java集合基础练习题

1.将数组保存到List中，并按倒序、从大到小的顺序显示出来

@Test
    public void listTest() {

        List list = new ArrayList();

        int[] arr = {1, 2, 3, 4, 8, 7, 6, 5};
        for (int i = 0; i < arr.length; i++) {
            list.add(arr[i]);
        }

        Collections.sort(list);
        Collections.reverse(list);
        System.out.println("排序后的顺序：");

        Iterator iterator = list.iterator();
        while (iterator.hasNext()) {
            System.out.print(iterator.next() + " ");
        }
    }

2.请把学生名与考试分数录入到集合中，并按分数显示前三名成绩学员的名字。

@Test
    public void treeSetTest(){
        //TreeSet<String> student = new TreeSet<>();
        HashMap<String, Integer> student = new HashMap<>();

        student.put("AA",80);
        student.put("BB",90);
        student.put("CC",70);
        student.put("DD",100);

        //将map转化为set
        Set<Map.Entry<String, Integer>> entries = student.entrySet();

        //把set转化为list
        ArrayList<Map.Entry<String, Integer>> entries1 = new ArrayList<>(entries);

        //用Collections 的 sort(list,comparator)方法对list进行排序
        Collections.sort(entries1, new Comparator<Map.Entry<String, Integer>>() {//重写comparator方法
            @Override
            public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
                return o2.getValue() - o1.getValue();
            }
        });

        for (int i = 0; i < 3; i++) {
            System.out.println(entries1.get(i));
        }

    }