Java transform ArrayList of type object to another ArrayList of type object

Question

I have a class with member variables:

class Filter{
  private String key;
  private String operator;
  private Object value;
}

And List of Type Filter as below:

[
{
  key: status,
  operator:equal,
  value: active
},
{
  key: status,
  operator:equal,
  value: inactive
},
{
  key: name,
  operator:equal,
  value: jhon
},
{
  key: id,
  operator:equal,
  value: 123
}
] 

I want to transform the above List into a new List as below where if the key is same key: status then in new list need to have only one entry with value is in list type value: [active, inactive] and operator will be IN

[
{
  key: status,
  operator:in,
  value: [active, inactive]
},
{
  key: name,
  operator:equal,
  value: jhon
},
{
  key: id,
  operator:equal,
  value: 123
}
]
 

What is the best and efficient way to achieve this solution, preferably using Java 8 stream and lambdas? Wanted to avoid traditional for loop.

Answer 1

You can use Collectors.partitioningBy to split the List into two parts based on whether or not the Filter's key is "status". ^Demo

final Map<Boolean, List<Filter>> parts = list.stream()
        .collect(Collectors.partitioningBy(f -> "status".equals(f.getKey())));
final Object[] statuses = parts.get(true).stream().map(Filter::getValue).toArray();
final List<Filter> result = parts.get(false);
if (statuses.length != 0) {
    result.add(0, new Filter("status", "in", statuses));
}

For multiple keys, you can use Collectors.groupingBy. ^Demo

final Map<String, List<Filter>> map = list.stream().collect(Collectors.groupingBy(Filter::getKey, LinkedHashMap::new, Collectors.toList()));
final List<Filter> result = map.entrySet().stream()
        .map(e -> e.getValue().size() > 1
                ? new Filter(e.getKey(), "in", e.getValue().stream().map(Filter::getValue).toArray())
                : e.getValue().get(0))
        .collect(Collectors.toList());