Java Stream.map vs Collectors.mapping – Key Differences

javajava-8java-stream

I've noticed many functionalities exposed in Stream are apparently duplicated in Collectors, such as Stream.map(Foo::bar) versus Collectors.mapping(Foo::bar, ...), or Stream.count() versus Collectors.counting(). What's the difference between these approaches? Is there a performance difference? Are they implemented differently in some way that affects how well they can be parallelized?

Best Answer

The collectors that appear to duplicate functionality in Stream exist so they can be used as downstream collectors for collector combinators like groupingBy().

As a concrete example, suppose you want to compute "count of transactions by seller". You could do:

Map<Seller, Long> salesBySeller = 
    txns.stream()
        .collect(groupingBy(Txn::getSeller, counting()));

Without collectors like counting() or mapping(), these kinds of queries would be much more difficult.

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Java – Differences Between Public, Protected, Package-Private, and Private Access Modifiers

The official tutorial may be of some use to you.

	Class	Package	Subclass (same pkg)	Subclass (diff pkg)	World
`public`	+	+	+	+	+
`protected`	+	+	+	+
no modifier	+	+	+
`private`	+

+ : accessible
blank : not accessible

Java – Differences Between HashMap and Hashtable

There are several differences between HashMap and Hashtable in Java:

Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.
One of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.

Since synchronization is not an issue for you, I'd recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.

Best Answer

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Java – Differences Between Public, Protected, Package-Private, and Private Access Modifiers

Java – Differences Between HashMap and Hashtable

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