How to group a list of objects by an attribute and then iterate the results using (Key, Value) pair?

Question

I want to Group a list of objects by an attribute and then iterate the results using (Key, Value) pair.

I have found the way in Java 8 to group the list of objects with an attribute as follows

// filteredPageLog has the filtered results from PageLog entity.

Map<String, List<PageLog>> results = 
filteredPageLog.stream().collect(Collectors.groupingBy(p -> p.getSessionId()));

But the results will have only entry sets(has values inside entrySet attribute). The keySet and valueSet will be having null values. I want to iterate something like

results.forEach((key,value) -> {
//logic
});

done in your desired way: `results.forEach((key,value) -> { //logic });` — Vishwa Ratna, Feb 19 '19 at 11:53

score 3 · Answer 1 · answered Feb 19 '19 at 08:32

3

Use

results.entrySet().forEach(entry -> {
     var key = entry.getKey();
     var value = entry.getValue();
//logic
});

answered Feb 19 '19 at 08:32

talex

17,973
3
29
66

getting null pointer exception on this line "results.entrySet().forEach(entry -> {" @talex – Mohammed Shirhaan Feb 19 '19 at 09:10
@MohammedShirhaan this mean `results` is null. – talex Feb 19 '19 at 09:13
The entrySet has values when I debug. @talex – Mohammed Shirhaan Feb 19 '19 at 09:36
@What is `null` then? – talex Feb 19 '19 at 10:04
for (Map.Entry> entry : results.entrySet()) { String sessionId = entry.getKey(); List pageLogs = entry.getValue(); //logic } // this worked – Mohammed Shirhaan Feb 19 '19 at 10:40

score 2 · Answer 2 · edited Feb 19 '19 at 13:44

Dummy map:

Map<String,String> map = new HashMap() {{
             put("1","20");
             put("2","30");
           }};

You can do it in two ways:

1. Map<K, V>.forEach() expects a BiConsumer<? super K,? super V> as argument, and the signature of the BiConsumer<T, U> abstract method is accept(T t, U u).

map.forEach((keys,values) -> { String k = keys ;
                               String v= values;
                                //logic goes here
                              });

2. Map<K, V>.entrySet().forEach() expects a Consumer<? super T> as argument, and the signature of the Consumer<T> abstract method is accept(T t).

map.entrySet().forEach((entry) -> { String k = entry.getKey();
                                    String v= entry.getValue();
                                    //logic goes here
                                   });

score 0 · Answer 3 · answered Feb 19 '19 at 08:55

0

There are no tuples in Java. Here you can find more detailed explanation. You can iterate over keys, values or entries, but not tuples.

answered Feb 19 '19 at 08:55

ilinykhma

980
1
8
14

There is no tuples means? – Vishwa Ratna Feb 19 '19 at 13:32
Means there is no syntactic sugar like in Scala, for example, see https://www.alexecollins.com/java-tuples. In Java you can't write (key, value) instead of entry. Only as a lambda to overloaded method, which receives suitable functional interface. – ilinykhma Feb 19 '19 at 14:11
cant write `can't write (key, value) instead of entry` you sure? – Vishwa Ratna Feb 19 '19 at 14:23
I forgot about map.forEach(BiConsumer super K,? super V>), your answer is perfect. So, in this case lambda looks like two element tuple ) – ilinykhma Feb 19 '19 at 14:24
exactly! don't worry i was checking your concept. :) – Vishwa Ratna Feb 19 '19 at 14:25

How to group a list of objects by an attribute and then iterate the results using (Key, Value) pair?

3 Answers3