Java save map to file

Writing from hashmap to a txt file

I have a hashMap of Integer,String (K,V) and want to write only the string values to a file (not the key Integer) and I want to write only some 1st n entries (no specific order) to the file and not the entire map. I have tried looking around a lot but could not find a way to write 1st n entries to file.(There are examples where I can convert the value to array of Strings and then do it] but then it does not provide the correct format in which I want to write the file)

then it does not provide the correct format in which I want to write the file what is this format you speak of?

I want the strings to be printed as they are. 1 on each line. and not in the form of a list [] i.e seperated by comma

3 Answers 3

This sounds like homework.

public static void main(String[] args) throws IOException < // first, let's build your hashmap and populate it HashMapmap = new HashMap(); map.put(1, "Value1"); map.put(2, "Value2"); map.put(3, "Value3"); map.put(4, "Value4"); map.put(5, "Value5"); // then, define how many records we want to print to the file int recordsToPrint = 3; FileWriter fstream; BufferedWriter out; // create your filewriter and bufferedreader fstream = new FileWriter("values.txt"); out = new BufferedWriter(fstream); // initialize the record count int count = 0; // create your iterator for your map Iterator it = map.entrySet().iterator(); // then use the iterator to loop through the map, stopping when we reach the // last record in the map or when we have printed enough records while (it.hasNext() && count < recordsToPrint) < // the key/value pair is stored here in pairs Map.Entrypairs = it.next(); System.out.println("Value is " + pairs.getValue()); // since you only want the value, we only care about pairs.getValue(), which is written to out out.write(pairs.getValue() + "\n"); // increment the record count once we have printed to the file count++; > // lastly, close the file and end out.close(); > 

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How to write a Map object to a file

You can write to a file with no problem if your Student class has a proper toString method. This code is not good. Print a stack trace or pass the exception along; don’t write a message to System.out. That’s less information than a stack trace. Returning true or false doesn’t help a user handle the issue.

there is half a dozen different ways to put an object graph on disk. look at serialisation. look at xml format. look at json format. etc. This question is too broad and we can’t reply.

Should it be possible to read the file and get the same object back? Should it be possible to read the file in some other program language? The fact that none of these things is mentioned in the Question contributes to it being Too Broad.

Simple: you first have to clarify your requirements. You can write all kinds of stuff into files. Whatever you do is fine. It only gets tricky if you ever want to read what was written there. So that is the thing you have to clarify first!

4 Answers 4

You can use tools like Jackson to write java maps out as json and read it in as map. As others have mentioned, this is only one possible way.

Full working Read/Write Example

@Test public void loadMapFromFileAndSaveIt() < Mapmap = loadMap("map.json"); map.put("8", "8th"); map.remove("7"); save(map,"/path/to/map2.txt"); > private Map loadMap(String string) < ObjectMapper mapper = new ObjectMapper(); try (InputStream in = Thread.currentThread().getContextClassLoader().getResourceAsStream("map.json")) < return mapper.readValue(in, HashMap.class); >catch (Exception e) < throw new RuntimeException(e); >> private void save(Map map,String path) < try (PrintWriter out = new PrintWriter(path)) < out.println(toString(map)); >catch (Exception e) < throw new RuntimeException(e); >> public String toString(Object obj) < try (StringWriter w = new StringWriter();) < new ObjectMapper().configure(SerializationFeature.INDENT_OUTPUT, true).writeValue(w, obj); return w.toString(); >catch (Exception e) < throw new RuntimeException(e); >> 

If the file map.json on your classpath contains

The code above will modify it and write it to a file /path/to/map2.txt that will contain

There is concept in java for saving state of object and retrieve it back later called ‘Serialization’.
To Write Object

File fileToSaveObject=new File("path"); Object objectToSave=new Object(); FileOutputStream fileOut = new FileOutputStream(fileToSaveObject); ObjectOutputStream out = new ObjectOutputStream(fileOut); out.writeObject(objectToSave); // It will save 'objectToSave' in given file out.close(); fileOut.close(); 

File fileToReadObject=new File("path"); Object objectToRead; FileInputStream fileIn = new FileInputStream(fileToReadObject); ObjectInputStream in = new ObjectInputStream(fileIn); objectToRead= (Object) in.readObject(); // It will return you the saved object in.close(); fileIn.close(); 

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Solved: save map to file java

In the world of programming, one common task developers often encounter is saving data to a file for future use, especially when it comes to maps and geographical data. In the Java programming language, there are numerous libraries and functions designed to help developers achieve this goal. In this article, we will explore a solution to save a map to a file using Java, provide a step-by-step explanation of the code, and delve into related libraries and functions that might be useful in tackling similar problems.

