Performing Integers operation on List<Objects>

Question

I have a List of Objects which I want to convert to a List of Integers

List<Object> list = new ArrayList<>();

list.add("StringTest");
list.add(10);
list.add(5.9);
list.add(2.5f);
list.add(12L);
...            // different datatypes

List<Integer> result = new ArrayList<>();
// convert items from list and add to result

What could be a good custom implementation to add values from different datatypes of List<Object> to a List<Integer>

1. Strings may be appended based on their length.
2. Floating numbers needs to be rounded

I know standard implmentation on How to cast an Object to an int in java? but I am looking for a good structure to write a generic one.

Answer 1

You can use streams to process the list of Object types as shown in the below code (follow the comments):

//Collect all Number types to ints
List<Integer> output1 = list.stream().
    filter(val -> !(val instanceof String)).//filter non-String types
    map(val -> ((Number)val).floatValue()).//convert to Number & get float value
    map(val -> (int)Math.round(val)).//apply Math.round
    collect(Collectors.toList());//collect as List

//Collect all String types to int
List<Integer> output2 = list.stream().
      filter(val -> (val instanceof String)).//filter String types
       map(val -> (((String)val).length())).//get String lengths
       collect(Collectors.toList());//collect as List

//Now, merge both the lists
output2.addAll(output1);

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Floating numbers needs to be rounded

You need to first convert the Number value to float using floatValue() method and then apply Math.round as shown above.