Iterate and filter a list using Java streams instead of for-each loop

Question

In a kind of chat with a chatbot that I'm implementing, I have a List of Conversation and every Conversation has a List of Messages. I want to add those conversations that its last message is not a chatbot message and is not viewed by the user to a List of Conversations. To do that I'm coding it as bellow:

List<ConversationDto> conversationsToBeSent = new ArrayList<>();

for (ConversationDto conversation : super.allConversations) {
    if (conversation.getMessages() != null) {
        if (conversation.getMessages().get(0).getMessageType().getId() != MessageTypeEnum.CHATBOT.getValue()
                && conversation.getMessages().get(0).getToViewedAt() == null) {
            conversationsToBeSent.add(conversation);
        }
    }

}

This is working. The question is how can I code that using Lambda and stream instead of for each.

Hi! Please show us what you've tried so far, so that we can see what you need help with — fps
– fps, Commented Dec 2, 2020 at 12:50

Alex Shesterov · Accepted Answer · 2020-12-02 12:59:44Z

Your problem is a classical "Filter a collection" task. This is what the Stream.filter() method does.

A code block:

List<Item> newList = new ArrayList<>();
for (Item item : collection) {
    if (predicate(item)) {
        newList.add(item);
    }
}

can generally be converted to:

List<Item> newList = 
    collection
        .stream()
        .filter(item -> predicate(item))
        .collect(Collectors.toList());

In your case, the code transforms as follows. Note the comments in the following code block for explanations:

List<ConversationDto> conversationsToBeSent = super.allConversations // Start with the original list
    .stream() // Get a stream of list items
    .filter(conversation -> // Filter the list items
        conversation.getMessages() != null
        && !conversation.getMessages().isEmpty() // YOU PROBABLY WANT THIS CHECK AS WELL!
        && conversation.getMessages().get(0).getMessageType().getId() != MessageTypeEnum.CHATBOT.getValue()
        && conversation.getMessages().get(0).getToViewedAt() == null
    )
    .collect(Collectors.toList()); // Collect the stream items to a new list

Zinc · Accepted Answer · 2020-12-02 12:52:28Z

3

To use stream on a list, just do super.allConversations.stream() Then if you want to select only some elements, you can use the filter method like in the code below:

super.allConversations.stream().filter(
  conversation ->
    conversion != null && 
    conversation.getMessages().get(0).getMessageType().getId() != MessageTypeEnum.CHATBOT.getValue() &&
    conversation.getMessages().get(0).getToViewedAt() == null
);

answered Dec 2, 2020 at 12:52

Zinc

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