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I want to extract a List<E> from a Map<String, List<E>> (E is a random Class) using stream().
I want a simple one-line method using java 8's stream.
What I have tried until now :
HashMap<String,List<E>> map = new HashMap<>(); List<E> list = map.values(); // does not compile list = map.values().stream().collect(Collectors.toList()); // does not compile 
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HashMap and List in Java. Java provides us with different data structures with various properties and characteristics to store objects. Among those, HashMap is a collection of key-value pairs that maps a unique key to a value. Also, a List holds a sequence of objects of the same type.
Different ways to iterate through Map :Using keySet(); method and Iterator interface. Using entrySet(); method and for-each loop. Using entrySet(); method and Iterator interface. Using forEach(); in Java 1.8 version.
map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:
List<E> result = map.values() .stream() .flatMap(List::stream) .collect(Collectors.toList()); If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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