javafx.util.Pair and other classes from javafx.util are not included in OpenJDK. Options for resolving:

SWITCH CLASS
Map.Entry<K, V> from java.util is similar to javafx.util.Pair.

INSTALL JAVA-OPENJFX
Install java-openjfx through your package manager. (or whatever means you used to install Java on your machine) Note that java-openjfx is compatible with OpenJDK8, not previous versions. After installing java-openjfx, you may have to add it manually to your IDE SDK classpath. Eg in IntelliJ, you may have to go to Project Structure | SDKs | <select your SDK> | Classpath | + (the Classpath +, not the SDKs +) | and add /usr/lib/jvm/java-8-openjdk/jre/lib/ext/jfxrt.jar (which should be there now that java-openjfx has been installed) | OK .

SWITCH JDK
Switch to the Oracle JDK. (bleh)

KEYWORDS
How to tutorial setup openjdk openjfx classnotfoundexception javafx.util javafx.util.* javafx.util.Pair IntelliJ IDEA compilation error doesn't compile not found