Java Tutorial/File/BufferedWriter

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Appending to a File

import java.io.BufferedWriter;
import java.io.FileWriter;
public class Main {
  public static void main(String[] argv) throws Exception {
    BufferedWriter out = new BufferedWriter(new FileWriter("filename", true));
    out.write("aString");
    out.close();
  }
}





BufferedWriter

You can rap your Writer with a BufferedWriter for better performance. BufferedWriter has the following constructors:



public BufferedWriter (Writer writer)
public BufferedWriter (Writer writer, in bufferSize)





Create BufferedWriter from FileWriter

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.EOFException;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.StringReader;
public class MainClass {
  public static void main(String[] args) throws IOException {
    try {
      BufferedReader in4 = new BufferedReader(new StringReader("asdf"));
      PrintWriter out1 = new PrintWriter(new BufferedWriter(new FileWriter("IODemo.out")));
      int lineCount = 1;
      String s = null;
      while((s = in4.readLine()) != null )
        out1.println(lineCount++ + ": " + s);
      out1.close();
    } catch(EOFException e) {
      System.err.println("End of stream");
    }
  }
}





Nested creation of FileWriter, BufferedWriter and PrintWriter

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.PrintWriter;
public class Main {
  public static void main(String[] argv) throws Exception {
    FileWriter file = new FileWriter("a.html");
    BufferedWriter buffer = new BufferedWriter(file);
    PrintWriter out = new PrintWriter(buffer);
    out.println("<html>\n\t<body>");
    out.println("\t</body>\n</html>");
    out.close();
    buffer.close();
    file.close();
  }
}





Read Input From User and Write to File

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.InputStream;
public class Main {
  public static void main(String[] args) throws Exception {
    InputStream in = System.in;
    BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt"));
    int letter;
    while ((letter = in.read()) != -1) {
      bw.write((char) letter);
      bw.flush();
    }
  }
}





Use a BufferedReader and a BufferedWriter to copy a text file, inverting the case of letters in the process

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
public class Main {
  public static void main(String[] argv) throws Exception {
    BufferedReader br = new BufferedReader(new FileReader("in.txt"));
    BufferedWriter bw = new BufferedWriter(new FileWriter("out.txt"));
    int i;
    do {
      i = br.read();
      if (i != -1) {
        if (Character.isLowerCase((char) i))
          bw.write(Character.toUpperCase((char) i));
        else if (Character.isUpperCase((char) i))
          bw.write(Character.toLowerCase((char) i));
        else
          bw.write((char) i);
      }
    } while (i != -1);
    br.close();
    bw.close();
  }
}





Write to file using a BufferedWriter

import java.io.BufferedWriter;
import java.io.FileWriter;
public class Main {
  public static void main(String[] args) throws Exception {
    BufferedWriter bufferedWriter = null;
    bufferedWriter = new BufferedWriter(new FileWriter("yourFile.txt"));
    bufferedWriter.write("string");
    bufferedWriter.newLine();
    bufferedWriter.write("string");
  }
}





Writing to a File: If the file does not exist, it is automatically created.

import java.io.BufferedWriter;
import java.io.FileWriter;
public class Main {
  public static void main(String[] argv) throws Exception {
    BufferedWriter out = new BufferedWriter(new FileWriter("outfilename"));
    out.write("asdf");
    out.close();
  }
}


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