1，永遠不要根據列舉的序數匯出與它關聯的值，而是要將他儲存在一個例項域中。（ordinal（））

public enum Ensemble {
SOLO, DUET, TRIO, QUARTET, QUINTET, SEXTET, SEPTET, OCTET, NONET, DECTET;

public int numberOfMusicians() {
return ordinal()+1;
}
}
無法新增

所以可以採用以下方式：

public enum Ensemble {
SOLO(1), DUET(2), TRIO(3), QUARTET(4), QUINTET(5), SEXTET(6), SEPTET(7), OCTET(8),
DOUBLE_QUARTET(8), NONET(9), DECTET(10), TRIPLE_QUARTET(12);

private final int numberOfMusicians;

Ensemble(int size) {
this.numberOfMusicians = size;
}

public int numberOfMusicians() {
return numberOfMusicians;
}
}