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编写博客目的：本系列博客均根据B站黑马程序员系列视频学习和编写目的在于记录自己的学习点滴，方便后续回忆和查找相关知识点，不足之处恳请各位有缘的朋友指正。

一、查找id对应的集合索引

package www.itheima;import java.util.ArrayList;public class ArrayListDemo7 {public static void main(String[] args) {ArrayList<User> list = new ArrayList<>();User u1 = new User("001", "zhangsan", "123456");User u2 = new User("002", "wangwu", "123456");User u3 = new User("003", "lisi", "123456");list.add(u1);list.add(u2);list.add(u3);System.out.println(getIndex(list,"002"));}public static int getIndex(ArrayList<User> list, String id) {for (int i = 0; i < list.size(); i++) {User u = list.get(i);String uid = u.getId();if (uid.equals(id)) {return i;}}return -1;}
}

二、返回手机价格低于三千的手机

package www.itheima;import java.util.ArrayList;public class ArrayListDemo8 {public static void main(String[] args) {ArrayList<Phone> list = new ArrayList<>();Phone p1 = new Phone("小米", 1000);Phone p2 = new Phone("苹果", 8000);Phone p3 = new Phone("锤子", 2999);list.add(p1);list.add(p2);list.add(p3);ArrayList<Phone> phoneInfoList = getPhoneInfo(list);for (int i = 0; i < phoneInfoList.size(); i++) {Phone phone = phoneInfoList.get(i);System.out.println(phone.getBrand() + "," + phone.getPrice());}}// 定义一个方法，返回手机价格低于3000的手机public static ArrayList<Phone> getPhoneInfo(ArrayList<Phone> list) {ArrayList<Phone> resultList = new ArrayList<>();for (int i = 0; i < list.size(); i++) {Phone p = list.get(i);int price = p.getPrice();if (price < 3000) {resultList.add(p);}}return resultList;}
}