Java Stream List to Map

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In this post, we will see how to convert List to Map using Stream in java 8.

Collectors’s toMap() can be used with Stream to convert List to Map in java.

Consider a class Named Movie which have 3 fields – id, name and genre

Create a list of movies and convert with to map with name as key and genre as value.

Create a class Named ConvertListToMapMain

Output:

{Interstellar=SciFi, Matrix=SciFi, The Hangover=Comedy, Forest gump=Comedy, 3 idiots=Comedy}

moviesMap contains name as key and genre as value.
We can also use method references instead of lamda expressions.

What happens in case of a duplicate key?

In case, duplicate keys are present, then it will throw java.lang.IllegalStateException exception.

Let’s say we have two movies with name Matrix

When you run ConvertListToMapMain again, you will get below output:
Output:

Exception in thread “main” java.lang.IllegalStateException: Duplicate key Sci Fi
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.HashMap.merge(HashMap.java:1254)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)
at org.arpit.java2blog.ConvertListToMapMain.main(ConvertListToMapMain.java:14)

To resolve this issue, we need to pass merge BinaryOperator function to Collectors.toMap().

When you run ConvertListToMapMain again, you will get below output:
Output:

{Matrix=Comedy, Interstellar=SciFi, Forest gump=Comedy, 3 idiots=Comedy}

What if you want specific Map such as TreeMap?

You can pass constructor method reference to Collectors.toMap() to get specific map.

When you run ConvertListToMapMain again, you will get below output:
Output:

{3 idiots=Comedy, Forest gump=Comedy, Interstellar=SciFi, Matrix=Comedy}

As you can see, we have TreeMap which is sorted by name.

Get multiMap with Collectors.groupingBy()

In case, you want movie grouped by genre as key and name as value, then you can use Collectors.groupingBy() and Collectors.mapping as below:

Output:

{Comedy=[3 idiots, Forest gump, The Hangover], SciFi=[Interstellar, Matrix]}

As you can see, moviesGenMap has genre as key and list of movies name as value.

Excercise

Given a list of Employee objects, you need to convert list to Map with name as key and age as value. In case, there are duplicate keys, then you need to preserve old value.
Here is the definition of Employee class.

That’s all about Java Stream List to Map

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One Response

  1. Suman June 10, 2020

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