如何使用Java从文本文档中提取内容？

Java有几种方法可以读取文本文件，如BufferReader、FileReader和Scanner。每一种方法都提供了一种独特的文本文件读取方式。

方法：

1. 使用 Files 类
2. 使用 FileReader 类
3. 使用 BufferReader 类
4. 使用 Scanner 类

让我们通过示例详细了解每种方法，以便更好地理解这些方法，以便稍后实现从文本文档中提取内容的方法。

方法一：使用 Files 类

正如Java提供的 Java.nio.file 。应用程序接口 我们可以使用Java.nio.file.Files 类将文件的所有内容读入数组。要读取文本文件，我们可以使用 Files 类的readAllBytes() 方法，当您需要内存中的所有文件内容以及处理小文件时，可以使用该方法。

例子：

Java

// Java Program to Extract Content From a Text Document
// Using Files class
 
// Importing java.nio package for network linking
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
// Importing List class from java.util package
import java.util.List;
 
// Main class
public class GFG {
    // Method 1
    // To read the file using class
    private static void readUsingClass(String fileName)
        throws IOException
    {
        // Creating an object of Path class
        Path path = Paths.get(fileName);
 
        // To read file to byte array
        byte[] bytes = Files.readAllBytes(path);
 
        // Display message only
        System.out.println(
            "Read text file using Files class");
 
        // Reading the file to String List
        @SuppressWarnings("unused")
 
        // Creating a List class object of string type
        // as data in file to be read is words
        List allLines = Files.readAllLines(
            path, StandardCharsets.UTF_8);
        System.out.println(new String(bytes));
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
        throws IOException
    {
        // Custom input directory passed where text
        String fileName
            = "/Users/mayanksolanki/Desktop/file.txt";
 
        // read using FileReader, no encoding support, not
        // efficient
        readUsingClass(fileName);
    }
}

输出：

方法二：使用 FileReader 类 

我们可以使用Java.io.FileReader从文件中读取数据（以字符）。这是一种非常有效的逐行读取文件的方法。

句法：

FileReader input = new FileReader(String name);

例子：

Java

// Java Program to Extract Content From a Text Document
// Using FileReader class
 
// Importing required libraries
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
 
// Main class
public class GFG {
    // Method
    // To read the file using File Reader
    private static void readUsingFileReader(String fileName)
        throws IOException
    {
        File file = new File(fileName);
        FileReader fr = new FileReader(file);
        BufferedReader br = new BufferedReader(fr);
        String line;
        System.out.println(
            "Reading text file using FileReader");
        while ((line = br.readLine()) != null) {
            // process the line
            System.out.println(line);
        }
        br.close();
        fr.close();
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
        throws IOException
    {
        String fileName
            = "/Users/mayanksolanki/Desktop/file.txt";
 
        // read using FileReader, no encoding support, not
        // efficient
        readUsingFileReader(fileName);
    }
}

输出：

方法 3：使用 BufferedReader 类 

如果你想阅读 逐行文件，如果要处理该文件，则必须使用 BufferedReader。它也用于处理大文件，它也支持编码。 BufferReader 上的读取操作非常高效。

Note: Either specify the size of the BufferReader or keep the size as a Default size of BufferReader which is 8KB.  
 

句法：

BufferedReader in = new BufferedReader(Reader in, int size);

执行：

Hello I am learning web- development.
I am writing article for GFG.
I am cloud enthusiast.
I am an open-source contributor.

Note: Before starting create a text file by using .txt extension on your local machine and use the path of that file whenever you necessary while practicing. 

例子：

Java

// Java Program to Extract Content From a Text Document
// Using BufferedReader class
 
// Importing required libraries
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
 
// Main class
public class GFG {
    // Method
    // To read the file using Buffered Reader
    private static void
    readUsingBufferedReader(String fileName)
        throws IOException
    {
        File file = new File(fileName);
        FileReader fr = new FileReader(file);
        // read file line by line
        BufferedReader br = new BufferedReader(fr);
        String line;
        System.out.println(
            "Read text file using BufferedReader");
        while ((line = br.readLine()) != null) {
            // process the line
            System.out.println(line);
        }
        // close resources
        br.close();
        fr.close();
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
        throws IOException
    {
        String fileName
            = "/Users/mayanksolanki/Desktop/file.txt";
 
        // read using FileReader, no encoding support, not
        // efficient
        readUsingBufferedReader(fileName);
    }
}

输出：

方法 4：使用 Scanner 类

如果我们想根据一些表达式读取文档&如果要逐行读取文档，那么我们使用 Scanner 类。 Scanner 将输入分解为标记，默认情况下与空白匹配。

例子 ：

Java

// Java Program to Extract Content From a Text Document
// Using Scanner class
 
// Importing required libraries
import java.io.IOException;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Scanner;
 
// Main class
public class GFG {
   
    // Method 1
    // To read the file using Scanner
    private static void readUsingScanner(String fileName)
        throws IOException
    {
 
        // Again,  creating Path class object in main()
        // method
        Path path = Paths.get(fileName);
 
        // Creating Scanner class object to take input
        Scanner scanner = new Scanner(path);
 
        // Display message for readability
        System.out.println("Read text file using Scanner");
 
        // Now reading file line by line
        // using hasNextLine() method
        while (scanner.hasNextLine()) {
 
            // Processing each line
            String line = scanner.nextLine();
 
            // Printing processed lines
            System.out.println(line);
        }
 
        // close() method is used to close all the read
        // write connections
        scanner.close();
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
        throws IOException
    {
        // Custom input text document
        // present already on the machine
        String fileName
            = "/Users/mayanksolanki/Desktop/file.txt";
 
        // Now lastly reading using FileReader
        // no encoding support, not efficient
        readUsingScanner(fileName);
    }
}

输出：

执行：

这里“ my_file.txt”是用于下面演示的程序的演示文件，其中示例行如下：

Hello I am learning web- development.
I am writing article of GFG.
I am cloud enthusiast.
I am an open-source contributor.

