java - 如何使用 HashMap 读写文件？

Question

我有HashMap两个 Strings Map<String, String> ldapContent = new HashMap<String, String>。

现在我想将其保存Map在一个外部文件中以供Map以后使用，而无需再次初始化它......

那么我必须如何保存Map以便以后再次使用它呢？

Answer 1

The simplest solution that I can think of is using Properties class.

Saving the map:

Map<String, String> ldapContent = new HashMap<String, String>();
Properties properties = new Properties();

for (Map.Entry<String,String> entry : ldapContent.entrySet()) {
    properties.put(entry.getKey(), entry.getValue());
}

properties.store(new FileOutputStream("data.properties"), null);

Loading the map:

Map<String, String> ldapContent = new HashMap<String, String>();
Properties properties = new Properties();
properties.load(new FileInputStream("data.properties"));

for (String key : properties.stringPropertyNames()) {
   ldapContent.put(key, properties.get(key).toString());
}

EDIT:

if your map contains plaintext values, they will be visible if you open file data via any text editor, which is not the case if you serialize the map:

ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data.ser"));
out.writeObject(ldapContent);
out.close();

EDIT2:

instead of for loop (as suggested by OldCurmudgeon) in saving example:

properties.putAll(ldapContent);

however, for the loading example this is the best that can be done:

ldapContent = new HashMap<Object, Object>(properties);

Answer 2

由于HashMap实现了Serializable接口，您可以简单地使用ObjectOutputStream类将整个写入文件，然后使用类 Map再次读取它ObjectInputStream

下面的简单代码解释了ObjectOutStream和的用法ObjectInputStream

import java.util.*;
import java.io.*;
public class A{

    HashMap<String,String> hm;
    public A() {
        hm=new HashMap<String,String>();

        hm.put("1","A");
        hm.put("2","B");
        hm.put("3","C");

        method1(hm);

    }

public void method1(HashMap<String,String> map) {
    //write to file : "fileone"
    try {
        File fileOne=new File("fileone");
        FileOutputStream fos=new FileOutputStream(fileOne);
        ObjectOutputStream oos=new ObjectOutputStream(fos);

        oos.writeObject(map);
        oos.flush();
        oos.close();
        fos.close();
    } catch(Exception e) {}

    //read from file 
    try {
        File toRead=new File("fileone");
        FileInputStream fis=new FileInputStream(toRead);
        ObjectInputStream ois=new ObjectInputStream(fis);

        HashMap<String,String> mapInFile=(HashMap<String,String>)ois.readObject();

        ois.close();
        fis.close();
        //print All data in MAP
        for(Map.Entry<String,String> m :mapInFile.entrySet()){
            System.out.println(m.getKey()+" : "+m.getValue());
        }
    } catch(Exception e) {}
  }

public static void main(String args[]) {
        new A();
}

}

或者，如果您想将数据作为文本写入文件，您可以简单地迭代Map并逐行写入键和值，然后逐行再次读取并添加到HashMap

import java.util.*;
import java.io.*;
public class A{

    HashMap<String,String> hm;
    public A(){
        hm=new HashMap<String,String>();

        hm.put("1","A");
        hm.put("2","B");
        hm.put("3","C");

        method2(hm);

    }

public void method2(HashMap<String,String> map) {
    //write to file : "fileone"
    try {
        File fileTwo=new File("filetwo.txt");
        FileOutputStream fos=new FileOutputStream(fileTwo);
        PrintWriter pw=new PrintWriter(fos);

        for(Map.Entry<String,String> m :map.entrySet()){
            pw.println(m.getKey()+"="+m.getValue());
        }

        pw.flush();
        pw.close();
        fos.close();
    } catch(Exception e) {}

    //read from file 
    try {
        File toRead=new File("filetwo.txt");
        FileInputStream fis=new FileInputStream(toRead);

        Scanner sc=new Scanner(fis);

        HashMap<String,String> mapInFile=new HashMap<String,String>();

        //read data from file line by line:
        String currentLine;
        while(sc.hasNextLine()) {
            currentLine=sc.nextLine();
            //now tokenize the currentLine:
            StringTokenizer st=new StringTokenizer(currentLine,"=",false);
            //put tokens ot currentLine in map
            mapInFile.put(st.nextToken(),st.nextToken());
        }
        fis.close();

        //print All data in MAP
        for(Map.Entry<String,String> m :mapInFile.entrySet()) {
            System.out.println(m.getKey()+" : "+m.getValue());
        }
    }catch(Exception e) {}
  }

public static void main(String args[]) {
        new A();
}

}

注意：上面的代码可能不是完成这项任务的最快方法，但我想展示一些类的应用

参见ObjectOutputStream、ObjectInputStream、HashMap、Serializable、StringTokenizer

Answer 3

HashMap实现Serializable，因此您可以使用正常的序列化将 hashmap 写入文件

这是Java的链接-序列化示例

Answer 4

writeObject您可以使用ObjectOutputStream将对象写入文件

对象输出流