clone()之浅拷贝与深拷贝

简述

我们在Java开发时，有时会涉及到对象拷贝复制，就是将一个对象的所有属性（成员变量）复制到另一个对象中，而拷贝又分为浅拷贝和深拷贝，造成这种分法的主要原因就是【值传递】和【引用传递】的不同

【浅拷贝】

• 若成员变量为基本数据类型，拷贝时为【值传递】，是两份不同的数据，改变A类中的该变量，B类不会变化
• 若成员变量为引用类型，拷贝时为【引用传递】，拷贝的是引用变量，实际上两个引用变量指向的是一个实例对象，若改变A类中的该变量，B会变化，即为【浅拷贝】

【深拷贝】

• 深拷贝就是对引用类型的成员变量拷贝时，拷贝整个对象，而不是只拷贝引用

clone()方法

• Object提供了clone()方法帮助我们进行拷贝，但它只是浅拷贝，如果需要深拷贝，需要重写clone()方法的逻辑
• 由源码protected native Object clone() throws CloneNotSupportedException可知，我们无法直接调用clone()方法，我们需要重写该方法并使用super.clone()方法去调用Object的clone方法来实现
• 使用clone方法的类必须实现Cloneable接口，否则会抛出异常CloneNotSupportedException

【上代码！！！】

【浅拷贝】

public class Teacher {
    
    private String name;
    
    private Subject subject;

    public String getName() {
        return name;
    }

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

    public Subject getSubject() {
        return subject;
    }

    public void setSubject(Subject subject) {
        this.subject = subject;
    }
    
    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    @Override
    public String toString() {
        return "Teacher [name=" + name + ", subject=" + subject + "]";
    }
}

public class Subject {
    
    private String name;

    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return "Subject [name=" + name + "]";
    }
}

    public static void main(String[] args) {
        Subject subject = new Subject();
        subject.setName("数学");
        Teacher teacher1 = new Teacher();
        teacher1.setName("张老师");
        teacher1.setSubject(subject);
        
        Teacher teacher2 = null;
        try {
            System.out.println("克隆后------------------");
            teacher2 = (Teacher) teacher1.clone();
            System.out.println("teacher 2:" + teacher2);
            System.out.println("改变teacher1后------------------");
            teacher1.getSubject().setName("语文");
            System.out.println("teacher 2:" + teacher2);
        } catch (CloneNotSupportedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

可以发现改变teacher1的subject后，克隆的对象teacher2的subject也发生了变化，这就是【浅拷贝】

【深拷贝】

public class Teacher implements Cloneable {
    
    private String name;
    
    private Subject subject;

    public String getName() {
        return name;
    }

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

    public Subject getSubject() {
        return subject;
    }

    public void setSubject(Subject subject) {
        this.subject = subject;
    }
    
    @Override
    public Object clone() throws CloneNotSupportedException {
        Teacher teacher = (Teacher) super.clone();
        Subject subject = (Subject) teacher.getSubject().clone();
        teacher.setSubject(subject);    
        return teacher;
    }

    @Override
    public String toString() {
        return "Teacher [name=" + name + ", subject=" + subject + "]";
    }
}

public class Subject implements Cloneable {
    
    private String name;

    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return "Subject [name=" + name + "]";
    }
    
    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

    public static void main(String[] args) {
        Subject subject = new Subject();
        subject.setName("数学");
        Teacher teacher1 = new Teacher();
        teacher1.setName("张老师");
        teacher1.setSubject(subject);
        
        Teacher teacher2 = null;
        try {
            System.out.println("克隆后------------------");
            teacher2 = (Teacher) teacher1.clone();
            System.out.println("teacher 2:" + teacher2);
            System.out.println("改变teacher1后------------------");
            teacher1.getSubject().setName("语文");
            System.out.println("teacher 2:" + teacher2);
        } catch (CloneNotSupportedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

其实就是让subject也实现克隆，然后在teacher的clone方法里，重新setSubject为subject的克隆，就完成了深拷贝