JAVA——集合

为什么要有集合？

集合是长度可变的容器，当添加集合里面的元素时，集合会自动扩容。

•  集合中五寸存储基本数据类型，若想存储，必须将其变为对应的包装类。

• 

集合

•  泛型：限定集合中存储数据的类型
• ArrayList<String> list = new ArrayList<>();
  • 尖括号就是泛型，表示集合里面存储数据的类型
  • 此时创建的是ArrayList的对象，而ArrayList是java已经写好的一个类
  • 这个类在底层做了一些处理
  • 打印对象不是地址值，而是集合中存储数据的内容
  • 在展示的时候会拿[ ]把所有的数据进行包裹

package com.itheima.test23;

import java.util.ArrayList;

public class ArrayListDemo {
    public static void main(String[] args) {

        ArrayList<String> list = new ArrayList<>();

        // 2.添加元素
        boolean result01 = list.add("aaa");
        System.out.println(result01);
        list.add("aaa");
        list.add("bbb");
        list.add("ccc");

        // 3.删除元素
        boolean result02 = list.remove("aaa");
        System.out.println(result02);

        boolean result03 = list.remove("ddd");
        System.out.println(result03);

        String str = list.remove(0);
        System.out.println(str);

        // 4.修改元素
        String result04 = list.set(1, "ddd");
        System.out.println(result04);

        // 5.查询元素
        String s = list.get(0);
        System.out.println(s);

        // 6.遍历
        for (int i = 0; i < list.size(); i++) {
            String st = list.get(i);
            System.out.println(st);
        }

        System.out.println(list);
    }
}

综合练习

集合的遍历方式

package com.itheima.test23;

import java.util.ArrayList;

public class ArrayListDemo01 {
    public static void main(String[] args) {

        ArrayList<String> list = new ArrayList<>();
        list.add("a");
        list.add("b");
        list.add("c");
        list.add("d");

        System.out.print("[");
        for (int i = 0; i < list.size(); i++) {
            if (i == list.size() - 1) {
                System.out.print(list.get(i));
            } else {
                System.out.print(list.get(i) + ", ");
            }
        }
        System.out.println("]");
    }
}

添加数字并遍历

package com.itheima.test23;

import java.util.ArrayList;

public class ArrayListDemo02 {
    public static void main(String[] args) {

        ArrayList<Integer> list = new ArrayList<>();

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

        System.out.print("[");
        for (int i = 0; i < list.size(); i++) {
            if (i == list.size() - 1) {
                System.out.print(list.get(i));
            } else {
                System.out.print(list.get(i) + ", ");
            }
        }
        System.out.println("]");
    }
}

添加学生对象并遍历

package com.itheima.test23;

import java.util.ArrayList;

public class ArrayListDemo03 {
    public static void main(String[] args) {

        ArrayList<Student> list = new ArrayList<>();

        Student s1 = new Student("lhj", 20);
        Student s2 = new Student("fjq", 21);

        list.add(s1);
        list.add(s2);

        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i).getName() + "," + list.get(i).getAge());
        }
    }
}

package com.itheima.test23;

import java.util.ArrayList;
import java.util.Scanner;

public class ArrayListDemo04 {
    public static void main(String[] args) {

        ArrayList<Student> list = new ArrayList<>();

        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 3; i++) {
            Student s = new Student();
            System.out.println("请输入学生的姓名：");
            String name = sc.next();
            s.setName(name);
            System.out.println("请输入学生的年龄：");
            int age = sc.nextInt();
            s.setAge(age);

            list.add(s);
        }

        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i).getName() + "," + list.get(i).getAge());
        }
    }
}

添加用户对象并判断是否存在

package com.itheima.test24;

import java.util.ArrayList;

public class UserTest {
    public static void main(String[] args) {

        ArrayList<User> list = new ArrayList<>();

        User u1 = new User("heima001", "lhj", "1234");
        User u2 = new User("heima002", "fjq", "12345");
        User u3 = new User("heima003", "hwd", "123456");

        list.add(u1);
        list.add(u2);
        list.add(u3);

        boolean flag = contain(list, "heima001");
        System.out.println(flag);
    }

    public static boolean contain(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 true;
            }
        }
        return false;
    }
}

添加手机对象并返回要求的数据

package com.itheima.test25;

import java.util.ArrayList;

public class PhoneTest {
    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> p = lowPrice(list, 3000);

        for (int i = 0; i < p.size(); i++) {
            System.out.println(p.get(i).getBrand() + "," + p.get(i).getPrice());
        }
    }

    public static ArrayList<Phone> lowPrice(ArrayList<Phone> list, int price) {
        ArrayList<Phone> ls = new ArrayList<>();
        for (int i = 0; i < list.size(); i++) {
            Phone p = list.get(i);
            if (p.getPrice() < price) {
                ls.add(p);
            }
        }
        return ls;
    }
}