Propertiesクラス

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.time.LocalDateTime;
import java.util.Properties;
import java.util.Set;

public class ProSample {
	static Path path = Path.of("ProSample.xml");
	public static void main(String[] args) {
		var p = new Properties();
		read(p);
		Set<Object> keys = p.keySet();
		for(Object k : keys) {
			System.out.println(k + ":" + p.getProperty((String)k));
		}
		p.setProperty("" + System.currentTimeMillis(), LocalDateTime.now().toString());
		write(p);
	}
	static void read(Properties p) {
		try(InputStream in = Files.newInputStream(path) ){
			p.loadFromXML(in);
			
		}
		catch(IOException e) {
			System.err.println("read:" + e);
		}
	}
	static void write(Properties p) {
		try(OutputStream out = Files.newOutputStream(path)){
			p.storeToXML(out, LocalDateTime.now().toString());
		}
		catch(IOException e) {
			System.err.println("write:" + e);
		}
	}

}