Comparator

不推荐：（它可能会导致整数最大长度溢出和 IEEE 754 浮点运算失真的危险）

Collections.sort(list2,new Comparator<Person2>(){

            @Override
            public int compare(Person2 o1, Person2 o2)
            {
                if(o1 == null || o2 == null)
                    return 0;
                return o1.getAge()-o2.getAge();
            }

        });

// BROKEN difference-based comparator - violates transitivity!
static Comparator<Object> hashCodeOrder = new Comparator<>() {
    public int compare(Object o1, Object o2) {
        return o1.hashCode() - o2.hashCode();
    }
};

推荐：

// Comparator based on static compare method
static Comparator<Object> hashCodeOrder = new Comparator<>() {
    public int compare(Object o1, Object o2) {
        return Integer.compare(o1.hashCode(), o2.hashCode());
    }
};

// Comparator based on Comparator construction method
static Comparator<Object> hashCodeOrder =
        Comparator.comparingInt(o -> o.hashCode());

Comparable

// Single-field Comparable with object reference field
public final class CaseInsensitiveString
        implements Comparable<CaseInsensitiveString> {
    public int compareTo(CaseInsensitiveString cis) {
        return String.CASE_INSENSITIVE_ORDER.compare(s, cis.s);
    }
    ... // Remainder omitted
}

lambda：写法

// Comparable with comparator construction methods
private static final Comparator<PhoneNumber> COMPARATOR =
        comparingInt((PhoneNumber pn) -> pn.areaCode)
          .thenComparingInt(pn -> pn.prefix)
          .thenComparingInt(pn -> pn.lineNum);
public int compareTo(PhoneNumber pn) {
    return COMPARATOR.compare(this, pn);
}

总而言之，无论何时实现具有合理排序的值类，你都应该让该类实现 Comparable 接口，以便在基于比较的集合中轻松对其实例进行排序，搜索和使用。 比较 compareTo 方法的实现中的字段值时，请避免使用「<」和「>」运算符。 相反，使用包装类中的静态 compare 方法(Intege.compare(x,y))或 Comparator 接口中的构建方法(Comparator.comparingInt(o -> o.hashCode()))。