Java: Read file content as string

Approach 1: Using Scanner

String content = new Scanner(new File(filePath)).useDelimiter("\\Z").next();

Approach 2: Using Files.readAllBytes.

String content = new String(Files.readAllBytes(Paths.get(filePath)), StandardCharsets.UTF_8);

Approach 3: Using FileInputStream.

public static String readFileContentViaStream(String filePath) throws IOException {
 validateInput(filePath);

 try (FileInputStream fis = new FileInputStream(new File(filePath))) {
  StringBuilder builder = new StringBuilder();

  int content;
  while ((content = fis.read()) != -1) {
   builder.append((char) content);
  }

  return builder.toString();

 }

}

Approach 4: Using FileChannel.

public static String readFileContentViaChannel(String filePath) throws IOException {
 validateInput(filePath);
 try (FileInputStream fin = new FileInputStream(filePath)) {
  FileChannel fileChannel = fin.getChannel();

  ByteBuffer buffer = ByteBuffer.allocate(1024);

  int noOfBytesRead;
  StringBuilder builder = new StringBuilder();

  while ((noOfBytesRead = fileChannel.read(buffer)) != -1) {
   String str = new String(buffer.array());
   builder.append(str);
   buffer.flip();

  }

  return builder.toString();
 }

}

Find the below working application.

App.java    

package com.sample.app;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Scanner;

public class App {

 private static void validateInput(String filePath) {
  if (filePath == null || filePath.isEmpty()) {
   throw new IllegalArgumentException("filePath is null or empty");
  }

  File file = new File(filePath);

  if (!file.exists()) {
   throw new RuntimeException("File is not exist");
  }

  if (file.isDirectory()) {
   throw new RuntimeException("File is a directory");
  }
 }

 public static String readFileContentViaScanner(String filePath) throws FileNotFoundException {
  validateInput(filePath);
  return new Scanner(new File(filePath)).useDelimiter("\\Z").next();
 }

 public static String readFileContentViaReadingAllBytes(String filePath) throws IOException {
  validateInput(filePath);
  return new String(Files.readAllBytes(Paths.get(filePath)), StandardCharsets.UTF_8);
 }

 public static String readFileContentViaStream(String filePath) throws IOException {
  validateInput(filePath);

  try (FileInputStream fis = new FileInputStream(new File(filePath))) {
   StringBuilder builder = new StringBuilder();

   int content;
   while ((content = fis.read()) != -1) {
    builder.append((char) content);
   }

   return builder.toString();

  }

 }

 public static String readFileContentViaChannel(String filePath) throws IOException {
  validateInput(filePath);
  try (FileInputStream fin = new FileInputStream(filePath)) {
   FileChannel fileChannel = fin.getChannel();

   ByteBuffer buffer = ByteBuffer.allocate(1024);

   int noOfBytesRead;
   StringBuilder builder = new StringBuilder();

   while ((noOfBytesRead = fileChannel.read(buffer)) != -1) {
    String str = new String(buffer.array());
    builder.append(str);
    buffer.flip();

   }

   return builder.toString();
  }

 }

 public static void main(String args[]) throws IOException {
  String filePath = "data.txt";

  String fileContent1 = readFileContentViaScanner(filePath);
  String fileContent2 = readFileContentViaReadingAllBytes(filePath);
  String fileContent3 = readFileContentViaStream(filePath);
  String fileContent4 = readFileContentViaChannel(filePath);

  System.out.println(fileContent1);
  System.out.println(fileContent2);
  System.out.println(fileContent3);
  System.out.println(fileContent4);
 }

}

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