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How to Read A File In Java?

7 minutes read

To read a file in Java, you can use the FileReader and BufferedReader classes. First, create a FileReader object and pass the file path as a parameter. Then, create a BufferedReader object and pass the FileReader object as a parameter to read the file line by line. Use the readLine() method of the BufferedReader to read each line of the file. Make sure to handle exceptions such as FileNotFound and IOException when reading the file. Close the BufferedReader and FileReader objects after reading the file to free up system resources.

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How to read a file and write to another file in Java?

To read a file and write to another file in Java, you can use the FileReader and FileWriter classes along with BufferedReader and BufferedWriter classes. Here is an example code snippet to demonstrate this:

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import java.io.*;

public class FileReadWriteExample {
    public static void main(String[] args) {
        try {
            // Open the input file for reading
            BufferedReader reader = new BufferedReader(new FileReader("input.txt"));
            
            // Open the output file for writing
            BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"));
            
            String line;
            
            // Read each line from the input file and write it to the output file
            while ((line = reader.readLine()) != null) {
                writer.write(line);
                writer.newLine();
            }
            
            // Close the input and output streams
            reader.close();
            writer.close();
            
            System.out.println("File read and write operation completed successfully.");
        } catch (IOException e) {
            System.err.println("Error occurred while reading/writing the file: " + e.getMessage());
            e.printStackTrace();
        }
    }
}


In this code snippet, we first open an input file "input.txt" for reading using a BufferedReader. Then, we open an output file "output.txt" for writing using a BufferedWriter. We read each line from the input file and write it to the output file. Finally, we close the input and output streams once the operation is completed.


Make sure to handle any exceptions that may be thrown while reading/writing the files to ensure the program runs smoothly.


How to read a JSON file in Java?

To read a JSON file in Java, you can use the Jackson library which provides various methods for reading and parsing JSON data. Here's a simple example of how to read a JSON file in Java using Jackson:

  1. Add Jackson dependencies to your project:
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<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.12.5</version>
</dependency>


  1. Create a Java class to represent the structure of your JSON data. For example, if your JSON file looks like this:
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{
  "name": "John Doe",
  "age": 30,
  "city": "New York"
}


You can create a corresponding Java class like this:

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public class Person {
    private String name;
    private int age;
    private String city;

    // Getters and setters
}


  1. Now, you can read the JSON file using Jackson:
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import com.fasterxml.jackson.databind.ObjectMapper;

public class ReadJsonFile {

    public static void main(String[] args) {
        try {
            ObjectMapper objectMapper = new ObjectMapper();
            Person person = objectMapper.readValue(new File("data.json"), Person.class);
            System.out.println("Name: " + person.getName());
            System.out.println("Age: " + person.getAge());
            System.out.println("City: " + person.getCity());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}


Make sure to replace "data.json" with the path to your JSON file. This code reads the JSON file and maps it to the Person class, allowing you to access the data using getters.


That's it! You have successfully read a JSON file in Java using Jackson.


How to read a file and count occurrences of a specific word in Java?

Here is an example code to read a file and count occurrences of a specific word in Java:

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

public class WordCounter {
    public static void main(String[] args) {
        File file = new File("file.txt");
        String wordToSearch = "hello";
        int count = 0;

        try {
            FileReader reader = new FileReader(file);
            BufferedReader br = new BufferedReader(reader);
            String line;

            while ((line = br.readLine()) != null) {
                String[] words = line.split(" ");

                for (String word : words) {
                    if (word.toLowerCase().equals(wordToSearch.toLowerCase())) {
                        count++;
                    }
                }
            }

            br.close();
            System.out.println("The word '" + wordToSearch + "' occurs " + count + " times in the file.");

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}


In this code snippet, we read a file named file.txt and search for the word "hello" in that file. The count variable is used to keep track of how many times the word occurs in the file. We split each line of the file into words and compare each word with the word we are searching for. If they match, we increment the count variable. Finally, we print out the total count of occurrences of the word in the file.


How to read a file from the classpath in Java?

There are several ways to read a file from the classpath in Java. One common approach is to use the getResourceAsStream method provided by the ClassLoader class. Here's an example:

  1. Place the file you want to read in the src/main/resources directory of your project.
  2. Use the following code snippet to read the file: 
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import java.io.InputStream;

public class ReadFileFromClasspath {
    public static void main(String[] args) {
        // Get the class loader
        ClassLoader classLoader = ReadFileFromClasspath.class.getClassLoader();
        
        // Get the input stream of the file using the class loader
        InputStream inputStream = classLoader.getResourceAsStream("filename.txt");
        
        // Read the file
        try {
            int data = inputStream.read();
            while(data != -1) {
                System.out.print((char)data);
                data = inputStream.read();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}


Replace "filename.txt" with the name of the file you want to read. This code snippet will read the contents of the file and print it to the console.

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