I'm making a custom class that implements comparable, and I'd like to throw some kind of exception if somebody tries to compare two objects that are not comparable by my definition. Is there a suitable exception already in the API, or do I need to make my own?
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2 Answers
Not that I know of.
The most accurate Exception to represent this is probably an IllegalArgumentException:
http://docs.oracle.com/javase/7/docs/api/java/lang/IllegalArgumentException.html
You should probably also be implementing Comparable<CustomClass> which will prevent callers from providing an instance of the wrong class.
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Consider ClassCastException, it is what Java Collection Framework throws for such situations. This is what happens when we try to add a non-comparable Test1 to a TreeSet
Exception in thread "main" java.lang.ClassCastException: Test1 cannot be cast to java.lang.Comparable
at java.util.TreeMap.compare(TreeMap.java:1188)
at java.util.TreeMap.put(TreeMap.java:531)
at java.util.TreeSet.add(TreeSet.java:255)
at java.util.AbstractCollection.addAll(AbstractCollection.java:334)
at java.util.TreeSet.addAll(TreeSet.java:312)
at java.util.TreeSet.<init>(TreeSet.java:160)
at Test1.main(Test1.java:9)
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philo
+1 for ClassCastException. This is referenced in the compareTo javadoc: docs.oracle.com/javase/8/docs/api/java/lang/…. But ideally, as Cory Kendall says above, you should try to come up with a class hierarchy that prevents this.