Counter.java

public class Counter {

    private int counter;

    private Counter(int i) {
        this.safeSetCounter(i);
    }

    public void click() {
        this.safeSetCounter(this.counter + 1);
    }

    private void safeSetCounter(int i) {
        this.counter = i % 10000;
    }

    public void reset() {
        this.counter = 0;
    }

    public int value() {
        return this.counter;
    }

    public static Counter zero() {
        return new Counter(0);
    }

    /**
     * Returns a new Counter with de defined value included between 0 and 9999. Throws an IllegalArgumentException
     * if the value is out of the range
     *
     * @param value the initial value between 0 and 9999
     * @return a counter with the given value set
     * @throws IllegalArgumentExcpetion if value is not included in the specific range
     */
    public static Counter withValue(int value) {
        if (value < 0 || value > 9999) {
            /* Nous préférons retourner une exception si l'utilisateur fait une bêtise */
            throw new IllegalArgumentException("Value must be between 0 and 9999");
        }
        return new Counter(value);
    }

    private String toFourDigits(int i) {
        final String tooMuchDigits = ("0000" + this.counter);
        return tooMuchDigits.substring( tooMuchDigits.length()-4, tooMuchDigits.length() );
    }

    public String toString() {
        return "Counter(" + toFourDigits(this.counter) + ")";
    }

    public static void main(String[] args) {
      Counter counter = Counter.withValue(10); 
      System.out.println( counter );
    }
}