练习、

代码：

public class User{
    private String id;
    private String username;
    private int password;

    public User() {
    }

    public User(String id, String username, int password) {
        this.id = id;
        this.username = username;
        this.password = password;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public int getPassword() {
        return password;
    }

    public void setPassword(int password) {
        this.password = password;
    }
}

import java.util.ArrayList;
import java.util.Scanner;
public class practice {
    public static void main(String[] args) {
        //1、定义一个集合，
        Scanner input = new Scanner(System.in);
        ArrayList<User> list = new ArrayList<>();
        // 存入三个用户对象
        for (int i = 0; i < 3; i++) {
            User ui = new User();
            System.out.println("输入用户的id：");
            String id = input.next();
            System.out.println("输入用户的username：");
            String username = input.next();
            System.out.println("输入用户的密码：");
            int password = input.nextInt();
            ui.setId(id);
            ui.setUsername(username);
            ui.setPassword(password);
            list.add(ui);
        }
        for (int i = 0; i < list.size(); i++) {
            User u = list.get(i);
            System.out.println(u.getId() + "," + u.getUsername() + "," + u.getPassword());
        }
        System.out.println("输入要查找的id：");
        String id = input.next();
        boolean b = func(id, list);
        System.out.println(b);

    }

    public static boolean func(String id, ArrayList<User> list) {
        for (int i = 0; i < list.size(); i++) {
            if (id.equals(list.get(i).getId())) {
                return true;
            }
        }
        return false;
    }
}

结果：

注意点：带有返回值的方法在返回值返回后，会立即结束该方法的执行。任何在返回值后面的代码都不会执行。这是因为返回值是方法的结果，一旦返回结果，方法的任务就完成了，没有必要继续执行后续的代码。

升级练习、

代码：

javabean代码一样

import java.util.ArrayList;
import java.util.Scanner;
public class practice {
    public static void main(String[] args) {
        //1、定义一个集合，
        Scanner input = new Scanner(System.in);
        ArrayList<User> list = new ArrayList<>();
        // 存入三个用户对象
        for (int i = 0; i < 3; i++) {
            User ui = new User();
            System.out.println("输入用户的id：");
            String id = input.next();
            System.out.println("输入用户的username：");
            String username = input.next();
            System.out.println("输入用户的密码：");
            int password = input.nextInt();
            ui.setId(id);
            ui.setUsername(username);
            ui.setPassword(password);
            list.add(ui);
        }
        for (int i = 0; i < list.size(); i++) {
            User u = list.get(i);
            System.out.println(u.getId() + "," + u.getUsername() + "," + u.getPassword());
        }
        System.out.println("输入要查找的id：");
        String id = input.next();
        int b = func(id, list);
        System.out.println(b);

    }

    public static int func(String id, ArrayList<User> list) {
        for (int i = 0; i < list.size(); i++) {
            if (id.equals(list.get(i).getId())) {
                return i;
            }
        }
        return -1;
    }
}

截图：

视频代码：直接在contains方法中调用getIndex方法，这样不用再写一次循环