How do I use the new computeIfAbsent function

Question

I very much want to use Map computeIfAbsent but it has been too long since lambdas in undergrad     Almost directly from the docs  it gives an example of the old way to do things   Map lt String  Boolean gt  whoLetDogsOut   new ConcurrentHashMap lt  gt     String key    snoop   if  whoLetDogsOut get key     null      Boolean isLetOut   tryToLetOut key     if  isLetOut    null      map putIfAbsent key  isLetOut       And the new way   map computeIfAbsent key  k - gt  new Value f k       But in their example  I think I m not quite  getting it    How would I transform the code to use the new lambda way of expressing this

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Another example  When building a complex map of maps  the computeIfAbsent   method is a replacement for map s get   method  Through chaining of computeIfAbsent   calls together  missing containers are constructed on-the-fly by provided lambda expressions        Stores regional movie ratings   Map lt String  Map lt Integer  Set lt String gt  gt  gt  regionalMovieRatings   new TreeMap lt  gt           This will throw NullPointerException    regionalMovieRatings get  New York   get 5  add  Boyhood          This will work   regionalMovieRatings      computeIfAbsent  New York   region - gt  new TreeMap lt  gt          computeIfAbsent 5  rating - gt  new TreeSet lt  gt          add  Boyhood

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multi-map  This is really helpful if you want to create a multimap  without resorting to the Google Guava library for its implementation of MultiMap   For example  suppose you want to store a list of students who enrolled for a particular subject   The normal solution for this using JDK library is   Map lt String List lt String gt  gt  studentListSubjectWise   new TreeMap lt  gt     List lt String gt lis   studentListSubjectWise get  a    if lis    null        lis   new ArrayList lt  gt       lis add  John       continue       Since it have some boilerplate code  people tend to use Guava Mutltimap   Using Map computeIfAbsent  we can write in a single line without guava Multimap as follows   studentListSubjectWise computeIfAbsent  a    x - gt  new ArrayList lt  gt      add  John      Stuart Marks  amp  Brian Goetz did a good talk about this https   www youtube com watch v 9uTVXxJjuco

User · Answer

Suppose you have the following code   import java util Map  import java util concurrent ConcurrentHashMap   public class Test       public static void main String   s            Map lt String  Boolean gt  whoLetDogsOut   new ConcurrentHashMap lt  gt             whoLetDogsOut computeIfAbsent  snoop   k - gt  f k            whoLetDogsOut computeIfAbsent  snoop   k - gt  f k              static boolean f String s            System out println  creating a value for     s               return s isEmpty              Then you will see the message creating a value for  snoop  exactly once as on the second invocation of computeIfAbsent there is already a value for that key  The k in the lambda expression k - gt  f k  is just a placeolder  parameter  for the key which the map will pass to your lambda for computing the value  So in the example the key is passed to the function invocation   Alternatively you could write  whoLetDogsOut computeIfAbsent  snoop   k - gt  k isEmpty     to achieve the same result without a helper method  but you won   t see the debugging output then   And even simpler  as it is a simple delegation to an existing method you could write  whoLetDogsOut computeIfAbsent  snoop   String  isEmpty   This delegation does not need any parameters to be written   To be closer to the example in your question  you could write it as whoLetDogsOut computeIfAbsent  snoop   key - gt  tryToLetOut key     it doesn   t matter whether you name the parameter k or key   Or write it as whoLetDogsOut computeIfAbsent  snoop   MyClass  tryToLetOut   if tryToLetOut is static or whoLetDogsOut computeIfAbsent  snoop   this  tryToLetOut   if tryToLetOut is an instance method

User · Answer

Recently I was playing with this method too  I wrote a memoized algorithm to calcualte  Fibonacci numbers which could serve as another illustration on how to use the method   We can start by defining a map and putting the values in it for the base cases  namely  fibonnaci 0  and fibonacci 1    private static Map lt Integer Long gt  memo   new HashMap lt  gt     static      memo put 0 0L     fibonacci 0     memo put 1 1L     fibonacci 1      And for the inductive step all we have to do is redefine our Fibonacci function as follows   public static long fibonacci int x       return memo computeIfAbsent x  n - gt  fibonacci n-2    fibonacci n-1        As you can see  the method computeIfAbsent will use the provided lambda expression to calculate the Fibonacci number when the number is not present in the map  This represents a significant improvement over the traditional  tree recursive algorithm

[java] How do I use the new computeIfAbsent function?

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