import java.io.FileInputStream;
import java.io.IOException;
import java.util.Properties;
public class Main {
public static void main(String[] argv) throws Exception {
Properties properties = new Properties();
try {
properties.load(new FileInputStream("filename.properties"));
} catch (IOException e) {
}
String string = properties.getProperty("a.b");
properties.setProperty("a.b", "new value");
}
}