GroupingBy in Java8-Streams

Question

I am using Grouping-By in Streams:

1: I want to know how can i use "Grouping-By" twice in Collect method.

2: Secondly,What is the strategy to define return types in grouping?

1:

Map<String,String> sum1 = Items.items().stream()
                       .collect(Collectors.groupingBy(Items::getName,
                               Collectors.groupingBy(Items::getName)));

error message:

1:Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - incompatible types: inference variable D has incompatible equality constraints java.lang.String,java.lang.Integer at collectorsinjava.CollectorsInJava.main(CollectorsInJava.java:33)

2:

     Map<String, Map<String,Integer>> sum1 = Items.items().stream()
                .collect(Collectors.groupingBy(Items::getName,
                            Collectors.groupingBy(Items::getQuantity)));

error message:

2:Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - incompatible types: inference variable K has incompatible bounds equality constraints: java.lang.String lower bounds: java.lang.Integer at collectorsinjava.CollectorsInJava.main(CollectorsInJava.java:37)

This is my list of items:

List<Items> item = Arrays.asList(new Items("A",22,new BigDecimal("23.3")),
 new Items("B",33,new BigDecimal("19.99")),

 new Items("C",31,new BigDecimal("23.3")),

 new Items("D",22,new BigDecimal("19.99")),

 new Items("B",33,new BigDecimal("23.3")),

 new Items("C",31,new BigDecimal("19.99")),

 new Items("D",22,new BigDecimal("23.3"))
);

can someone please suggest how I can avoid the compilation errors?

What is the desired output in these two snippets? And why are you grouping twice by the same property in the first snippet? — Eran
@Eran i want to use GroupingBy twice ,what is the correct way to define types in collect method? — Beenish Sajjad
@Aominè the second condition is till not working with your strategy. — Beenish Sajjad
@BeenishSajjad as is, your code doesn't compile. are you trying to remove the compilation errors? if yes then post the error message. when you chain grouping collectors, you'll get back a multi-level map hence my suggestion above and it should work when used properly. — Ousmane D.
@Aominè Yup i am getting error because i have used incorrect data types in Map and again whats the way to define return types in multi-level map? — Beenish Sajjad

Ousmane D. Ousmane D. · Accepted Answer · 2017-12-18T11:57:59

You can group twice or however many times you want by chaining groupingBy collectors as you've done but the issue here is that the receiver type is incorrect.

the receiver type for the first query should be:

Map<String, List<Items>> resultSet = Items.items().stream()
            .collect(Collectors.groupingBy(Items::getName));

as there is no need to group by the same property twice.

when you have two or more grouping collectors chained together, you'll receive a multi-level map so therefore the receiver type for the second query should be:

Map<String, Map<Integer, List<Items>>> anotherResultSet

Otherwise, if you want to adapt a Collector to perform an additional finishing transformation then you're looking for a collectingAndThen.

GroupingBy in Java8-Streams

1 Answers