Java 简明教程

Java - Reading File

Reading a File in Java

在Java中，我们可以使用多种方法读取一个文件。以下是使用Java创建文件的三种最流行方法 −

We can read a file in Java using multiple ways. Following are three most popular ways to create a file in Java −

Using FileInputStream() constructor
Using FileReader.read() method
Using Files.readAllLines() method

让我们了解一下每一种阅读Java文件中内容的方法。

Let’s take a look at each of the way to read file in Java.

Reading File Using FileInputStream() Constructor

FileInputStream 用于从文件读取数据。可以使用关键字 new 创建对象，并且有几种类型的构造函数可用。

FileInputStream is used for reading data from the files. Objects can be created using the keyword new and there are several types of constructors available.

Syntax

以下 constructor 将一个文件名作为字符串，创建一个输入流对象来读取文件——

Following constructor takes a file name as a string to create an input stream object to read the file −

InputStream f = new FileInputStream("C:/java/hello.txt");

Syntax

以下构造函数获取文件对象以创建输入流对象来读取文件。我们首先使用 File() 方法创建一个文件对象，如下所示 −

Following constructor takes a file object to create an input stream object to read the file. First we create a file object using File() method as follows −

File f = new File("C:/java/hello.txt");
InputStream f = new FileInputStream(f);

Example: Reading File Using FileInputStream() Constructor

以下示例演示了 FileInputStream 从当前目录读取文件 −

Following is the example to demonstrate FileInputStream to read a file from current directory −

package com.tutorialspoint;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class FileTest {
   public static void main(String args[]) {
      try {
         byte bWrite [] = {65, 66, 67, 68, 69};
         OutputStream os = new FileOutputStream("test.txt");
         for(int x = 0; x < bWrite.length ; x++) {
            os.write( bWrite[x] );   // writes the bytes
         }
         os.close();

         InputStream is = new FileInputStream("test.txt");
         int size = is.available();

         for(int i = 0; i < size; i++) {
            System.out.print((char)is.read() + "  ");
         }
         is.close();
      } catch (IOException e) {
         System.out.print("Exception");
      }
   }
}

上面的代码将创建文件 test.txt，并以二进制格式写入给定的数字。使用 FileInputStream 会读取相同内容，输出打印在 stdout 屏幕上。

The above code would create file test.txt and would write given numbers in binary format. Same would be read using FileInputStream and the output is printed on the stdout screen.

A  B  C  D  E

Reading File Using FileReader.read() Method

FileReader class 的 FileReader.read() 方法允许从文件读取字符，如下所示——

FileReader.read() method of FileReader class allows to read chars from a file as shown below −

Syntax

// get an existing file
File file = new File("d://test//testFile1.txt");

// read content
FileReader reader = new FileReader(file);

int c;
while ((c = reader.read()) != -1) {
   char ch = (char) c;
   System.out.print(ch);
}

Example: Reading File Using FileReader.read() Method

以下示例演示文件，该文件使用 FileReader.read() 方法读取给定目录中的文件 −

Following is the example to demonstrate File to read a file in given directory using FileReader.read() method −

package com.tutorialspoint;

import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class FileTest {
   public static void main(String args[]) {
      try {
         File file = new File("d://test//testFile1.txt");

         //Create the file
         if (file.createNewFile()) {
            System.out.println("File is created!");
         } else {
            System.out.println("File already exists.");
         }

         // Write Content
         FileWriter writer = new FileWriter(file);
         writer.write("Test data");
         writer.close();


         // read content
         FileReader reader = new FileReader(file);

         int c;
         while ((c = reader.read()) != -1) {
            char ch = (char) c;
            System.out.print(ch);
         }
      } catch (IOException e) {
         System.out.print("Exception");
      }
   }
}

上面的代码将创建文件 test.txt，并将指定的字符串写入文本格式。stdout 屏幕上将有相同输出。

The above code would create file test.txt and would write given string in text format. Same would be the output on the stdout screen.

File is created!
Test data

Reading File Using Files.readAllLines() Method

Files.readAllLines() 是一个较新的方法，可以按字符串列表的形式读取文件的所有内容，如下所示 −

Files.readAllLines() is a newer method to read all content of a file as a List of String as shown below −

Syntax

List<String> content = Files.readAllLines(Paths.get("d://test/testFile3.txt"));

Example: Reading File Using Files.readAllLines() Method

以下示例演示了文件，使用 readAllLines() 方法从给定目录读取文件 −

Following is the example to demonstrate File to read a file from a given directory using readAllLines() method −

package com.tutorialspoint;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.Arrays;
import java.util.List;

public class FileTest {
   public static void main(String args[]) {
      try {
    	  String data = "Test data";
          Files.write(Paths.get("d://test/testFile3.txt"), data.getBytes());
          List<String> lines = Arrays.asList("1st line", "2nd line");
          Files.write(Paths.get(
             "file6.txt"), lines, StandardCharsets.UTF_8,
             StandardOpenOption.CREATE, StandardOpenOption.APPEND);

          List<String> content = Files.readAllLines(Paths.get("d://test/testFile3.txt"));

          System.out.println(content);

          content = Files.readAllLines(Paths.get("file6.txt"));

          System.out.println(content);
      } catch (IOException e) {
         System.out.print("Exception");
      }
   }
}

上面的代码将创建文件 test.txt，并将指定的字符串写入文本格式。stdout 屏幕上将有相同输出。

The above code would create file test.txt and would write given strings in text format. Same would be the output on the stdout screen.

[Test data]
[1st line, 2nd line]