java - 反序列化为大小为 0 的数组

Question

在 Treets 类的 load() 方法中，我声明了一个大小为 0 的 Treet 对象数组。但是，在反序列化时，我将 readObject() 方法的结果保存到同一个数组中。碰巧我首先序列化了 2 个对象，所以反序列化时我也得到了 2 个对象。当我将 2 个对象插入先前初始化的大小为 0 的数组时，为什么没有收到 ArrayIndexOutOfBoundsException 异常？

Treet.java

import java.util.Date;
import java.io.Serializable;
public class Treet implements Comparable<Treet>, Serializable {
    private String mAuthor;
    private String mDescription;
    private Date mCreationDate;
    Treet(String author, String description, Date creationDate) {
        mAuthor = author;
        mDescription = description;
        mCreationDate = creationDate;
    }
    public Date getCreationDate() {
        return mCreationDate;
    }
    @Override
    public String toString() {
        return String.format("Treet by %s: \"%s\" on %s", mAuthor, mDescription, mCreationDate);
    }
    @Override
    public int compareTo(Treet anotherTreet) {
        if(equals(anotherTreet)) {
            return 0;
        }
        return mCreationDate.compareTo(anotherTreet.getCreationDate());
    }
}

Treets.java

import java.io.*;
public class Treets {
    public static void save(Treet[] treets) {
        try(
            FileOutputStream fos = new FileOutputStream("treets.ser");
            ObjectOutputStream oos = new ObjectOutputStream(fos);
        )
        {
            oos.writeObject(treets);
        } catch(IOException ioe) {
            System.out.println(ioe.getMessage());
        }
    }
    public static Treet[] load() {
        Treet[] treets = new Treet[0];
        try(
            FileInputStream fis = new FileInputStream("treets.ser");
            ObjectInputStream ois = new ObjectInputStream(fis);
        )
        {           
            treets = (Treet[])ois.readObject();
        } catch(IOException ioe) {
            System.out.println(ioe.getMessage());
        } catch(ClassNotFoundException cnfe) {
            System.out.println(cnfe.getMessage());
        }
        return treets;
    }
}

Main.java

import java.util.Date;
import java.util.Arrays;
public class Main {
    public static void main(String[] args) {
        Treet treetOne = new Treet("John", "This is treet one", new Date());
        Treet treetTwo = new Treet("Jane", "This is treet two", new Date(1526074720000L));
        Treet[] treets = {treetOne, treetTwo};
        Arrays.sort(treets);
        Treets.save(treets);
        Treet[] loadedTreets = Treets.load();
        for(Treet treet : loadedTreets) {
            System.out.println(treet.toString());
        }
    }
}

Answer 1

您误解了 ObjectInputStream.read 的概念。

它不会填充您声明的数组，而是返回一个包含文件内容的新数组

//A
Treet[] treets = new Treet[0]; //at this moment you are declaring the variable and initializing it with an array of 0 spaces
    try(
        FileInputStream fis = new FileInputStream("treets.ser");
        ObjectInputStream ois = new ObjectInputStream(fis);
    )
    {   //B        
        treets = (Treet[])ois.readObject(); //at this moment you are replacing the pointer of the previous 0 length array to a new one read from the file

    } catch(IOException ioe) {
        System.out.println(ioe.getMessage());
    } catch(ClassNotFoundException cnfe) {
        System.out.println(cnfe.getMessage());
    }
    return treets;
}

换句话说:在“A”上，变量treets指向内存X中的一个地址......在“B”上，变量更改为指向内存 Y 中的新地址

程序结束后，X 的内容仍然是一个长度为零的数组，但没有变量指向它。

如果你想测试你可以这样做:

Treet[] treets = new Treet[0]; 
Treet[] treetsOld = treets;
    try(
System.out.println(treetsOld == treets);
        FileInputStream fis = new FileInputStream("treets.ser");
        ObjectInputStream ois = new ObjectInputStream(fis);
    )
    {          
        treets = (Treet[])ois.readObject(); 
System.out.println(treetsOld == treets);
    } catch(IOException ioe) {
        System.out.println(ioe.getMessage());
    } catch(ClassNotFoundException cnfe) {
        System.out.println(cnfe.getMessage());
    }
    return treets;
}