How to write and read a file with a HashMap?

Question

I have a HashMap with two Strings Map<String, String> ldapContent = new HashMap<String, String>.

Now I want to save the Map in an external file to use the Map later without initializing it again...

So how must I save the Map to use it later again?

What have you tried? You can use Java serialization, XML, CSV, binary data manually... — Tomasz Nurkiewicz, Oct 05 '12 at 14:04
i just know the simple filereader. therefore thank for the serialization tip. i don't know it before... — , Oct 05 '12 at 14:15

linski · Answer 1 · 2012-10-05T14:55:27.063

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);

As `Properties` implements `Map` you should be able to use `putAll` to populate it. Also, since `HashMap` has a `HashMap(Map extends K,? extends V> m)` constructor you should be able to populate it on construction for the same reason. — OldCurmudgeon, Oct 05 '12 at 14:21
This solution does not maintain the order from Hashmap. Is there something that can write to the file from a TreeMap where insertion order is maintained in the map? — Swapnil Jaju, Aug 29 '18 at 02:52

score 23 · Answer 2 · edited Jan 08 '20 at 15:27

since HashMap implements Serializable interface, you can simply use ObjectOutputStream class to write whole Map to file, and read it again using ObjectInputStream class

below simple code that explain usage of ObjectOutStream and 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();
}

}

or if you want to write data as text to file you can simply iterate through Map and write key and value line by line, and read it again line by line and add to 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();
}

}

NOTE: above code may not be the fastest way to doing this task, but i want to show some application of classes

See ObjectOutputStream , ObjectInputStream, HashMap, Serializable, StringTokenizer

Amit Deshpande · Accepted Answer · 2012-10-05T14:10:21.303

21

HashMap implements Serializable so you can use normal serialization to write hashmap to file

Here is the link for Java - Serialization example

edited Oct 05 '12 at 14:10

answered Oct 05 '12 at 14:04

Amit Deshpande

19,001
4
46
72

score 2 · Answer 4 · edited Oct 05 '12 at 14:08

2

You can write an object to a file using writeObject in ObjectOutputStream

ObjectOutputStream

edited Oct 05 '12 at 14:08

Michael Schmidt

9,090
13
56
80

answered Oct 05 '12 at 14:05

arutaku

5,937
1
24
38

How to write and read a file with a HashMap?

4 Answers4

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