List利用Comparator进行Sort

代码

public static void main(String[] args) {
    List<Integer> a = new ArrayList(10);

    a.add(1);
    a.add(4);
    a.add(2);
    a.add(5);
    a.add(3);

    Collections.sort(a, new Comparator<Integer>() {
        @Override
        public int compare(Integer o1, Integer o2) {
            return o2 - o1;//倒叙
        }
    });

    a.stream().forEach(x -> System.out.println(x));
}

输出：

5
4
3
2
1

分析

Comparator

• 比较器
• java.util包
• 接口
• 用于排序和分组
• 常用于数组和列表
  • Arrays.sort(T[],Comparator<? super T> c)
  • Collections.sort(List list,Comparator<? super T> c)

原理

List.java

default void sort(Comparator<? super E> c) {
    Object[] a = this.toArray();
    Arrays.sort(a, (Comparator) c);
    ListIterator<E> i = this.listIterator();
    for (Object e : a) {
        i.next();
        i.set((E) e);
    }
}

• 调用List默认接口实现
• 实际调用Arrays.sort(a, (Comparator) c);

Arrays.java

public static <T> void sort(T[] a, Comparator<? super T> c) {
    if (c == null) {
        sort(a);
    } else {
        if (LegacyMergeSort.userRequested)
            legacyMergeSort(a, c);
        else
            TimSort.sort(a, 0, a.length, c, null, 0, 0);
    }
}

• LegacyMergeSort归并排序，默认不使用
• TimSort利用合并和插入排序

指定Comparator逆序

Collections.sort(result, Collections.reverseOrder());

源码

public static <T> Comparator<T> reverseOrder() {
    return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
}
private static class ReverseComparator
    implements Comparator<Comparable<Object>>, Serializable {

    private static final long serialVersionUID = 7207038068494060240L;

    static final ReverseComparator REVERSE_ORDER
        = new ReverseComparator();

    //逆序
    public int compare(Comparable<Object> c1, Comparable<Object> c2) {
        return c2.compareTo(c1);
    }

    private Object readResolve() { return Collections.reverseOrder(); }

    @Override
    public Comparator<Comparable<Object>> reversed() {
        return Comparator.naturalOrder();
    }
}