Java的可变参数与Collections类的功能示例解析

public class ChangeArgs {
    public static void main(String[] args) {
        int sum = getSum(6, 7, 2, 12, 2121);
        System.out.println(sum);
    }
    public static int getSum(int... arr) {
           int sum = 0;
            for (int a : arr) {
         sum += a;
        }
            return sum;
    }
}

public class CollectionsDemo {
    public static void main(String[] args) {
      ArrayList<Integer> list = new ArrayList<Integer>();
      //原来写法
      //list.add(12);
      //list.add(14);
      //list.add(15);
      //list.add(1000);
      //采用工具类 完成 往集合中添加元素  
      Collections.addAll(list, 5, 222, 1，2);
      System.out.println(list);
}

public class CollectionsDemo {
    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        list.add(100);
        list.add(300);
        list.add(200);
        list.add(50);
        //排序方法 
        Collections.sort(list);
        System.out.println(list);
    }
}
结果：
[50,100, 200, 300]

public class Student{
    private String name;
    private int age;
    //构造方法
    //get/set
     //toString
}

public class Demo {
    public static void main(String[] args) {
        // 创建四个学生对象 存储到集合中
        ArrayList<Student> list = new ArrayList<Student>();
        list.add(new Student("rose",18));
        list.add(new Student("jack",16));
        list.add(new Student("abc",20));
        Collections.sort(list, new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
            return o1.getAge()-o2.getAge();//以学生的年龄升序
            }
        });
        for (Student student : list) {
            System.out.println(student);
        }
    }
}
Student{name='jack', age=16}
Student{name='rose', age=18}
Student{name='abc', age=20}

public class Test1 {
    public static void main(String[] args) {
        /* 班级里有N个学生，学生属性:姓名，年龄，性别。
        实现随机点名器。*/
        //1.定义集合
        ArrayList<String> list = new ArrayList<>();
        //2.添加数据
        Collections.addAll(list,"范闲","范建","范统","杜子腾","杜琦燕","宋合泛","侯笼藤","朱益群","朱穆朗玛峰","袁明媛");
        //3.随机点名
        /* Random r = new Random();
        int index = r.nextInt(list.size());
        String name = list.get(index);
        System.out.println(name);*/
        //打乱
        Collections.shuffle(list);
        String name = list.get(0);
        System.out.println(name);
    }
}

public class Test2 {
    public static void main(String[] args) {
        /* 班级里有N个学生
        要求：
        70%的概率随机到男生
        30%的概率随机到女生
        "范闲","范建","范统","杜子腾","宋合泛","侯笼藤","朱益群","朱穆朗玛峰",
        "杜琦燕","袁明媛","李猜","田蜜蜜",
        */
        //1.创建集合
        ArrayList<Integer> list = new ArrayList<>();
        //2.添加数据
        Collections.addAll(list,1,1,1,1,1,1,1);
        Collections.addAll(list,0,0,0);
        //3.打乱集合中的数据
        Collections.shuffle(list);
        //4.从list集合中随机抽取0或者1
        Random r = new Random();
        int index = r.nextInt(list.size());
        int number = list.get(index);
        System.out.println(number);
        //5.创建两个集合分别存储男生和女生的名字
        ArrayList<String> boyList = new ArrayList<>();
        ArrayList<String> girlList = new ArrayList<>();
        Collections.addAll(boyList,"范闲","范建","范统","杜子腾","宋合泛","侯笼藤","朱益群","朱穆朗玛峰");
        Collections.addAll(girlList,"杜琦燕","袁明媛","李猜","田蜜蜜");
        //6.判断此时是从boyList里面抽取还是从girlList里面抽取
        if(number == 1){
            //boyList
            int boyIndex = r.nextInt(boyList.size());
            String name = boyList.get(boyIndex);
            System.out.println(name);
        }else{
            //girlList
            int girlIndex = r.nextInt(girlList.size());
            String name = girlList.get(girlIndex);
            System.out.println(name);
        }
    }
}

public class Test3 {
    public static void main(String[] args) {
       /* 班级里有5个学生
        要求：
        被点到的学生不会再被点到。
        但是如果班级中所有的学生都点完了，需要重新开启第二轮点名。*/
        //1.定义集合
        ArrayList<String> list1 = new ArrayList<>();
        //2.添加数据
        Collections.addAll(list1, "范闲", "范建", "范统", "杜子腾", "杜琦燕", "宋合泛", "侯笼藤", "朱益群", "朱穆朗玛峰", "袁明媛");
        //创建一个临时的集合，用来存已经被点到学生的名字
        ArrayList<String> list2 = new ArrayList<>();
        //外循环：表示轮数
        for (int i = 1; i <= 10; i++) {
            System.out.println("=========第" + i + "轮点名开始了======================");
            //3.获取集合的长度
            int count = list1.size();
            //4.随机点名
            Random r = new Random();
            //内循环：每一轮中随机循环抽取的过程
            for (int j = 0; j < count; j++) {
                int index = r.nextInt(list1.size());
                String name = list1.remove(index);
                list2.add(name);
                System.out.println(name);
            }
            //此时表示一轮点名结束
            //list1 空了 list2 10个学生的名字
            list1.addAll(list2);
            list2.clear();
        }
    }
}

public class Test4 {
    public static void main(String[] args) {
        /* 需求
        定义一个Map集合，键用表示省份名称province，值表示市city，但是市会有多个。
        添加完毕后，遍历结果格式如下：
                江苏省 = 南京市，扬州市，苏州市，无锡市，常州市
                湖北省 = 武汉市，孝感市，十堰市，宜昌市，鄂州市
                河北省 = 石家庄市，唐山市，邢台市，保定市，张家口市*/
        //1.创建Map集合
        HashMap<String, ArrayList<String>> hm = new HashMap<>();
        //2.创建单列集合存储市
        ArrayList<String> city1 = new ArrayList<>();
        city1.add("南京市");
        city1.add("扬州市");
        city1.add("苏州市");
        city1.add("无锡市");
        city1.add("常州市");
        ArrayList<String> city2 = new ArrayList<>();
        city2.add("武汉市");
        city2.add("孝感市");
        city2.add("十堰市");
        city2.add("宜昌市");