Cactoos提供了一个声明性的一行代码:

new TextOf(new File("a.txt")).asString();

这基本上与Jesus Ramos的回答完全相同，除了使用File而不是FileReader加上迭代来逐级遍历文件的内容。

Scanner in = new Scanner(new File("filename.txt"));

while (in.hasNext()) { // Iterates each line in the file
    String line = in.nextLine();
    // Do something with line
}

in.close(); // Don't forget to close resource leaks

．.． 抛出FileNotFoundException

Guava提供了一个简单的例子:

import com.google.common.base.Charsets;
import com.google.common.io.Files;

String contents = Files.toString(filePath, Charsets.UTF_8);

try {
  File f = new File("filename.txt");
  Scanner r = new Scanner(f);  
  while (r.hasNextLine()) {
    String data = r.nextLine();
    JOptionPane.showMessageDialog(data);
  }
  r.close();
} catch (FileNotFoundException ex) {
  JOptionPane.showMessageDialog("Error occurred");
  ex.printStackTrace();
}

Java 11 Files.readString中引入了最直观的方法

import java.io.*;
import java.nio.file.Files;
import java.nio.file.Paths;

public class App {
    public static void main(String args[]) throws IOException {
        String content = Files.readString(Paths.get("D:\\sandbox\\mvn\\my-app\\my-app.iml"));
        System.out.print(content);
    }
}

PHP几十年来一直享有这种特权!☺

Cactoos提供了一个声明性的一行代码:

new TextOf(new File("a.txt")).asString();