引用拷貝、淺拷貝、深拷貝

引用拷貝

@Data
class Father {
    String name;
    Integer age;
    Son son;

    Father(String name, Integer age, Son son) {
        this.name = name;
        this.age = age;
        this.son = son;
    }
}

@Data
class Son {
    String name;

    Son(String name) {
        this.name = name;
    }
}


public class CloneTest {
    public static void main(String[] args) {
        Father father = new Father("father", 25, new Son("son"));
        Father copyFather = father;
        copyFather.name="copyName";
        copyFather.age = 85;
        copyFather.son.name="copySon";
        System.out.println(father);
        System.out.println(copyFather);
    }
}

Father(name=copyName, age=85, son=Son(name=copySon))
Father(name=copyName, age=85, son=Son(name=copySon))

淺拷貝

實現 Cloneable 接口，重寫clone方法。

@Data
class Father implements Cloneable{
    String name;
    int age;
    Son son;

    Father(String name, int age, Son son) {
        this.name = name;
        this.age = age;
        this.son = son;
    }

    @Override
    protected Object clone(){
        try {
            return super.clone();
        } catch (CloneNotSupportedException e) {
            throw new RuntimeException(e);
        }
    }
}

@Data
class Son {
    String name;

    Son(String name) {
        this.name = name;
    }
}


public class CloneTest {
    public static void main(String[] args) {
        Father father = new Father("father", 25, new Son("son"));
        Father copyFather = (Father) father.clone();
        copyFather.name="copyName";
        copyFather.age = 85;
        copyFather.son.name="copySon";
        System.out.println(father);
        System.out.println(copyFather);
    }
}

Father(name=father, age=25, son=Son(name=copySon))
Father(name=copyName, age=85, son=Son(name=copySon))

你可以發現 son 不是深拷貝。

JAVA爲什麼String，Integer等對象在淺克隆時會被克隆，而其它Object不行？
String，Integer 這種不可變類在 深拷貝 中也是特例。如果你實現了深拷貝，你會發現這兩類成員依然不會創建克隆，而是在深拷貝時傳遞引用。看似這操作違反了Java的深拷貝定義，但它們其實完全不需要也不能被深拷貝。最主要的原因是：String，Integer等包裝類本沒有實現Cloneable接口，故根本無法克隆，只能傳遞引用。我們自己重寫clone方法時，對於String、Integer這些成員也不必去理會。—— JasperFang

深拷貝

@Data
class Father implements Cloneable{
    String name;
    int age;
    Son son;

    Father(String name, int age, Son son) {
        this.name = name;
        this.age = age;
        this.son = son;
    }

    @Override
    protected Object clone(){
        try {
            Father father= (Father) super.clone();
            father.setSon((Son) father.getSon().clone());
            return father;
        } catch (CloneNotSupportedException e) {
            throw new RuntimeException(e);
        }
    }
}

@Data
class Son implements Cloneable{
    String name;

    Son(String name) {
        this.name = name;
    }

    @Override
    protected Object clone() {
        try {
            return super.clone();
        } catch (CloneNotSupportedException e) {
            throw new RuntimeException(e);
        }
    }
}

public class CloneTest {
    public static void main(String[] args) {
        Son son=new Son("son");
        Father father = new Father("father", 25, son);

        Father copyFather = (Father) father.clone();
        copyFather.name="copyName";
        copyFather.age = 85;
        copyFather.son.name="copySon";
        System.out.println(father);
        System.out.println(copyFather);
    }
}

Father(name=father, age=25, son=Son(name=son))
Father(name=copyName, age=85, son=Son(name=copySon))

注：還有一種深拷貝是序列化。

@Data
class Father implements Serializable {
    String name;
    int age;
    Son son;

    Father(String name, int age, Son son) {
        this.name = name;
        this.age = age;
        this.son = son;
    }
}

@Data
class Son implements Serializable {
    String name;

    Son(String name) {
        this.name = name;
    }
}

public class CloneTest {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Son son = new Son("son");
        Father father = new Father("father", 25, son);

        ObjectOutputStream outputStream = new ObjectOutputStream(new FileOutputStream("D:\\a.txt"));
        outputStream.writeObject(father);
        outputStream.close();

        ObjectInputStream inputStream = new ObjectInputStream(new FileInputStream("D:\\a.txt"));
        Father copyFather = (Father) inputStream.readObject();
        System.out.println(copyFather);
        copyFather.name = "copyName";
        copyFather.age = 85;
        copyFather.son.name = "copySon";
        System.out.println(copyFather);
    }
}

Father(name=father, age=25, son=Son(name=son))
Father(name=copyName, age=85, son=Son(name=copySon))