Counter with lambda over java8 card

Question

I am trying to convert this:

Map<String,Long> parties = new HashMap<>(); parties.add("a", 1); ... Long counter = 0l; for (Long votes : parties.values()){ counter += votes; } 

In lambda in Java8, I try it with a decrease:

 parties.entrySet().stream().reduce((stringLongEntry, stringLongEntry2) -> /*Here I Stack*/) 

But I do not know how to proceed.

PS: I know that I can do this: parties.values().stream().count(); but I want to find a different approach

Answer 1

Try the following expression:

 counter = parties.values().stream().map((votes) -> votes).reduce(counter, (a, i) -> a+i); 

In addition, there are several errors in your code:

• Using Map<String,Long> parties = new HashMap<>(); is the right way, but yours is wrong.

• HashMap does not have a .add(..) method, but .put(..) :

   parties.put("a",1L); 

• Since your value is Long , you should use 1L or 1L instead of the integer 1 to indicate the Long value.

Answer 2

If you always save 1 as a value for each key, then the total will always match the size of the card. You can get it simply with parties.size() .

If you save different values ​​for each key, then the calculation of the number of values ​​that you have on the map is incorrect. You must summarize them:

 long total = parties.values().stream().mapToLong(v -> v).sum(); 

Answer 3

• parties.values().stream().mapToLong(l -> l).sum();
• parties.values().stream().reduce(0L, (a, b) -> a + b);

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3. parties.entrySet().stream().mapToLong(Map.Entry::getValue).sum();
4. parties.entrySet().stream().mapToLong(Map.Entry::getValue).reduce(0L, (a, b) -> a + b);

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Explanation of the issue in the comments. Here we can write either (Map.Entry<String, Long> i) -> i.getValue() , or i -> i.getValue() . But it will be more readable if we replace it with a reference to a method , for example Map.Entry::getValue .

Answer 4

if you insist on entrySet -

 parties.entrySet().stream().map(e -> e.getValue()).reduce(0L, (longValue1, longValue2) -> longValue1 + longValue2)