ClassCastException when converting return value of a Java method to boolean

Script:

var x = new java.util.HashMap()
x.put('test', new java.io.File('test'))
if (x.get('test')) {
    print('Found!')
}

Exception:


 
Exception in thread "main" java.lang.ClassCastException: java.io.File cannot be cast to java.lang.Number
at sun.invoke.util.ValueConversions.primitiveConversion(ValueConversions.java:199)
at sun.invoke.util.ValueConversions.unboxBoolean(ValueConversions.java:95)
at jdk.nashorn.internal.scripts.Script$t.runScript(t.js:5)
at jdk.nashorn.internal.runtime.ScriptFunctionData.invoke(ScriptFunctionData.java:498)
at jdk.nashorn.internal.runtime.ScriptFunction.invoke(ScriptFunction.java:207)
at jdk.nashorn.internal.runtime.ScriptRuntime.apply(ScriptRuntime.java:378)
at jdk.nashorn.tools.Shell.apply(Shell.java:381)
at jdk.nashorn.tools.Shell.runScripts(Shell.java:310)
at jdk.nashorn.tools.Shell.run(Shell.java:166)
at jdk.nashorn.tools.Shell.main(Shell.java:130)
at jdk.nashorn.tools.Shell.main(Shell.java:109)

This was reported by Tal Liron @ nashorn-dev alias.