Iterating Map Entries Effciently

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Maps

The java.util.Map interface represents a mapping between keys and their values. A map cannot contain duplicate keys; and each key can map to at most one value.

Since Map is an interface, then you need to instantiate a concrete implementation of that interface in order to use it; there are several Map implementations, and mostly used are the java.util.HashMap and java.util.TreeMap

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Iterating Map Entries Effciently

This section provides code and benchmarks for ten unique example implementations which iterate over the entries of a Map and generate the sum of the Integer values. All of the examples have an algorithmic complexity of Θ(n), however, the benchmarks are still useful for providing insight on which implementations are more efficient in a "real world" environment.

1.Implementation using Iterator with Map.Entry

 Iterator<Map.Entry<Integer, Integer>> it = map.entrySet().iterator();
 while (it.hasNext()) {
 Map.Entry<Integer, Integer> pair = it.next();
 sum += pair.getKey() + pair.getValue();
 }
 
2. Implementation using for with Map.Entry

 for (Map.Entry<Integer, Integer> pair : map.entrySet()) {
 sum += pair.getKey() + pair.getValue();
 }
 
3. Implementation using Map.forEach (Java 8+)

 map.forEach((k, v) -> sum[0] += k + v);
 
4. Implementation using Map.keySet with for

 for (Integer key : map.keySet()) {
 sum += key + map.get(key);
 }
 
5. Implementation using Map.keySet with Iterator

 Iterator<Integer> it = map.keySet().iterator();
 while (it.hasNext()) {
 Integer key = it.next();
 sum += key + map.get(key);
 }
 
6. Implementation using for with Iterator and Map.Entry

 for (Iterator<Map.Entry<Integer, Integer>> entries =
 map.entrySet().iterator(); entries.hasNext(); ) {
 Map.Entry<Integer, Integer> entry = entries.next();
 sum += entry.getKey() + entry.getValue();
 }
 
7. Implementation using Stream.forEach (Java 8+)

 map.entrySet().stream().forEach(e -> sum += e.getKey() + e.getValue());
 
8. Implementation using Stream.forEach with Stream.parallel (Java 8+)

 map.entrySet()
 .stream()
 .parallel()
 .forEach(e -> sum += e.getKey() + e.getValue());
 
9. Implementation using IterableMap from Apache Collections

 MapIterator<Integer, Integer> mit = iterableMap.mapIterator();
 while (mit.hasNext()) {
 sum += mit.next() + it.getValue();
 }
 
10. Implementation using MutableMap from Eclipse Collections

 mutableMap.forEachKeyValue((key, value) -> {
 sum += key + value;
 });
 

Performance Tests

Average Performance of 10 Trials (100 elements) Best: 308±21 ns/op

Average Performance of 10 Trials (10000 elements) Best: 37.606±0.790 μs/op

Average Performance of 10 Trials (100000 elements) Best: 1184.767±332.968 μs/op

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Basic Programs

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• Creating Your First Java Program
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Sets

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Maps

• Iterating Map Entries Effciently
• Using Default Methods of Map from Java 8
• Usage of HashMap and Iterating through the contents of a Map
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TreeMap and TreeSet

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super keyword

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Properties Class

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Lambda Expressions

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Java Floating Point Operations

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Random Number Generation

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Singletons

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Autoboxing

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