Ignore null in Java stream

I am performing some transformations using a Java stream on some data. I would like to ignore any values in a map that contain a null value.

Example:

HashMap<String, Object> vals = new HashMap<>();
vals.put("null", null);
vals.put("string1", "1 ");

I've tried using filter:

Map<String, Object> nullMap = vals.entrySet().stream()
    .filter(e -> e.getValue() != null)
    .collect(Collectors.toMap(
            e -> e.getKey().trim(),
            e -> (e.getValue() instanceof String ? ((String) e.getValue()).trim() : e.getValue())))

However the returned object has the null key/value pair (in my example "null" -> null) completely removed. How can I retain the key "null" and ignore it during any subsequent processing after the .filter step in the Java stream?

In the above example I would like to get a Map returned with where the key and value pairs are trimmed and the null value is 'ignored' during the trimming step.

The toMap collector does not allow null values, so you can’t use it to produce a Map containing null values.

If your input map is mutable and changing it is an option, you could trim the values as simple as

HashMap<String, Object> vals = new HashMap<>();
vals.put("null", null);
vals.put("string1", "1 ");

vals.replaceAll((key,val) -> val instanceof String? ((String)val).trim(): val);

If modifying the source map is not an option or you really have to trim the keys to, you have to use a different collector. You could define a collector ad-hoc, e.g.

HashMap<String, Object> result = vals.entrySet().stream()
    .collect(HashMap::new, (m,e) -> m.put(e.getKey().trim(),
        e.getValue() instanceof String? ((String)e.getValue()).trim(): e.getValue()),
        Map::putAll);

For better readability, i.e. to avoid repeating the same expression, we may use the block lambda syntax and a local variable, like

HashMap<String, Object> result = vals.entrySet().stream()
    .collect(
        HashMap::new,
        (m,e) -> {
            Object value = e.getValue();
            m.put(e.getKey().trim(), value instanceof String? ((String)value).trim(): value);
        }, Map::putAll);

Mind that when applying trim() to the keys, the keys may clash even when they were unique in the original map, e.g. when having " key1" and "key1 ". The toMap collector would throw in such scenarios whereas the ad-hoc collector created above would simply overwrite one of the conflicting mappings without a warning.