public Object clone() throws CloneNotSupportedException{
return super.clone();
}
}
public class Student implements Cloneable {
private String name;
private int age;
private Professor professor;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public Professor getProfessor() {
return professor;
}
public void setProfessor(Professor professor) {
this.professor = professor;
}
@Override
public String toString() {
return "Student [name=" + name + ", age=" + age + ", professor="
+ professor + "]";
}
public Object clone() throws CloneNotSupportedException{
Student newStudent = (Student) super.clone();
newStudent.professor = (Professor) professor.clone();
return newStudent;
}
}
再次執行【程式例項2】得到的結果為：
Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]
複製後的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]
複製後的：s2 = Student [name=xiao hong, age=17, professor=Professor [name=Professor Li, age=45]]可以看到：修改clone()得到的s2的任何欄位都不會影響s1的欄位，這也就是深複製的作用。