The following code throws a ClassCastException:
TreeSet<Identity> members = new TreeSet<Identity>();
Star star = new Star();
members.add(star);
Jovian jov = new Jovian();
members.add(jov);
Both Jovian and Star inherit from CelestialBody, which, in turn, inherits from Identity.
why the ClassCastException?
sorry the ClassCastException is on the last line.
BTW ive tried:
TreeSet members = new TreeSet();
Star star = new Star();
members.add(star);
Jovian jov = new Jovian();
members.add(jov);
with the same result, and doing this:
TreeSet<? extends Identity> members = new TreeSet<Identity>();
Star star = new Star();
members.add(star);
Jovian jov = new Jovian();
members.add(jov);
results in compile time errors at lines 3 and 5.
-sigh-
here's the trace (not that it really illuminates anything):
java.lang.ClassCastException: com.zenithhouse.celestial.Jovian
at java.util.TreeMap.compare(TreeMap.java:1093)
at java.util.TreeMap.put(TreeMap.java:465)
at java.util.TreeSet.add(TreeSet.java:210)
at com.zenithhouse.celestial.foundry.RegularJovianDiscFactoryUnitTest.testAddingMember(RegularJovianDiscFactoryUnitTest.java:24)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:478)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:344)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)
Simple... Different authors at different times for different jobs.
You need to implement Comparable... which underlies all sort of sorting in the Collections framework.
A Comparator is an object which allows other objects to be compared.
A Comparable object is one which can be compared "inherently".
> actually, I've been reading the javadoc on Comparable
> and Comparator, and they both kinda look the same to
> me. What's the difference?
Implementing Comparable means that the objects of a class can be compared to each other, that they have some natural order. For example strings have a can be ordered lexicographically so that a < aa < ab < b < ba.
Comparators are for implementing an order different from the natural. You might for example want to reverse the natural order or base the comparison on some other characteristic. For example there are comparators that enforce a national standard ordering. For example in English you have a < ?< z while in Finnish and Swedish you have a < z < ?
