Java浅拷贝及深拷贝

前言

在Java中,有赋值的地方,就存在浅拷贝和深拷贝的问题.稍有不慎,就会为项目埋下隐藏的BUG.本文将对问题进行总结,以减少此类问题的产生.

浅拷贝及深拷贝定义

浅拷贝:在拷贝对象时,对基础数据类型进行赋值,对引用数据类型进行传递引用.
深拷贝:在拷贝对象时,对基础数据类型进行赋值,对引用数据类型进行创建新对象并复制其内的成员变量

java的基础数据类型:byte/short/int/long/float/double/char/boolean
引用数据类型:类实例对象

特别注意:
基本数据类型只能按值传递
每个基本数据类型对应的封装类是按引用传递的
基本数据类型的好处就是速度快(不涉及到对象的构造和回收),封装类的好处是工具方法多,用起来方便。
实际上,封装类型在深拷贝及浅拷贝上的体现并不明显,实验中它的表现效果跟基础数据类型差不多.

上面说[封装类型在深拷贝及浅拷贝上的体现并不明显,实验中它的表现效果跟基础数据类型差不多],这里我们使用一个实验来验证:

@Test
    public void check(){
        String userName = "张三";
        String userName2 = userName;
        System.out.println("userName:"+userName.hashCode()+" userName2:"+userName2.hashCode()+
" hashCode 一致说明确实是引用传值");
        //封装类型在深拷贝及浅拷贝上的体现并不明显,实验中它的表现效果跟基础数据类型差不多
        //其实相当于 userName2 = new String("李四");
        userName2 = "李四";
        System.out.println("userName:"+userName.hashCode()+" userName2:"+userName2.hashCode()+
" userName2 的hashCode 发生了改变");
    }
运行结果

示例分析(java代码)

浅拷贝的实现

1.我们定义了一个用户实体类.类中成员变量很简单,一个是String类型的用户名,一个是地址子类(Address)

package keytop.com.demo.copy;

/**
 * 用户实体类(浅拷贝测试)
 * Created by fengwenhua on 2018/3/7.
 */

public class UserEntity implements Cloneable{

    private String mUserName;

    private Address mAddress;


    public UserEntity(String id,Address address){
        this.mUserName = id;
        this.mAddress = address;
    }

    public String getmUserName() {
        return mUserName;
    }

    public void setmUserName(String mUserName) {
        this.mUserName = mUserName;
    }

    public Address getmAddress() {
        return mAddress;
    }

    public void setmAddress(Address mAddress) {
        this.mAddress = mAddress;
    }


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

    @Override
    public String toString() {
        return "UserEntity{" +
                "mUserName='" + mUserName + '\'' +
                ", mAddress=" + mAddress +
                '}';
    }
}

Address.java

package keytop.com.demo.copy;

/**
 * 当前用户地址
 * Created by fengwenhua on 2018/3/7.
 */

public class Address {
    /**国家*/
    private String country;
    /**省份*/
    private String province;

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

    public String getCountry() {
        return country;
    }

    public void setCountry(String country) {
        this.country = country;
    }

    public String getProvince() {
        return province;
    }

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

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

很好.我们需要测试的原材料已经准备好了.接下来是我们的测试用例:

package keytop.com.demo.copy;

import org.junit.Test;

/**
 * 浅拷贝及深拷贝测试
 * Created by fengwenhua on 2018/3/7.
 */

public class MainTest {

    @Test
    public void testShallowCopy() throws CloneNotSupportedException {
        shallowCopy();
    }

    /**
     * 浅拷贝
     * @throws CloneNotSupportedException 类型不支持
     */
    private void shallowCopy() throws CloneNotSupportedException {
        Address address = new Address("中国","福建");

        UserEntity entity = new UserEntity("张三",address);


        //尝试修改数据的内容
        UserEntity copyData = (UserEntity) entity.clone();
        copyData.setmUserName("李四");
        copyData.getmAddress().setProvince("北京");

        System.out.println("entity hashcode:"+entity.hashCode()+" address hashCode"
        +entity.getmAddress().hashCode()+" 数据内容:"+entity.toString());
        System.out.println("copyData hashcode:"+copyData.hashCode()+" address hashCode"
        +copyData.getmAddress().hashCode()+" 数据内容:"+copyData.toString());
    }

}
运行结果

你没看错,修改李四所在的省份地址.张三的省份地址也动到了.因为张三李四的地址其实指向的是同一个实例

如何实现深拷贝

前面我们总结过了.浅拷贝是因为基础数据类型是值传递,引用数据类型仍然是传值引用.
那么实现深拷贝可以 对实例对象的引用数据类型 变量再进行 clone() 处理.

