Return from lambda forEach in java

Question

I am trying to change some for-each loops to lambda forEach  -methods to discover the possibilities of lambda expressions  The following seems to be possible   ArrayList lt Player gt  playersOfTeam   new ArrayList lt Player gt           for  Player player   players        if  player getTeam   equals teamName             playersOfTeam add player             With lambda forEach    players forEach player- gt  if  player getTeam   equals teamName    playersOfTeam add player         But the next one doesn t work   for  Player player   players        if  player getName   contains name             return player            with lambda  players forEach player- gt  if  player getName   contains name    return player        Is there something wrong in the syntax of the last line or is it impossible to return from forEach   method

User · Answer

I suggest you to first try to understand Java 8 in the whole picture, most importantly in your case it will be streams, lambdas and method references.

You should never convert existing code to Java 8 code on a line-by-line basis, you should extract features and convert those.

What I identified in your first case is the following:

You want to add elements of an input structure to an output list if they match some predicate.

Let's see how we do that, we can do it with the following:

List<Player> playersOfTeam = players.stream()
    .filter(player -> player.getTeam().equals(teamName))
    .collect(Collectors.toList());

What you do here is:

Turn your input structure into a stream (I am assuming here that it is of type Collection<Player>, now you have a Stream<Player>.
Filter out all unwanted elements with a Predicate<Player>, mapping every player to the boolean true if it is wished to be kept.
Collect the resulting elements in a list, via a Collector, here we can use one of the standard library collectors, which is Collectors.toList().

This also incorporates two other points:

Code against interfaces, so code against List<E> over ArrayList<E>.
Use diamond inference for the type parameter in new ArrayList<>(), you are using Java 8 after all.

Now onto your second point:

You again want to convert something of legacy Java to Java 8 without looking at the bigger picture. This part has already been answered by @IanRoberts, though I think that you need to do players.stream().filter(...)... over what he suggested.

User · Answer

This what helped me   List lt RepositoryFile gt  fileList   response getRepositoryFileList    RepositoryFile file1   fileList stream   filter f - gt  f getName   contains  my-file txt    findFirst   orElse null     Taken from Java 8 Finding Specific Element in List with Lambda

User · Answer

You can also throw an exception   Note   For the sake of readability each step of stream should be listed in new line   players stream           filter player - gt  player getName   contains name           findFirst           orElseThrow MyCustomRuntimeException  new     if your logic is loosely  exception driven  such as there is one place in your code that catches all exceptions and decides what to do next  Only use exception driven development when you can avoid littering your code base with multiples try-catch and throwing these exceptions are for very special cases that you expect them and can be handled properly

User · Answer

The return there is returning from the lambda expression rather than from the containing method   Instead of forEach you need to filter the stream   players stream   filter player - gt  player getName   contains name           findFirst   orElse null     Here filter restricts the stream to those items that match the predicate  and findFirst then returns an Optional with the first matching entry   This looks less efficient than the for-loop approach  but in fact findFirst   can short-circuit - it doesn t generate the entire filtered stream and then extract one element from it  rather it filters only as many elements as it needs to in order to find the first matching one   You could also use findAny   instead of findFirst   if you don t necessarily care about getting the first matching player from the  ordered  stream but simply any matching item   This allows for better efficiency when there s parallelism involved

User · Answer

If you want to return a boolean value  then you can use something like this  much faster than filter    players stream   anyMatch player - gt  player getName   contains name

[java] Return from lambda forEach() in java

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