创建测试类

package StudentMange;

import java.util.ArrayList;
import java.util.List;

public class Test {
public static void main(String[] args) {
User user = new User("张三", 23, "小黑", 123456);
User user2 = new User("李四", 24, "小白", 654321);
User user3 = new User("王五", 25, "小紫", 321456);
User user4 = new User("赵六", 26, "小绿", 123654);

ArrayList users = new ArrayList<>();
users.add(user);
users.add(user2);
users.add(user3);
users.add(user4);

for (int i = 0; i < users.size(); i++) {
User user1 = users.get(i);
System.out.print("姓名 " + user1.getName() + " ");
System.out.print("年龄 " + user1.getAge() + " ");
System.out.print("用户名 " + user1.getUsername() + " ");
System.out.print("密码 " + user1.getPassword() + " ");
System.out.println("");

}
for (int i = 0; i < users.size(); i++) {
User use = users.get(i);
if (user.getUsername().equals("小黑") && user.getPassword()==123456){
System.out.println("找到用户名为小黑且密码为12345的用户");
System.out.println("姓名 "+user.getName()+" 年龄 "+user.getAge()+" 用户名 "+user.getUsername()+" 密码 "+user.getPassword());
return;
}
}
System.out.println("用户不存在");
}

}