在以上试验的基础上,我们新增加Score.java类

package keytop.com.demo.copy;

/**
 * 成绩
 * Created by fengwenhua on 2018/3/7.
 */

public class Score implements Cloneable{
    /**英语成绩*/
    private String mEnglish;
    /**语文成绩*/
    private String mChinese;

    public Score(String mEnglish,String mChinese){
        this.mEnglish = mEnglish;
        this.mChinese = mChinese;
    }

    public String getmEnglish() {
        return mEnglish;
    }

    public void setmEnglish(String mEnglish) {
        this.mEnglish = mEnglish;
    }

    public String getmChinese() {
        return mChinese;
    }

    public void setmChinese(String mChinese) {
        this.mChinese = mChinese;
    }

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

    @Override
    public String toString() {
        return "Score{" +
                "mEnglish='" + mEnglish + '\'' +
                ", mChinese='" + mChinese + '\'' +
                '}';
    }
}

为UserEntity 添加Score成员属性,并在clone()上做新的处理(焦点)

package keytop.com.demo.copy;

/**
 * 用户实体类(浅拷贝测试)
 * Created by fengwenhua on 2018/3/7.
 */

public class UserEntity implements Cloneable{

    private String mUserName;

    private Address mAddress;

    private Score mScore;

    public UserEntity(String id,Address address){
        this.mUserName = id;
        this.mAddress = address;
    }

    public UserEntity(String id,Address address,Score score){
        this.mUserName = id;
        this.mAddress = address;
        this.mScore = score;
    }

    public String getmUserName() {
        return mUserName;
    }

    public void setmUserName(String mUserName) {
        this.mUserName = mUserName;
    }

    public Address getmAddress() {
        return mAddress;
    }

    public void setmAddress(Address mAddress) {
        this.mAddress = mAddress;
    }

    public Score getmScore() {
        return mScore;
    }

    public void setmScore(Score mScore) {
        this.mScore = mScore;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        UserEntity userEntity = (UserEntity) super.clone();
        userEntity.setmScore((Score) this.mScore.clone());//对 引用数据类型 变量再进行 clone() 处理
        return userEntity;
    }

    @Override
    public String toString() {
        return "UserEntity{" +
                "mUserName='" + mUserName + '\'' +
                ", mAddress=" + mAddress +
                ", mScore=" + mScore +
                '}';
    }
}

扩展我们的测试用例:

package keytop.com.demo.copy;

import org.junit.Test;

/**
 * 浅拷贝及深拷贝测试
 * Created by fengwenhua on 2018/3/7.
 */

public class MainTest {

    @Test
    public void testShallowCopy() throws CloneNotSupportedException {
        shallowCopy();
    }

    /**
     * 浅拷贝及深拷贝测试
     * @throws CloneNotSupportedException 类型不支持
     */
    private void shallowCopy() throws CloneNotSupportedException {
        Address address = new Address("中国","福建");
        Score score = new Score("99.9","100");
        UserEntity entity = new UserEntity("张三",address,score);


        //尝试修改数据的内容
        UserEntity copyData = (UserEntity) entity.clone();
        copyData.setmUserName("李四");
        copyData.getmAddress().setProvince("北京");
        copyData.getmScore().setmEnglish("50"); //--->修改李四的英语成绩为50

        System.out.println("address hashCode:"+entity.getmAddress().hashCode()+" score hashcode:"
                +entity.getmScore().hashCode()+" 数据内容:"+entity.toString());
        System.out.println("address hashCode:"+copyData.getmAddress().hashCode()+" score hashcode:"
                +copyData.getmScore().hashCode()+" 数据内容:"+copyData.toString());
    }

}
运行结果

很好,这次我们的引用数据类型变量不再相互影响.李四也不用变成学渣了

扩展阅读:细说 Java 的深拷贝和浅拷贝

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