Question

So if we do like this:

 Properties props = new Properties();
 OutputStream osr = new FileOutputStream(Store.class.getResource("my.properties").getFile());

 props.setProperty("wallboard_text_rgb", "aaa");
 props.setProperty("wallboard_back_rgb", "bbb");

 props.store(osr, "");

other keys in existing properties will be deleted, how to avoid that?

Answer 1

Load properties from that file before modifying it. In other words, replace

Properties props = new Properties();

with

Properties props = Properties.load(new FileInputStream(Store.class.getResource("my.properties").getFile()));