How to filter a dataframe?

Okay, so I looked at their documentation (partly for self too to see how are they using it compared to Python). What you are trying above is a Python type code in Java. That would not work out.

Before going to how, about the error:

Quote:

The type of the expression must be an array type but it resolved to DataFrame<Object>

This is because, shortAndTall is of type DataFrame<object>. Object -> not am array or a list. Thus, when you try to mask and get comparison code, you will get an error. General case, you would need to cast before use. But, not in this case.

Now, based on the documentation there, it seems you need to use Predicate Interface for any filter. If it's difficult for you, do a Cntrl+F for Predicate word: DataFrame| Joinery[^]
An example of filter there is:

DataFrame<Object> df = new DataFrame<>("name", "value");
for (int i = 0; i < 10; i++)
    df.append(Arrays.asList("name" + i, i));

df.select(new Predicate<Object>() {
        @Override
        public Boolean apply(List<Object> values) {
             return Integer.class.cast(values.get(1)).intValue() % 2 == 0;
        
     })
  .col(1);
// Output [0, 2, 4, 6, 8] }  

You have chosen the library for working with dataframe and thus you need to follow their documentation and the code expected to get the results.

It would be better to go through their documentation in detail, understand it and then refer it if you plan to use it.