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 · Accepted 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

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

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 lt Player gt  playersOfTeam   players stream        filter player - gt  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 lt Player gt   now you have a Stream lt Player gt   Filter out all unwanted elements with a Predicate lt Player gt   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 lt E gt  over ArrayList lt E gt   Use diamond inference for the type parameter in new ArrayList lt  gt     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

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

[java] Return from lambda forEach() in java

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