How to Add Optional to List Only If Present and Exists in Java

How do we add an Optional value to a list only if it is present?

Suppose we have an Optional opt that we want to add to this list lst only if a value exists.

ArrayList<String> lst = new ArrayList<>();
Optional<String> opt = getOptionalFromSomewhere();

Add to list using `isPresent()`

We can use the isPresent() function on the Optional instance to check if a value is present.

if (opt.isPresent()) {
  lst.add(opt.get())
}

We can be slightly more concise by using ifPresent(), which will perform some action if a value is present.

opt.ifPresent(lst::add);

We can modify this to run some callback function if a value exists.

opt.ifPresent(value -> {
  // do something with value
});

Note that the callback should not return anything itself. Any return value will be lost.