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Java对象传递与返回的细节问题详析

CodePanda@GPF 人气:0

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

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

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:

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:

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

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

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

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

 protected native Object clone() throws CloneNotSupportedException;
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

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:

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

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

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

总结

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