反序列化HashMap dosent給我所有的元素

Question

我想從.dat文件中檢索多個hashmaps 但是它只給我第一個存儲！

public class TestSerialize 
{ 
    public static void main(String args[]) throws IOException 
    { 
     HashMap<String, String> students = new HashMap<String, String>(); 

     // This part I changed everytime to keep the data stored in "students.dat" 
     // I want to retrieve all the keys that have been run 
     students.put("22", "xxx"); 
     students.put("33", "yyy"); 

     ObjectOutputStream out = new ObjectOutputStream(
      new FileOutputStream("students.dat", true)); 
     out.writeObject(students); 
     out.close();  
    } 
} 

下面的代碼是用來反序列化

public class TestSerialize2 
{ 
    public static void main(String args[]) throws IOException, ClassNotFoundException 
    { 
     ObjectInputStream in = new ObjectInputStream(
      new FileInputStream("students.dat")); 
     HashMap<String, String> students = new HashMap<String, String>(); 

     students = (HashMap<String, String>) in.readObject(); 
     in.close();  

     for (String s : students.keySet()) 
      System.out.println(s); 
    } 
} 

我希望這是明確的..

（在萬阿英，蔣達清出現運行（TestSerialize類惠蔭）第二次與提供不同的密鑰學生..（TestSerialize2類）的輸出將是舊的值只有22,33）希望這是幫助

---

在您的意見我試試這個： 但同樣的問題仍然存在！

package week8; 
 

 
import java.io.File; 
 
import java.io.FileInputStream; 
 
import java.io.FileOutputStream; 
 
import java.io.IOException; 
 
import java.io.ObjectInputStream; 
 
import java.io.ObjectOutputStream; 
 
import java.util.ArrayList; 
 
import java.util.HashMap; 
 

 
public class TestSerialize 
 
{ 
 
\t public static void main(String args[]) throws IOException, ClassNotFoundException 
 
\t { 
 
\t \t HashMap<String, String> students = new HashMap<String, String>(); 
 

 
\t \t // This part I changed everytime to keep the data stored in "students.dat" 
 
\t \t // I want to retrieve all the keys that have been run 
 
\t \t students.put("66", "xxx"); 
 
\t \t students.put("99", "yyy"); 
 

 
\t \t ObjectOutputStream out = new ObjectOutputStream(
 
\t \t \t \t new FileOutputStream("students.dat", true)); 
 

 

 
\t \t File file = new File("students.dat"); 
 
\t \t if (file.length() == 0){ 
 
\t \t \t System.out.print("sss"); 
 
\t \t \t out.writeObject(students); 
 
\t \t \t out.close();  
 
\t \t }else 
 
\t \t { 
 

 
\t \t \t students =readThenWrite(students); 
 
\t \t \t for (String s : students.keySet()) 
 
\t    System.out.println(s); 
 
\t \t \t 
 
\t \t \t out.writeObject(students); 
 
\t \t \t out.close(); 
 
\t \t \t 
 
\t \t \t 
 
\t   
 

 
\t \t } 
 

 
\t } 
 

 
\t private static HashMap<String, String> readThenWrite(HashMap<String, String> students) { 
 
     HashMap<String, String> ss = new HashMap<String, String>(); 
 

 
     ObjectInputStream in; 
 
\t \t try { 
 
\t \t \t in = new ObjectInputStream(
 
\t \t \t    new FileInputStream("students.dat")); 
 
\t \t 
 

 
\t \t   ss = (HashMap<String, String>) in.readObject(); 
 
\t \t  
 
\t \t   in.close();  
 

 
\t \t   for (String s : students.keySet()) 
 
\t \t    ss.put(s, students.get(s)); 
 
\t \t   
 
\t \t   
 
\t \t   
 
\t \t   
 
\t \t } catch (IOException | ClassNotFoundException e) { 
 
    \t \t \t e.printStackTrace(); 
 
\t \t } 
 
\t \t 
 
\t \t return ss; 
 
\t } 
 
}

Answer 1

new FileOutputStream("students.dat", true)); 

的問題是在這裏。第二個參數表示您添加新數據，而不是覆蓋它，但是您的閱讀代碼不會嘗試從文件中讀取多個地圖。事實上，不能，因爲你不能附加到對象流，至少不會這樣。

刪除true參數。