06-merge-map.asc

Maps and Streams

Objective

Develop code that uses the merge(), flatMap(), and map() methods on Java Streams

About the objectives of this section, only the merge method has not yet been introduced in other sections.

1. You can put a new value on a map, or change the value that was already present using the merge method.

   src/org/j6toj8/collections/mergemap/Collections_Merge.java

   link:../../../src/org/j6toj8/collections/mergemap/Collections_Merge.java[role=include]

   console output

   Map before merge: {1=String1-, 2=String2-}
   Map after merge: {1=String1-StringA, 2=String2-StringB, 3=StringC, 4=StringD}

   Note that for keys that were already present in Map, the lambda function was applied. For keys that were not already present, only the String passed as value was entered.

2. You can transform values into a collection with the map method.

   src/org/j6toj8/collections/mergemap/Collections_Map.java

   link:../../../src/org/j6toj8/collections/mergemap/Collections_Map.java[role=include]

   console output

   2
   4
   6
   8
   10
   12
   14
   16
   18

3. You can traverse another Stream in sequence with the current Stream using the flatMap method.

   src/org/j6toj8/collections/mergemap/Collections_FlatMap.java

   link:../../../src/org/j6toj8/collections/mergemap/Collections_FlatMap.java[role=include]

   console output

    With map:
   java.util.stream.ReferencePipeline$Head@5f184fc6
   
    With flatMap:
   M
   a
   n
   o
   e
   l

   Note that a transformation that results in another Stream is traversed as if it were the original Stream itself.

References

• Additions in Java 8

  Boyarsky, Jeanne; Selikoff, Scott. OCP: Oracle Certified Professional Java SE 8 Programmer II Study Guide (p. 152). Wiley. Kindle Edition.

• Using Streams

  Boyarsky, Jeanne; Selikoff, Scott. OCP: Oracle Certified Professional Java SE 8 Programmer II Study Guide (p. 185). Wiley. Kindle Edition.

• The Java 8 Stream API Tutorial.

• Merging Two Maps with Java 8.