淺拷貝

什麼是淺拷貝

被複制物件的所有變數都含有與原來的物件相同的值，而所有的對其他物件的引用仍然指向原來的物件。即物件的淺拷貝會對“主”物件進行拷貝，但不會複製主物件裡面的物件。”裡面的物件“會在原來的物件和它的副本之間共享。

淺拷貝例項

public class ShallowCopy {
    public static void main(String[] args) throws CloneNotSupportedException {
        Teacher teacher = new Teacher();
        teacher.setName("Delacey");
        teacher.setAge(29);

        Student2 student1 = new Student2();
        student1.setName("Dream");
        student1.setAge(18);
        student1.setTeacher(teacher);

        Student2 student2 = (Student2) student1.clone();
        System.out.println("拷貝後");
        System.out.println(student2.getName());
        System.out.println(student2.getAge());
        System.out.println(student2.getTeacher().getName());
        System.out.println(student2.getTeacher().getAge());
        System.out.println("修改老師的資訊後-------------");

        // 修改老師的資訊
        teacher.setName("Jam");
        System.out.println(student1.getTeacher().getName());
        System.out.println(student2.getTeacher().getName());
    }

}

class Teacher 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;
    }
}

class Student2 implements Cloneable{
    private String name;
    private int age;
    private Teacher teacher;

    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 Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    @Override
    public Object clone() throws CloneNotSupportedException {
        Object object = super.clone();
        return object;
    }

}
 

輸出結果：

拷貝後
Dream
18
Delacey
29
修改老師的資訊後-------------
Jam
Jam

結果分析： 兩個引用student1和student2指向不同的兩個物件，但是兩個引用student1和student2中的兩個teacher引用指向的是同一個物件，所以說明是淺拷貝。

深拷貝

定義

深拷貝是一個整個獨立的物件拷貝，深拷貝會拷貝所有的屬性,並拷貝屬性指向的動態分配的記憶體。當物件和它所引用的物件一起拷貝時即發生深拷貝。深拷貝相比於淺拷貝速度較慢並且花銷較大。

簡而言之，深拷貝把要複製的物件所引用的物件都複製了一遍。

深拷貝例項

public class DeepCopy {
    public static void main(String[] args) throws Exception {
        Teacher2 teacher = new Teacher2();
        teacher.setName("Delacey");
        teacher.setAge(29);

        Student3 student1 = new Student3();
        student1.setName("Dream");
        student1.setAge(18);
        student1.setTeacher(teacher);

        Student3 student2 = (Student3) student1.clone();
        System.out.println("拷貝後");
        System.out.println(student2.getName());
        System.out.println(student2.getAge());
        System.out.println(student2.getTeacher().getName());
        System.out.println(student2.getTeacher().getAge());
        System.out.println("修改老師的資訊後-------------");

        // 修改老師的資訊
        teacher.setName("Jam");
        System.out.println(student1.getTeacher().getName());
        System.out.println(student2.getTeacher().getName());
    }
}

class Teacher2 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 Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

}

class Student3 implements Cloneable {
    private String name;
    private int age;
    private Teacher2 teacher;

    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 Teacher2 getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher2 teacher) {
        this.teacher = teacher;
    }

    @Override
    public Object clone() throws CloneNotSupportedException {
        // 淺複製時：
        // Object object = super.clone();
        // return object;

        // 改為深複製：
        Student3 student = (Student3) super.clone();
        // 本來是淺複製，現在將Teacher物件複製一份並重新set進來
        student.setTeacher((Teacher2) student.getTeacher().clone());
        return student;
    }

}
 

輸出結果：

拷貝後
Dream
18
Delacey
29
修改老師的資訊後-------------
Jam
Delacey

結果分析：
兩個引用student1和student2指向不同的兩個物件，兩個引用student1和student2中的兩個teacher引用指向的是兩個物件，但對teacher物件的修改只能影響student1物件,所以說是深拷貝。

teacher姓名Delacey更改前:

teacher姓名Jam更改後:

參考資源

【Java深入】深拷貝與淺拷貝詳解