Convert List<Map<String, Object>> to Map<String, List<Map<String, Object>>> using Java 8 streams API

Question

My question is little similar to what has already been asked here in stackoverflow. My input is as follows:

Map<String, Object> m1 = new HashMap<>();
m1.put("group", "1");
m2.put("A", 10l);
m2.put("B", 20l);
m2.put("C", 100l);

Map<String, Object> m2 = new HashMap<>();
m1.put("group", "1");
m2.put("A", 30l);
m2.put("B", 40l);
m2.put("C", 500l);

List<Map<String, Object>> beforeFormatting = new ArrayList<>();
beforeFormatting.add(m1);
beforeFormatting.add(m2);

My expected output is:

Map<String, List<Map<String, Object>>> afterFormatting;

Output:

1 -> [m1, m2]

I have tried below, but getting compile errors: seems like I am doing something wrong with mapping:

Map<String, List<Map<String, Object>>> afterFormatting = beforeFormatting.stream()
                    .collect(groupingBy(map -> map.get("group_id"), toList()));

Error:(114, 33) java: incompatible types: inference variable K has incompatible bounds equality constraints: java.lang.String lower bounds: java.lang.Object — Gaurav Saini
– Gaurav Saini, Commented Jan 29, 2018 at 20:24

Eugene · Accepted Answer · 2018-01-29 20:27:11Z

2

Well map.get("group_id") returns an Object, but you try to map it to a String, change the return type:

Map<Object, List<Map<String, Object>>> afterFormatting = beforeFormatting.stream()
            .collect(Collectors.groupingBy(map -> map.get("group_id")));

And also you can drop toList, since it is implicitly used when groupingBy with a single parameter is used.

answered Jan 29, 2018 at 20:27

Eugene

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3 Comments

Gaurav Saini Over a year ago

Wow thanks!! I guess I got a bit lost & my mind went in a different direction. Is it ok to typecast like this : (String)map.get("group_id") ?

Eugene Over a year ago

@GauravSaini how about proving the correct types initially?

Gaurav Saini Over a year ago

'@Eugene my expected key needs to be a String type and I am less keen to keep keys open ended.

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