Why do I get `java.lang.NoClassDefFoundError: scala/Function1` when I run my code in ScalaIDE?

adding the Scala dependency to the maven build can fix the issue :

<dependency>
    <groupId>org.scala-lang</groupId>
    <artifactId>scala-library</artifactId>
    <version>2.10.3</version>
</dependency>

As for me, in the configuration of "Run/Debug configurations" [idealJ]

remember to choose the "Include dependencies with "Provided" scope"