Java append to file

Today we will look into how to append to a file in java. Java append to file is a common java IO operation. For example, whenever we print something to server logs, it gets appended to the existing file.

Java append to file

We can append to file in java using following classes.

1. Java append to file using FileWriter
2. Java append content to existing file using BufferedWriter
3. Append text to file in java using PrintWriter
4. Append to file in java using FileOutputStream

If you are working on text data and the number of write operations is less, use FileWriter and use its constructor with append flag value as true. If the number of write operations is huge, you should use the BufferedWriter. To append binary or raw stream data to an existing file, you should use FileOutputStream.

Java append to file using FileWriter

Here is the short program to append to file in java using FileWriter. We will look into a complete Java append to file example program later on.

File file = new File("append.txt");
FileWriter fr = new FileWriter(file, true);
fr.write("data");
fr.close();

Java append content to existing file using BufferedWriter

File file = new File("append.txt");
FileWriter fr = new FileWriter(file, true);
BufferedWriter br = new BufferedWriter(fr);
br.write("data");

br.close();
fr.close();

Append text to file in java using PrintWriter

We can also use PrintWriter to append to file in java.

File file = new File("append.txt");
FileWriter fr = new FileWriter(file, true);
BufferedWriter br = new BufferedWriter(fr);
PrintWriter pr = new PrintWriter(br);
pr.println("data");
pr.close();
br.close();
fr.close();

Append to file in java using FileOutputStream

You should use FileOutputStream to append data to file when it’s raw data, binary data, images, videos etc.

OutputStream os = new FileOutputStream(new File("append.txt"), true);
os.write("data".getBytes(), 0, "data".length());
os.close();

Java append to file example

Here is the final java append to file program showing all the different options we discussed above.

package com.journaldev.files;

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;

public class JavaAppendToFile {

	/**
	 * Java append to file example
	 * 
	 * @param args
	 */
	public static void main(String[] args) {
		String filePath = "/Users/pankaj/Downloads/append.txt";

		String appendText = "This String will be appended to the file, Byte=0x0A 0xFF";

		appendUsingFileWriter(filePath, appendText);

		appendUsingBufferedWriter(filePath, appendText, 2);

		appendUsingPrintWriter(filePath, appendText);

		appendUsingFileOutputStream(filePath, appendText);
	}

	private static void appendUsingPrintWriter(String filePath, String text) {
		File file = new File(filePath);
		FileWriter fr = null;
		BufferedWriter br = null;
		PrintWriter pr = null;
		try {
			// to append to file, you need to initialize FileWriter using below constructor
			fr = new FileWriter(file, true);
			br = new BufferedWriter(fr);
			pr = new PrintWriter(br);
			pr.println(text);
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				pr.close();
				br.close();
				fr.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	/**
	 * Use Stream for java append to file when you are dealing with raw data, binary
	 * data
	 * 
	 * @param data
	 */
	private static void appendUsingFileOutputStream(String fileName, String data) {
		OutputStream os = null;
		try {
			// below true flag tells OutputStream to append
			os = new FileOutputStream(new File(fileName), true);
			os.write(data.getBytes(), 0, data.length());
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				os.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	/**
	 * Use BufferedWriter when number of write operations are more
	 * 
	 * @param filePath
	 * @param text
	 * @param noOfLines
	 */
	private static void appendUsingBufferedWriter(String filePath, String text, int noOfLines) {
		File file = new File(filePath);
		FileWriter fr = null;
		BufferedWriter br = null;
		try {
			// to append to file, you need to initialize FileWriter using below constructor
			fr = new FileWriter(file, true);
			br = new BufferedWriter(fr);
			for (int i = 0; i < noOfLines; i++) {
				br.newLine();
				// you can use write or append method
				br.write(text);
			}

		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				br.close();
				fr.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	/**
	 * Use FileWriter when number of write operations are less
	 * 
	 * @param filePath
	 * @param text
	 * @param noOfLines
	 */
	private static void appendUsingFileWriter(String filePath, String text) {
		File file = new File(filePath);
		FileWriter fr = null;
		try {
			// Below constructor argument decides whether to append or override
			fr = new FileWriter(file, true);
			fr.write(text);

		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				fr.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

}

That’s all for append to file in java program.

You can checkout more Java IO examples from our GitHub Repository.

References:

• PrintWriter API Doc
• FileWriter API Doc
• BufferedWriter API Doc
• FileOutputStream API Doc