深克隆與淺克隆

轉載自：https://www.cnblogs.com/acode/p/6306887.html

Java中要想自定義類的對象可以被複制，自定義類就必須實作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，當自定義類的字段的類型不是基本資料類型時，上面實作了clone()方法會導緻問題，不信看下面的代碼：

【程式執行個體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("複制後的：s1 = " + s1);
            System.out.println("複制後的：s2 = " + s2);
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }

    }

}
           

【運作結果1】

1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]
2 複制後的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]
3 複制後的：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，于是我們順理成章地複制複制s1并複制給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的導師資訊也跟着修改了，這可不是我們期望的。

【問題分析】

程式執行個體1中的問題出在哪兒呢？我們已經對Student類實作了clone()方法，怎麼還是出問題了呢？我們在看下面的代碼：

【程式執行個體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("複制後的：s1 = " + s1);
            System.out.println("複制後的：s2 = " + s2);
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }

    }

}
           

【運作結果】

1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]
2 複制後的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Li, age=45]]
3 複制後的：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并沒有因為s2的修改而改變。

結合程式執行個體1和程式執行個體2，我們發現Student的字段如果不是一個引用時，修改clone()得到對象的該字段(name, age)時并不會影響原來的對象，但是當字段為一個引用時，修改clone()得到對象的該字段(professor)時并會影響原來的對象。上面實作的clone()方法為淺複制（shadow copy）。

如果想要clone()得到的新對象的修改不會影響被複制的對象的字段時，我們就需要實作深複制（deep copy），代碼修改如下：

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;
    }

}
           

再次運作【程式執行個體2】得到的結果為：

1 Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]
2 複制後的：s1 = Student [name=xiao ming, age=18, professor=Professor [name=Professor Zhang, age=30]]
3 複制後的：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的任何字段都不會影響s1的字段，這也就是深複制的作用。

參考資料：

1、http://pengcqu.iteye.com/blog/493120

2、http://www.cnblogs.com/shuaiwhu/archive/2010/12/14/2065088.html