IT俱乐部 Java Java对象传递与返回的细节问题详析

Java对象传递与返回的细节问题详析

1.传递引用

在一个方法中将一个对象的引用传递给另外一个方法,引用指向的对象是同一个

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public class Person {
    int age;
    String name;
    public Person(int age, String name) {
        this.age = age;
        this.name = name;
    }
     
    public static void main(String[] args) {
        Person p=new Person(18, "tom");
        System.out.println("main:  "+p);
        f(p);
    }
    public static void f(Person p) {
        System.out.println("f():  "+p);
    }
}

引用别名

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public static void main(String[] args) {
        Person p=new Person(18, "tom");
        Person p2=p;
        p2.age++;
        System.out.println(p.age);//19 
    }
    

引用p和p2指向的是同一个对象,p2对对象的属性进行操作,当使用引用p访问对象的属性时当然也改变,同样的情况也发生在对象引用在方法之间的传递,如下面的代码:

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public static void main(String[] args) {
        Person p=new Person(18, "tom");
        f(p);
        System.out.println(p.age);//19 
    }
    public static void f(Person p) {
        p.age++;
    }

2. 创建本地副本

几个概念:

  • 引用别名会在方法参数是对象类型时自动发生
  • 没有本地对象,只有本地引用(方法中创建的对象存在于堆中,只有引用变量存在于方法栈中)
  • 引用是有作用域的,而对象没有
  • Java中的对象的生命周期并不是一个问题(垃圾回收机制)

2.1 值传递

Java中方法之间只有值传递

传递基本类型时,传递的是基本类型的值的拷贝

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public static void main(String[] args) {
        int a=100;
        change(a);
        System.out.println(a);//100 不受影响
    }
    public static void change(int a) {
        a=99;
    }

传递对象时,传递的是对象的引用拷贝

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public static void main(String[] args) {
        Person p=new Person(18, "tom");
        f(p);
        System.out.println(p.age);//还是18 f()中p指向了一个新的对象 不影响main函数
    }
    public static void f(Person p) {
        p=new Person(20, "bob");
    }

传递对象时如果在另外一个方法中对对象的属性进行操作会对main方法产生“副作用”, 但是如果只是简单的对引用进行操作是没有影响的

2.2 对象克隆

步骤:

类实现Cloneable空接口,默认情况下不希望所有的类都有克隆能力,当需要某个类有克隆能力时就需要实现该接口作为一种“可克隆”的标记,否则克隆时会报错CloneNotSupportedException

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public interface Cloneable {
}

重写clone方法,clone方法是Object类中的,它在Object类中的是一个protected的本地方法,需要重写,加上public修饰符,否则只能在当前类中使用clone方法

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protected native Object clone() throws CloneNotSupportedException;
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public class Person implements Cloneable {
    int age;
    String name;
    public Person(int age, String name) {
        this.age = age;
        this.name = name;
    }
     
    public Person clone() throws CloneNotSupportedException  {
            return (Person) super.clone();
    }
     
    public static void main(String[] args) throws CloneNotSupportedException {
        Person p1=new Person(18, "tom");
        Person p2=p1.clone();
        System.out.println(p1+"   name: "+p1.name+"  age: "+p1.age);
        System.out.println(p2+"   name: "+p2.name+"  age: "+p2.age);
    }
}

重写clone方法时实际是就是调用Object类中的本地clone方法,Object类中的clone方法做了哪些工作?

Object类中clone方法负责创建正确大小的存储空间,并执行了从原始对象中所有二进制位到新对象内存中的按位复制。

2.3 浅拷贝问题

场景:Person类中增加一个引用类型的属性Country, 表示这个人所属的国家,然后进行克隆

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package test;
 
class Country{
    String nation;
 
    public Country(String nation) {
        super();
        this.nation = nation;
    }
     
}
public class Person implements Cloneable {
    int age;
    String name;
    Country country;
    public Person(int age, String name,Country country) {
        this.age = age;
        this.name = name;
        this.country=country;
    }
     
    public Person clone() throws CloneNotSupportedException  {
            return (Person) super.clone();
    }
     
    public static void main(String[] args) throws CloneNotSupportedException {
        Country country=new Country("China");
        Person p1=new Person(18, "tom",country);
        Person p2=p1.clone();
        System.out.println(p1+"   name: "+p1.name+"  age: "+p1.age+"  country: "+p1.country);
        System.out.println(p2+"   name: "+p2.name+"  age: "+p2.age+"  country: "+p2.country);
        p1.name="bob";
        p1.country.nation="America";
        System.out.println(p1+"   name: "+p1.name+"  age: "+p1.age+"  country: "+p1.country.nation);
        System.out.println(p2+"   name: "+p2.name+"  age: "+p2.age+"  country: "+p2.country.nation);
    }
}

