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POFinding partial duplicate values and keeping a specific
 

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    <p>I have a list which contains a value and another list, in this case a trainNumber with corresponding list of Stations. However there are duplicate trainNumbers, which might have different stations. From this I am trying to create a new list keeping the trainNumbers which have the most stations.</p> <p>For example if I have a list which contains: </p> <pre><code>String trainNumber = 1, List trainNumber = [Station1, Station3] String trainNumber = 2, List trainNumber = [Station1, Station2, Station3] String trainNumber = 3, List trainNumber = [Station1, Station3] String trainNumber = 3, List trainNumber = [Station3] </code></pre> <p>I want the new list to contain: </p> <pre><code>String trainNumber = 1, List trainNumber = [Station1, Station3] String trainNumber = 2, List trainNumber = [Station1, Station2, Station3] String trainNumber = 3, List trainNumber = [Station1, Station3] </code></pre> <p>I have seen usage of Set to remove duplicate list items, however I need to specify which item I want to keep.</p> <pre><code>HashSet&lt;Trains&gt; newList = new HashSet&lt;Trains&gt;(); for(Trains train: trainOverview){ String trainNumber = train.getTrainNumber(); int stationSize = train.getStations().size(); int largest = 0; for(Trains trainCopy: trainOverview){ if(trainNumber.equals(trainCopy.getTrainNumber())){ int stationCopySize = trainCopy.getStations().size(); if(stationCopySize &gt; largest) largest = stationCopySize; } } if(togSize &gt;= largest){ newList.add(tog); } } trainOverview.clear(); trainOverview.addAll(newList); </code></pre> <p>Now this kinda works, but I find it extremely messy. I am using HashSet to remove duplicates which have the same amount of stations (which also occurs). There surely must be a better way to go about this problem?</p> <p><strong>EDIT</strong>: Thanks for you answers, but i see that the put method replaces the value of the key. Im my case I would like to keep the values (stations) that were mapped the first time to the key(trainNumber) in addition to the new ones.</p> <p>I've never worked with a Map before, but this is my approach (not sure if Im using it correct):</p> <pre><code> Map&lt;String, List&lt;Station&gt;&gt; overview= new TreeMap&lt;String, List&lt;Station&gt;&gt;(); for(Trains train: trainOverview){ List&lt;Stations&gt; lista = overview.get(train.getTrainNumber()); //Merge lists if the key already exists, and replace the old value with the merged list if(overview.containsKey(train.getTrainNumber())){ Set setboth = new HashSet(lista); setboth.addAll(train.getStations()); lista.clear(); lista.addAll(setboth); overview.put(train.getTrainNumber(), lista); } //If no key exists, create a new entry else{ overview.put(train.getTrainNumber(), train.getStations()); } } </code></pre>
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    1. COCreate a mapping from trainNumbers to a set of stations and just fill it.
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