Introduction to Java Maps and File Handling

Java provides several data structures for handling and storing data efficiently. One such data structure is the Map, which stores data in key-value pairs. In the realm of geographical data, a map usually consists of coordinates, landmarks, and other relevant information. Saving this data to a file can help improve functionality, allowing applications to store and access the information more easily.

To achieve this, Java offers File Handling capabilities that allow developers to create, read, update, and delete files. Combining these capabilities with Maps, we can create a robust solution to save a map to a file.

Solution to Save a Map to File in Java

In our solution, we will utilize the HashMap class, provided by the java.util package, to create a map and then save it to a file using the ObjectOutputStream, which is part of the java.io package.

Step 1: Import necessary packages and create the main class.

import java.io.FileOutputStream; import java.io.IOException; import java.io.ObjectOutputStream; import java.util.HashMap; import java.util.Map; public class SaveMapToFile < public static void main(String[] args) < // code goes here >>

Step 2: Create a map and populate it with some sample data.

Map cityCoordinates = new HashMap<>(); cityCoordinates.put("New York", "40.7128,74.0060"); cityCoordinates.put("Los Angeles", "34.0522,118.2437"); cityCoordinates.put("Chicago", "41.8781,87.6298");

Step 3: Save the map to a file using ObjectOutputStream and FileOutputStream.

Now, the map has been successfully saved to a file named “mapData.ser”.

Java Libraries for Maps and File Handling

Various third-party libraries can provide additional functionality for working with maps and saving them to files. Some popular libraries include:

• Jackson: A high-performance JSON processor that can be used to serialize and deserialize Java Maps as JSON files.
• Google Gson: A library developed by Google that can convert Java Maps to JSON files or read JSON files into Java Maps.
• OpenStreetMap (OSM) Libraries: A collection of libraries that provide support for working with OpenStreetMap data, a popular source of map data used in various applications.

Exploring Alternative File Formats

In our solution, we utilized a binary file format (“.ser”) to save the map data. However, depending on the use case, alternative file formats might be more suitable. Some common formats include:

• JSON: A lightweight, human-readable format commonly used for storing and exchanging data.
• XML: A markup language used for encoding documents, allowing for more extensive metadata and organization of data.
• CSV: A simple text format for storing tabular data, ideal for importing and exporting data from spreadsheets and databases.
• KML: A file format specifically designed for displaying geographic data in mapping applications, such as Google Earth.

By understanding the various techniques and tools available in Java for saving maps to files, developers can create more versatile and powerful applications, catering to different use cases and requirements.

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How to write and read a file with a HashMap? [duplicate]

I have a HashMap with two Strings Map ldapContent = new HashMap . 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?

i just know the simple filereader. therefore thank for the serialization tip. i don’t know it before.

4 Answers 4

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

Map ldapContent = new HashMap(); Properties properties = new Properties(); for (Map.Entry entry : ldapContent.entrySet()) < properties.put(entry.getKey(), entry.getValue()); >properties.store(new FileOutputStream("data.properties"), null); 

Map ldapContent = new HashMap(); Properties properties = new Properties(); properties.load(new FileInputStream("data.properties")); for (String key : properties.stringPropertyNames())

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(); 

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(properties); 

As Properties implements Map you should be able to use putAll to populate it. Also, since HashMap has a HashMap(Map m) constructor you should be able to populate it on construction for the same reason.

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?

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 < HashMaphm; public A() < hm=new HashMap(); hm.put("1","A"); hm.put("2","B"); hm.put("3","C"); method1(hm); > public void method1(HashMap 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); HashMapmapInFile=(HashMap)ois.readObject(); ois.close(); fis.close(); //print All data in MAP for(Map.Entry 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 < HashMaphm; public A()< hm=new HashMap(); hm.put("1","A"); hm.put("2","B"); hm.put("3","C"); method2(hm); > public void method2(HashMap map) < //write to file : "fileone" try < File fileTwo=new File("filetwo.txt"); FileOutputStream fos=new FileOutputStream(fileTwo); PrintWriter pw=new PrintWriter(fos); for(Map.Entrym :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); HashMapmapInFile=new HashMap(); //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 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

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