CollectionUtils.retainAll方法不起作用（Java）

Question

I have two ArrayLists. I want to compare them, to get the number of common elements. Or just a list with the common elements.

When I try to do this with CollectionUtils.retainAll, it doesn't know this method I think?

It says it cannot find the symbol 'Variable CollectionUtils', so it also doesn't know the method? Or what do I wrong?

My code:

ArrayList<String> lijst1 = new ArrayList<String>(Arrays.asList(inhoud1.split("AT")));
ArrayList<String> lijst2 = new ArrayList<String>(Arrays.asList(inhoud2.split("AT")));

ArrayList matches = CollectionUtils.retainAll(lijst1,lijst2);

Sorry if it's something easy. It probably will be.

Answer 1

The CollectionUtils class is a part of the Apache Commons Collections library, and is not part of the Java API, hence unavailable by default. Include the library as a JAR on your classpath, or use Maven to add it, then import org.apache.commons.collections4.CollectionUtils .

You can also use lijst1.retainAll(lijst2) , but you'll have to use it slightly differently, since ArrayList#retainAll acts by side-effects (that is, it will update lijst1 instead of returning a new collection.