java - 如何计算并仅返回 ArrayList 中为真的对象？

Question

我想计算所有为 true 的对象并返回它(正如我在 countNational 中尝试做的那样。我用来将对象分配为 true/false 的方法在不同的类中并命名为 NationalMessage。我粘贴了它在主要方法下面。

public class Ex6 
{
 public static int countNational (ArrayList<Message> messageList) 
 {

 int countTrue = 0; //will count those that are true

 for (int i = 0; i<= messageList.size(); i++)
 {
     if(messageList.get(i)==true) //GETTING ERROR SAYING CAN'T COMPARE BOOLEAN
     countTrue = countTrue + 1;
 }
 return countTrue;
 }
 public static void main(String[] args)
 { 
 ArrayList<Message> messageList = new ArrayList<Message>();
 messageList.add(new NationalMessage("UKJohn","UKMark","aa"));
 messageList.add(new NationalMessage("UKJohn","FRJean","aa"));
 messageList.add(new Message("Mike","John","aa"));
 System.out.println(countNational(messageList));
 }
}

*************************** 下面是来自一个名为 National Message 的单独类的方法************ **********************

public NationalMessage(String ssender, String rreceiver, String ccontent)
{
    super(ssender,rreceiver,ccontent);

    if (ssender.equals("UK") && rreceiver.equals("UK"))
    {
        isNational = true;
    }

    else 
    {
        isNational = false;
    }
}

Answer 1

在 java8 中使用流就像过滤列表一样简单

long xx = myList.stream().filter(t -> t.isNational).collect(Collectors.counting());

或获取列表

List<NationalMessage> x = myList.stream().filter(t->t.isNational).collect(Collectors.toList());