Java对象的clone

Java对象的clone

​ 在程序开发中，有时可能好会遇到下列情况：已经存在一个对象A，现在需要一个与对象A完全相同的B对象，并对B对象的值进行修改，但是A对象的原有的属性值不能改变。这时，如果使用Java提供的对象赋值语句B=A，当修改B对象值后，A对象的值也会被修改，实际上是引用传递。那么应该如何实现创建一个和对象A完全相同的对象B，而且修改对象B时，对象A的属性值不被改变呢？

浅clone

所谓浅克隆就是说被克隆的对象的各个属性都是基本类型，而不是引用类型（接口、类、数组），若基本类型和引用类型都含有，且未重写clone方法，则只能克隆基本类型。如果存在引用类型的属性也需要clone，则需要进行深克隆。下面分别演示。

先创建一个Address，表示人员地址，并且作为人员类的成员变量

public class Address {
    public String province;

    public Address(String province) {
        this.province = province;
    }

    public String getProvince() {
        return province;
    }

    public void setProvince(String province) {
        this.province = province;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    @Override
    public String toString() {
        return "Address{" +
                "province='" + province + '\'' +
                '}';
    }
}

再创建people，且可以clone

public class People implements Cloneable{ //需要实现Cloneable接口
    public String name;
    public int age;
    public Address address;

    @Override
    protected Object clone() throws CloneNotSupportedException { //重写clone方法
        return super.clone();
    }

    public People(String name, int age, Address address) {
        this.name = name;
        this.age = age;
        this.address = address;
    }

    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 Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "People{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", address=" + address +
                '}';
    }
}

最后创建一个测试类

public class cloneTest {
    public static void main(String[] args) throws CloneNotSupportedException {
        People c1 = new People("Tom",22,new Address("山东"));
        Object c2 = c1.clone();
        System.out.println(c2.toString());
        c1.setAge(33);//尝试改变c1基本数据类型变量
        c1.getAddress().setProvince("北京");//尝试改变c1引用类型的成员变量
        System.out.println(c2.toString());
    }
}

结果

People{name='Tom', age=22, address=Address{province='山东'}}
People{name='Tom', age=22, address=Address{province='北京'}}

QA

通过运行结果我们发现，虽然c2是被c1 clone来的，但是改变c1的引用类型的成员变量的时候，clone而来的c2对应的变量也被改变了。同时，c1基本数据类型的改变，没有影响到c2，说明c1的基本数据类型变量被成功clone到c2，但引用类型变量未成功clone。则需要深clone。

深clone

public class Address implements Cloneable{
    public String province;

    public Address(String province) {
        this.province = province;
    }

    public String getProvince() {
        return province;
    }

    public void setProvince(String province) {
        this.province = province;
    }

    /*@Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }*/

    @Override
    protected Address clone() throws CloneNotSupportedException {
        return (Address) super.clone();
    }

    @Override
    public String toString() {
        return "Address{" +
                "province='" + province + '\'' +
                '}';
    }
}

public class People implements Cloneable{
    public String name;
    public int age;
    public Address address;

    /*@Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }*/

    @Override
    protected People clone() throws CloneNotSupportedException {
        People people;
        people= (People) super.clone();
        people.address=address.clone();
        return people;
    }

    public People(String name, int age, Address address) {
        this.name = name;
        this.age = age;
        this.address = address;
    }

    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 Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "People{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", address=" + address +
                '}';
    }
}

public class cloneTest {
    public static void main(String[] args) throws CloneNotSupportedException {
        People c1 = new People("Tom",22,new Address("山东"));
        Object c2 = c1.clone();
        System.out.println(c2.toString());
        c1.setAge(33);
        c1.getAddress().setProvince("北京");
//        Object c3 = c1.clone();
        System.out.println(c2.toString());
//        System.out.println(c3.toString());
    }
}

结果

People{name='Tom', age=22, address=Address{province='山东'}}
People{name='Tom', age=22, address=Address{province='山东'}}