import java.util.LinkedList;

public class Logs {           // classe che implementa il design pattern Singleton
	private static Logs obj;   // attributo della classe che tiene l'unica istanza creata
	private LinkedList< String > l; 

	private Logs() {           // costruttore privato
		empty();
	}
	
	public static Logs getInstance() {    // metodo della classe
		if (obj == null) obj = new Logs(); // crea l'istanza
		return obj;                        // restituisce l'istanza
	}
	
	public void record(String s) {
		l.add(s);
	}
	
	public String dumpLast() {
		return l.getLast();
	}
	
	public String dumpAll() {
		StringBuilder acc = new StringBuilder(); // accumulatore di stringhe
		for (String s : l)
			acc.append(s);                  // aggiunge in coda
		return acc.toString();
	}
	
	public void empty() {
		l = new LinkedList< >();
	}
}

public class TestLogs {
	private Logs lg = Logs.getInstance();

	public void testSingl() {
		initLogs();
		Logs lg2 = Logs.getInstance();
		lg2.record("uno");
		lg2.record("due");
		if (lg.dumpLast().equals("due"))
			System.out.println("OK test logs singl");
		else
			System.out.println("FAILED test logs singl");
	}

	public void testLast() {
		initLogs();
		if (lg.dumpLast().equals("three "))
			System.out.println("OK test logs last");
		else
			System.out.println("FAILED test logs last");
	}

	public void testAll() {
		initLogs();
		if (lg.dumpAll().equals("one two three "))
			System.out.println("OK test logs all");
		else
			System.out.println("FAILED test logs all");
	}

	private void initLogs() {
		lg.empty();
		lg.record("one ");
		lg.record("two ");
		lg.record("three ");
	}

	public static void main(String[] args) {
		TestLogs tl = new TestLogs();
		tl.testSingl();
		tl.testAll();
		tl.testLast();
	}
}