问题描述: 当Person类中有一个引用类型属性时,对于基本类型数据和String类型是对值进行拷贝的,因此改变p1的name不影响p2的name, 但是对于Country这种引用类型,在clone时仅仅是拷贝了一份对象的引用,拷贝的引用和原来的引用指向的是同一个对象,因此p1改变country的属性时,p2的country的属性也发生了改变

2.4 深拷贝

解决浅拷贝问题的关键点在于不仅仅是拷贝引用,而且要拷贝一份引用指向的对象,深拷贝有两种方式:

  • 逐个对引用指向的对象进行浅拷贝
  • 使用序列化方式进行深拷贝

2.4.1 引用类型逐个浅拷贝

如果一个类A中有多个引用类型,那么这些引用类型的类需要实现 Cloneable接口,在对A的对象进行克隆时,逐个浅拷贝其中的引用类型

eg: Person类中有Country引用类型,在进行clone时,单独对country进行浅拷贝

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package test;
 
class Country implements Cloneable{
    String nation;
 
    public Country(String nation) {
        super();
        this.nation = nation;
    }
    public Country clone() throws CloneNotSupportedException  {
        return (Country) super.clone();
}
     
}
public class Person implements Cloneable {
    int age;
    String name;
    Country country;
    public Person(int age, String name,Country country) {
        this.age = age;
        this.name = name;
        this.country=country;
    }
     
    public Person clone() throws CloneNotSupportedException  {
             
            Person p=null;
            p=(Person) super.clone();
            p.country=p.country.clone();//对country进行浅拷贝
            return p;
    }
     
    public static void main(String[] args) throws CloneNotSupportedException {
        Country country=new Country("China");
        Person p1=new Person(18, "tom",country);
        Person p2=p1.clone();
        System.out.println(p1+"   name: "+p1.name+"  age: "+p1.age+"  country: "+p1.country);
        System.out.println(p2+"   name: "+p2.name+"  age: "+p2.age+"  country: "+p2.country);
        p1.name="bob";
        p1.country.nation="America";
        System.out.println(p1+"   name: "+p1.name+"  age: "+p1.age+"  country: "+p1.country.nation);
        System.out.println(p2+"   name: "+p2.name+"  age: "+p2.age+"  country: "+p2.country.nation);
         
    }
}

2.4.2 序列化方式进行深拷贝

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package test;
 
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
 
class Country implements Serializable{
 
    private static final long serialVersionUID = 1L;
    String nation;
 
    public Country(String nation) {
        super();
        this.nation = nation;
    }
     
}
     
 
public class Person implements Cloneable,Serializable {
    private static final long serialVersionUID = 1L;
    int age;
    String name;
    Country country;
    public Person(int age, String name,Country country) {
        this.age = age;
        this.name = name;
        this.country=country;
    }
     
    public Person clone() throws CloneNotSupportedException  {
             
            Person p=null;
            ObjectInputStream ois=null;
            ObjectOutputStream oos=null;
            ByteArrayInputStream bais=null;
            ByteArrayOutputStream baos=null;
             
            try {
                baos=new ByteArrayOutputStream();
                oos=new ObjectOutputStream(baos);
                oos.writeObject(this);//Person对象序列化 序列化写入到baos流中
                 
                 
                bais=new ByteArrayInputStream(baos.toByteArray());
                ois=new ObjectInputStream(bais);//从baos流中读取数据到ois
                p=(Person) ois.readObject();//ois读取对象数据
            } catch (Exception e) {
                e.printStackTrace();
            }finally {
                if(bais!=null)
                    try {
                        bais.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                if(baos!=null)
                    try {
                        baos.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                if(oos!=null)
                    try {
                        oos.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                if(ois!=null)
                    try {
                        ois.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
            }
 
            return p;
    }
     
    public static void main(String[] args) throws CloneNotSupportedException {
        Country country=new Country("China");
        Person p1=new Person(18, "tom",country);
        Person p2=p1.clone();
        System.out.println(p1+"   name: "+p1.name+"  age: "+p1.age+"  country: "+p1.country);
        System.out.println(p2+"   name: "+p2.name+"  age: "+p2.age+"  country: "+p2.country);
        p1.name="bob";
        p1.country.nation="America";
        System.out.println(p1+"   name: "+p1.name+"  age: "+p1.age+"  country: "+p1.country.nation);
        System.out.println(p2+"   name: "+p2.name+"  age: "+p2.age+"  country: "+p2.country.nation);
         
    }
}

总结: 实现一个可克隆的类的步骤

  • 实现Cloneable接口
  • 重写clone方法
  • 在重写的clone方法中调用super.clone()方法
  • 在重写的clone方法中捕获异常

总结

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