Java對(duì)象傳遞與返回的細(xì)節(jié)問題詳析

更新時(shí)間：2022年11月04日 11:15:09 作者：CodePanda@GPF

我們知道這是一個(gè)核心概念,在Java中總是按值傳遞而不是按引用傳遞,下面這篇文章主要給大家介紹了關(guān)于Java對(duì)象傳遞與返回的細(xì)節(jié)問題的相關(guān)資料,需要的朋友可以參考下

1.傳遞引用

在一個(gè)方法中將一個(gè)對(duì)象的引用傳遞給另外一個(gè)方法，引用指向的對(duì)象是同一個(gè)

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);
	}
}

引用別名

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指向的是同一個(gè)對(duì)象，p2對(duì)對(duì)象的屬性進(jìn)行操作，當(dāng)使用引用p訪問對(duì)象的屬性時(shí)當(dāng)然也改變，同樣的情況也發(fā)生在對(duì)象引用在方法之間的傳遞，如下面的代碼：

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. 創(chuàng)建本地副本

幾個(gè)概念：

引用別名會(huì)在方法參數(shù)是對(duì)象類型時(shí)自動(dòng)發(fā)生
沒有本地對(duì)象，只有本地引用（方法中創(chuàng)建的對(duì)象存在于堆中，只有引用變量存在于方法棧中）
引用是有作用域的，而對(duì)象沒有
Java中的對(duì)象的生命周期并不是一個(gè)問題（垃圾回收機(jī)制）

2.1 值傳遞

Java中方法之間只有值傳遞

傳遞基本類型時(shí)，傳遞的是基本類型的值的拷貝

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

傳遞對(duì)象時(shí)，傳遞的是對(duì)象的引用拷貝

public static void main(String[] args) {
		Person p=new Person(18, "tom");
		f(p);
		System.out.println(p.age);//還是18 f()中p指向了一個(gè)新的對(duì)象 不影響main函數(shù)
	}
	public static void f(Person p) {
		p=new Person(20, "bob");
	}

傳遞對(duì)象時(shí)如果在另外一個(gè)方法中對(duì)對(duì)象的屬性進(jìn)行操作會(huì)對(duì)main方法產(chǎn)生“副作用”，但是如果只是簡單的對(duì)引用進(jìn)行操作是沒有影響的

2.2 對(duì)象克隆

步驟：

類實(shí)現(xiàn)Cloneable空接口，默認(rèn)情況下不希望所有的類都有克隆能力，當(dāng)需要某個(gè)類有克隆能力時(shí)就需要實(shí)現(xiàn)該接口作為一種“可克隆”的標(biāo)記，否則克隆時(shí)會(huì)報(bào)錯(cuò)CloneNotSupportedException

public interface Cloneable {
}

重寫clone方法，clone方法是Object類中的，它在Object類中的是一個(gè)protected的本地方法，需要重寫，加上public修飾符，否則只能在當(dāng)前類中使用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);
	}
}

重寫clone方法時(shí)實(shí)際是就是調(diào)用Object類中的本地clone方法，Object類中的clone方法做了哪些工作？

Object類中clone方法負(fù)責(zé)創(chuàng)建正確大小的存儲(chǔ)空間，并執(zhí)行了從原始對(duì)象中所有二進(jìn)制位到新對(duì)象內(nèi)存中的按位復(fù)制。

2.3 淺拷貝問題

場景：Person類中增加一個(gè)引用類型的屬性Country, 表示這個(gè)人所屬的國家，然后進(jìn)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);
	}
}

問題描述：當(dāng)Person類中有一個(gè)引用類型屬性時(shí)，對(duì)于基本類型數(shù)據(jù)和String類型是對(duì)值進(jìn)行拷貝的，因此改變p1的name不影響p2的name, 但是對(duì)于Country這種引用類型，在clone時(shí)僅僅是拷貝了一份對(duì)象的引用，拷貝的引用和原來的引用指向的是同一個(gè)對(duì)象，因此p1改變country的屬性時(shí)，p2的country的屬性也發(fā)生了改變

2.4 深拷貝

解決淺拷貝問題的關(guān)鍵點(diǎn)在于不僅僅是拷貝引用，而且要拷貝一份引用指向的對(duì)象，深拷貝有兩種方式：

逐個(gè)對(duì)引用指向的對(duì)象進(jìn)行淺拷貝
使用序列化方式進(jìn)行深拷貝

2.4.1 引用類型逐個(gè)淺拷貝

如果一個(gè)類A中有多個(gè)引用類型，那么這些引用類型的類需要實(shí)現(xiàn) Cloneable接口，在對(duì)A的對(duì)象進(jìn)行克隆時(shí)，逐個(gè)淺拷貝其中的引用類型

eg: Person類中有Country引用類型，在進(jìn)行clone時(shí)，單獨(dú)對(duì)country進(jìn)行淺拷貝

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();//對(duì)country進(jìn)行淺拷貝
			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 序列化方式進(jìn)行深拷貝

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對(duì)象序列化 序列化寫入到baos流中
				
				
				bais=new ByteArrayInputStream(baos.toByteArray());
				ois=new ObjectInputStream(bais);//從baos流中讀取數(shù)據(jù)到ois
				p=(Person) ois.readObject();//ois讀取對(duì)象數(shù)據(jù)
			} 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);
		
	}
}