How to replace a string from a file?

Here are two ways (basic without any error/exception handling and passing target and replacement as arguments) how you can do it.

If your file stores key/value pairs than the best way is to user java.util.Properties

public class ReplaceInFile { private final static String src = "test.txt"; private final static String dst_str = "test_new_str.txt"; private final static String dst_prop = "test_new_prop.txt"; public static void main(String[] args) throws IOException { usingStringOperations(); usingProperties(); } private static void usingProperties() throws IOException { File srcFile = new File(src); FileInputStream fis = new FileInputStream(srcFile); Properties properties = new Properties(); properties.load(fis); fis.close(); if(properties.getProperty("hostname") != null) { properties.setProperty("hostname", "127.0.0.1"); FileOutputStream fos = new FileOutputStream(dst_prop); properties.store(fos, "Using java.util.Properties"); fos.close(); } } private static void usingStringOperations() throws IOException { File srcFile = new File(src); FileInputStream fis = new FileInputStream(srcFile); int len = fis.available(); if(len > 0) { byte[] fileBytes = new byte[len]; fis.read(fileBytes, 0, len); fis.close(); String strContent = new String(fileBytes); int i = strContent.indexOf("localhost"); if(i != -1) { String newStrContent = strContent.substring(0, i) + "127.0.0.1" + strContent.substring(i + "localhost".length(), strContent.length()); FileOutputStream fos = new FileOutputStream(dst_str); fos.write(newStrContent.getBytes()); fos.close(); } } } }