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    <p><code>Collections.sort</code> requires a list of <code>Comparable</code>'s. <code>TreeSet</code> doesn't have that restriction in the typing. With the default constructor, the docs say it sorts by " natural ordering of its elements". What isn't clear is how it would sort <code>Object</code>, but your problem deals with the typing.</p> <p>Update: I missed the last part of the question. Without looking at the stacktrace, I would guess that since the default constructor for <code>TreeSet</code> tries to sort by "natural order", internally, it is doing a cast to <code>Comparable</code>, which would cause a <code>ClassCastException</code>.</p> <p>Update: I looked closer at the javadocs for <code>TreeSet</code> (<a href="http://docs.oracle.com/javase/6/docs/api/java/util/TreeSet.html#TreeSet%28%29" rel="nofollow">JDK 6</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html#TreeSet%28%29" rel="nofollow">JDK 7</a>) and it says</p> <blockquote> <p>Constructs a new, empty tree set, sorted according to the natural ordering of its elements. All elements inserted into the set must implement the Comparable interface. Furthermore, all such elements must be mutually comparable: e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the set. If the user attempts to add an element to the set that violates this constraint (for example, the user attempts to add a string element to a set whose elements are integers), the add call will throw a ClassCastException.</p> </blockquote> <p>for both. So, if no ClassCastException happens with JDK6, perhaps it is a bug.</p>
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    1. POHow come I get compile time error when use Collections.sort() but not with TreeSet.add(new Object())
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    1. POHow come I get compile time error when use Collections.sort() but not with TreeSet.add(new Object())
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    1. COBut how come I only get the ClassCastException when running with jdk7 and not jdk6?
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      1. USOlle Söderström
    2. COWhen I looked closer, the javadoc states that the `TreeSet` should throw an ClassCastException when using the default constructor and adding non-`Comparable` objects.
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    3. COIf you add a couple items, how are things being sorted in `TreeSet` in JDK 6?
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