Java——深拷贝/浅拷贝

一、作用

想拷贝一个新对象出来，并且对新对象的修改不会影响到旧对象。

二、概念

1. 浅拷贝

对基本数据类型进行值传递，对引用数据类型进行引用传递，并没有创建新对象。

2. 深拷贝

对基本数据类型进行值传递，对引用数据类型进行引用传递，创建新对象，并复制其内容。

三、使用

1. 普通的

JavaBean 中不包含引用类型数据（String 类型的成员可以有）

public class Student implements Cloneable {
    private int id;
    private int age;
    private String name;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Student() {}

    public Student(int id, int age) {
        this.id = id;
        this.age = age;
    }

    public Student(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }

    @NonNull
    @Override
    public Object clone() {
        Student o = null;
        try {
            o = (Student) super.clone();
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
        return o;
    }
}

@Test
public void clone2() {
    Student s = new Student(1, 10, "小明");
    Student s2 = (Student) s.clone();
    System.out.println("s.hashcode=" + s.hashCode());
    System.out.println("s2.hashcode=" + s2.hashCode());
    s2.setAge(11);
    s2.setName("小红");
    System.out.println("s.age=" + s.getAge());
    System.out.println("s2.age=" + s2.getAge());
    System.out.println("s.name=" + s.getName());
    System.out.println("s2.name=" + s2.getName());
}

s.hashcode=458209687
s2.hashcode=233530418
s.age=10
s2.age=11
s.name=小明
s2.name=小红

2. 类中包含引用类型成员变量

给 Student 中增加一引用类型成员

private Farther mFarther;

要想引用类型的实例成员也完成深拷贝，需要完成以下操作：

• Farther 类实现Cloneable接口，重写clone()方法

public class Farther implements Cloneable {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Farther() {}

    public Farther(String name) {
        this.name = name;
    }

    @NonNull
    @Override
    public Object clone() {
        Farther o = null;
        try {
            o = (Farther) super.clone();
            return o;
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
        return o;
    }
}

• 修改 Student 类的clone()方法

@NonNull
@Override
public Object clone() {
    Student o = null;
    try {
        o = (Student) super.clone();
        o.setFarther((Farther) mFarther.clone());
    } catch (CloneNotSupportedException e) {
        e.printStackTrace();
    }
    return o;
}

测试

@Test
public void clone3() {
    Farther f1 = new Farther("小明爸爸");
    Student s = new Student(1, 10, "小明", f1);
    Student s2 = (Student) s.clone();
    System.out.println("s.hashcode=" + s.hashCode());
    System.out.println("s2.hashcode=" + s2.hashCode());
    s2.setAge(11);
    s2.setName("小红");
    System.out.println("s.age=" + s.getAge());
    System.out.println("s2.age=" + s2.getAge());
    System.out.println("s.name=" + s.getName());
    System.out.println("s2.name=" + s2.getName());
    System.out.println("s.farther.hashCode=" + s.getFarther().hashCode());
    System.out.println("s2.farther.hashCode=" + s2.getFarther().hashCode());
}

s.hashcode=458209687
s2.hashcode=233530418
s.farther.hashCode=683287027
s2.farther.hashCode=1766822961

3. List 的深拷贝

List 的深拷贝，就是对其中的每个对象都完成深拷贝。

@Test
public void clone4() {
    List < Student > students = new ArrayList < > ();

    Farther f1 = new Farther("小明爸爸");
    Student s = new Student(1, 10, "小明", f1);
    students.add(s);

    Farther f2 = new Farther("小红爸爸");
    Student s2 = new Student(1, 10, "小红", f2);
    students.add(s2);

    List < Student > cloneStudents = new ArrayList < > ();
    for (int i = 0; i < students.size(); i++) {
        Student student = students.get(i);
        Student cloneStudent = (Student) student.clone();
        cloneStudents.add(cloneStudent);
    }

    for (int i = 0; i < students.size(); i++) {
        System.out.println("old：" + i + ":" + students.get(i).toString());
        System.out.println("new：" + i + ":" + cloneStudents.get(i).toString());
    }
}

测试结果

old：0:Student{student.hashCode=458209687, name='小明', mFarther,hashCode=233530418}
new：0:Student{student.hashCode=683287027, name='小明', mFarther,hashCode=1766822961}
old：1:Student{student.hashCode=254413710, name='小红', mFarther,hashCode=1496724653}
new：1:Student{student.hashCode=553264065, name='小红', mFarther,hashCode=897697267}

参考文献

深拷贝和浅拷贝