Java read file to String

Filed Under: Java

Sometimes while working with files, we need to read the file to String in Java. Today we will look into various ways to read the file to String in Java.

Java read file to String

There are many ways to read a file to String in Java. We will explore the following ways in this tutorial.

  1. Java read file to String using BufferedReader
  2. Read file to String in java using FileInputStream
  3. Java read file to string using Files class
  4. Read file to String using Scanner class
  5. Java read file to string using Apache Commons IO FileUtils class

java read file to string

Now let’s look into these classes and read a file to String.

Java read file to String using BufferedReader

We can use BufferedReader readLine method to read a file line by line. All we have to do is append these to a StringBuilder object with newline character. Below is the code snippet to read the file to String using BufferedReader.


BufferedReader reader = new BufferedReader(new FileReader(fileName));
StringBuilder stringBuilder = new StringBuilder();
String line = null;
String ls = System.getProperty("line.separator");
while ((line = reader.readLine()) != null) {
	stringBuilder.append(line);
	stringBuilder.append(ls);
}
// delete the last new line separator
stringBuilder.deleteCharAt(stringBuilder.length() - 1);
reader.close();

String content = stringBuilder.toString();

There is another efficient way to read file to String using BufferedReader and char array.


BufferedReader reader = new BufferedReader(new FileReader(fileName));
StringBuilder stringBuilder = new StringBuilder();
char[] buffer = new char[10];
while (reader.read(buffer) != -1) {
	stringBuilder.append(new String(buffer));
	buffer = new char[10];
}
reader.close();

String content = stringBuilder.toString();

Read file to String in java using FileInputStream

We can use FileInputStream and byte array to read file to String. You should use this method to read non-char based files such as image, video etc.


FileInputStream fis = new FileInputStream(fileName);
byte[] buffer = new byte[10];
StringBuilder sb = new StringBuilder();
while (fis.read(buffer) != -1) {
	sb.append(new String(buffer));
	buffer = new byte[10];
}
fis.close();

String content = sb.toString();

Java read file to string using Files class

We can use Files utility class to read all the file content to string in a single line of code.


String content = new String(Files.readAllBytes(Paths.get(fileName)));

Read file to String using Scanner class

The scanner class is a quick way to read a text file to string in java.


Scanner scanner = new Scanner(Paths.get(fileName), StandardCharsets.UTF_8.name());
String content = scanner.useDelimiter("\\A").next();
scanner.close();

Java read file to string using Apache Commons IO FileUtils class

If you are using Apache Commons IO in your project, then this is a simple and quick way to read the file to string in java.


String content = FileUtils.readFileToString(new File(fileName), StandardCharsets.UTF_8);

Java read file to String example

Here is the final program with proper exception handling and showing all the different ways to read a file to string.


package com.journaldev.files;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Scanner;

import org.apache.commons.io.FileUtils;

public class JavaReadFileToString {

	/**
	 * This class shows different ways to read complete file contents to String
	 * 
	 * @param args
	 * @throws IOException
	 */
	public static void main(String[] args) {
		String fileName = "/Users/pankaj/Downloads/myfile.txt";

		String contents = readUsingScanner(fileName);
		System.out.println("*****Read File to String Using Scanner*****\n" + contents);

		contents = readUsingApacheCommonsIO(fileName);
		System.out.println("*****Read File to String Using Apache Commons IO FileUtils*****\n" + contents);

		contents = readUsingFiles(fileName);
		System.out.println("*****Read File to String Using Files Class*****\n" + contents);

		contents = readUsingBufferedReader(fileName);
		System.out.println("*****Read File to String Using BufferedReader*****\n" + contents);

		contents = readUsingBufferedReaderCharArray(fileName);
		System.out.println("*****Read File to String Using BufferedReader and char array*****\n" + contents);

		contents = readUsingFileInputStream(fileName);
		System.out.println("*****Read File to String Using FileInputStream*****\n" + contents);

	}

	private static String readUsingBufferedReaderCharArray(String fileName) {
		BufferedReader reader = null;
		StringBuilder stringBuilder = new StringBuilder();
		char[] buffer = new char[10];
		try {
			reader = new BufferedReader(new FileReader(fileName));
			while (reader.read(buffer) != -1) {
				stringBuilder.append(new String(buffer));
				buffer = new char[10];
			}
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			if (reader != null)
				try {
					reader.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
		}

		return stringBuilder.toString();
	}

	private static String readUsingFileInputStream(String fileName) {
		FileInputStream fis = null;
		byte[] buffer = new byte[10];
		StringBuilder sb = new StringBuilder();
		try {
			fis = new FileInputStream(fileName);

			while (fis.read(buffer) != -1) {
				sb.append(new String(buffer));
				buffer = new byte[10];
			}
			fis.close();

		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			if (fis != null)
				try {
					fis.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
		}
		return sb.toString();
	}

	private static String readUsingBufferedReader(String fileName) {
		BufferedReader reader = null;
		StringBuilder stringBuilder = new StringBuilder();

		try {
			reader = new BufferedReader(new FileReader(fileName));
			String line = null;
			String ls = System.getProperty("line.separator");
			while ((line = reader.readLine()) != null) {
				stringBuilder.append(line);
				stringBuilder.append(ls);
			}
			// delete the last ls
			stringBuilder.deleteCharAt(stringBuilder.length() - 1);
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			if (reader != null)
				try {
					reader.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
		}

		return stringBuilder.toString();
	}

	private static String readUsingFiles(String fileName) {
		try {
			return new String(Files.readAllBytes(Paths.get(fileName)));
		} catch (IOException e) {
			e.printStackTrace();
			return null;
		}
	}

	private static String readUsingApacheCommonsIO(String fileName) {
		try {
			return FileUtils.readFileToString(new File(fileName), StandardCharsets.UTF_8);
		} catch (IOException e) {
			e.printStackTrace();
			return null;
		}
	}

	private static String readUsingScanner(String fileName) {
		Scanner scanner = null;
		try {
			scanner = new Scanner(Paths.get(fileName), StandardCharsets.UTF_8.name());
			// we can use Delimiter regex as "\\A", "\\Z" or "\\z"
			String data = scanner.useDelimiter("\\A").next();
			return data;
		} catch (IOException e) {
			e.printStackTrace();
			return null;
		} finally {
			if (scanner != null)
				scanner.close();
		}

	}

}

You can use any of the above ways to read file content to string in java. However, it’s not advisable if the file size is huge because you might face out of memory errors.

You can checkout more Java IO examples from our GitHub Repository.

References:

Comments

  1. K says:

    Very nice. Just a note. reader.close() should be added to readUsingBufferedReader()

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