示例 1：

Java

// Java Program to Implement Extraction of Content
// From a Text Document
 
// Importing required libraries
import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;
import java.util.Scanner;
 
// Main class
public class read_file {
    // Method 1
    // To read the file using File Reader
    private static void readUsingFileReader(String fileName)
        throws IOException
    {
        File file = new File(fileName);
        FileReader fr = new FileReader(file);
        BufferedReader br = new BufferedReader(fr);
        String line;
        System.out.println(
            "Reading text file using FileReader");
        while ((line = br.readLine()) != null) {
            // process the line
            System.out.println(line);
        }
        br.close();
        fr.close();
    }
 
    // Method 2
    // To read the file using Buffered Reader
    private static void
    readUsingBufferedReader(String fileName)
        throws IOException
    {
        File file = new File(fileName);
        FileReader fr = new FileReader(file);
        // read file line by line
        BufferedReader br = new BufferedReader(fr);
        String line;
        System.out.println(
            "Read text file using BufferedReader");
        while ((line = br.readLine()) != null) {
            // process the line
            System.out.println(line);
        }
 
        // Closing resources to
        // release memory spaces
        br.close();
        fr.close();
    }
 
    // Method 3
    // To read the file using Scanner
    private static void readUsingScanner(String fileName)
        throws IOException
    {
        Path path = Paths.get(fileName);
        Scanner scanner = new Scanner(path);
        System.out.println("Read text file using Scanner");
        // read line by line
        while (scanner.hasNextLine()) {
            // process each line
            String line = scanner.nextLine();
            System.out.println(line);
        }
        scanner.close();
    }
 
    // Method 4
    // To read the file using class
    private static void readUsingClass(String fileName)
        throws IOException
    {
        Path path = Paths.get(fileName);
        // read file to byte array
        byte[] bytes = Files.readAllBytes(path);
        System.out.println(
            "Read text file using Files class");
        // read file to String list
        @SuppressWarnings("unused")
        List allLines = Files.readAllLines(
            path, StandardCharsets.UTF_8);
        System.out.println(new String(bytes));
    }
 
    // Method 5
    // main driver method
    public static void main(String[] args)
        throws IOException
    {
        String fileName
            = "C:\\Users\\HP\\Desktop\\my_file.txt";
 
        // using Java 7 Files class to
        // process small files, get complete file data
        readUsingClass(fileName);
 
        // using Scanner class for
        // large files, to read line by line
        readUsingScanner(fileName);
 
        // read using BufferedReader, to read line by line
        readUsingBufferedReader(fileName);
 
        // read using FileReader, no encoding support, not
        // efficient
        readUsingFileReader(fileName);
    }
}

示例 2：从文本文件中读取特定行

现在如果你想从给定的文档中读取特定的行，那么我们使用 BufferReader 方法。根据文件 BufferReader() 方法的使用，我们的代码运行得更快更高效。在这个程序中，BufferReader中存储的Text文件是使用for循环遍历所有行，当条件成立时，我们将打印该行

执行：

“myfile.txt” is the demo file to be used. 

这是包含在此文件中的样本行，由行组成的随机任意词组成

3D printing or additive manufacturing is a process of making three dimensional solid objects from a digital file.

The creation of a 3D printed object is achieved using additive processes. In an additive process an object is created by laying down successive layers of material until the object is created. Each of these layers can be seen as a thinly sliced cross-section of the object.

3D printing is the opposite of subtractive manufacturing which is cutting out / hollowing out a piece of metal or plastic with for instance a milling machine.

3D printing enables you to produce complex shapes.

It uses less material than traditional manufacturing methods.

Java

// Java program to Read Specific Lines from a Text File
 
// Importing required libraries
import java.io.*;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Initially taking empty string
        String text = "";
 
        // Declaring a variable to lookup for
        // number of lines in afile
        int lineNumber;
 
        // Try block to check for exceptions
        try {
           
            // Creating a FileReader object so as to
            // get the directory of file to be read
            FileReader readfile = new FileReader(
                "C:\\Users\\HP\\Desktop\\Exam.txt");
 
            // Creating a BufferReader class object to
            // read file as passed above
            BufferedReader readbuffer
                = new BufferedReader(readfile);
 
            // Buffer reader declaration
 
            // Conditionality for specific line/s
 
            // Loop for the traversing line by line
            // into the text file
            for (lineNumber = 1; lineNumber < 10;
                 lineNumber++) {
                // If spefic line/s is found
                if (lineNumber == 7) {
                    // Store the content of this specific
                    // line
                    text = readbuffer.readLine();
                }
 
                else {
                    // Just keep on reading
                    readbuffer.readLine();
                }
            }
        }
 
        // Catching IOException exception
        catch (IOException e) {
 
            // Print the line number where exception occurred
            e.printStackTrace();
        }
 
        // Print the specific line from the file read
        System.out.println(" The specific Line is: "
                           + text);
    }
}

输出：具体行是：

3D printing enables you to produce complex shapes.