TreeSet集合在哪种情况下会报错

1.自然排序中的元素对象，都必须实现了Comparable接口，否则会抛出异常，案例如下：

public class MySetTree {

  public static void main(String args[]){
      TreeSet treeSet = new TreeSet();
      treeSet.add(new SetTreeTest());
      treeSet.add(new SetTreeTest());
      treeSet.add(new SetTreeTest());

      System.out.println(treeSet);
  }
}
class SetTreeTest{
}

修正后：

public class MySetTree {

  public static void main(String args[]){
      TreeSet treeSet = new TreeSet();
      treeSet.add(new SetTreeTest());
      treeSet.add(new SetTreeTest());
      treeSet.add(new SetTreeTest());

      System.out.println(treeSet);
  }
}
class SetTreeTest implements Comparable{
    @Override
    public int compareTo(Object o) {
        return 0;
    }
}

2.由于TreeSet会调用元素的compareTo方法，这就要求所有元素的类型都相同，否则也会发生异常

public class MySetTree {

  public static void main(String args[]){
      TreeSet treeSet = new TreeSet();
      treeSet.add("1");
      treeSet.add(0);
      System.out.println(treeSet);
  }
}

3.当在TreeSet中放置null值时，会报空指针异常

public class MySetTree {

  public static void main(String args[]){
      TreeSet treeSet = new TreeSet();
      treeSet.add(null);
      System.out.println(treeSet);
  }
}