泛型是多态的一种形式吗？(Are generics a form of polymorphism? [closed])

 我不确定你在找什么奇怪的东西。 您有两个 TreeSet实例，每个实例都有自己的CompareID实例作为Comparator ，每个CompareID实例都维护它自己的计数器。 
 
 因此，看到每个计数器值（0,1,2等）出现两次并不奇怪。 至于计数器值的出现顺序，取决于TreeSet的内部实现。 
 
 至于 
 
 
 
 等于一组树是否是正确的输出？ 
 
 
 是的，两个集都包含相同的元素。 订单无关紧要。 为了澄清 - 如果对于TreeSet的某个元素y ， compare(x,y)==0则contains和containsAll TreeSet的所有方法都会考虑将元素x包含在TreeSet ，其中compare是所提供的Comparator的compare方法。 因此，在你的例子中，只有id属性决定两个元素是否相等。 
 
 以下情形解释了输出： 
 
0 // compare method of 1st CompareID object executed
1 // compare method of 1st CompareID object executed
2 // compare method of 1st CompareID object executed
3 // compare method of 1st CompareID object executed
0 // compare method of 2nd CompareID object executed
1 // compare method of 2nd CompareID object executed      
2 // compare method of 2nd CompareID object executed
3 // compare method of 2nd CompareID object executed
4 // compare method of 1st CompareID object executed
5 // compare method of 1st CompareID object executed
6 // compare method of 1st CompareID object executed
7 // compare method of 1st CompareID object executed
8 // compare method of 1st CompareID object executed
4 // compare method of 2nd CompareID object executed
5 // compare method of 2nd CompareID object executed
6 // compare method of 2nd CompareID object executed
7 // compare method of 2nd CompareID object executed
8 // compare method of 2nd CompareID object executed
 
 编辑：首先，您要向第一个TreeSet添加元素（因此compare第一个Comparator连续多次调用），然后您将元素添加到第二个TreeSet （因此compare第二个Comparator连续多次调用），然后你调用treeSetA.containsAll(treeSetB) ，它导致第一个Comparator被连续多次调用，最后你调用treeSetB.containsAll(treeSetA) ，这导致第二个Comparator被多次调用一排。 

I'm not sure what you are finding strange. You have two TreeSet instances, each having its own CompareID instance serving as Comparator, and each CompareID instance maintains it's own counter.
 
Therefore it's not a surprise to see each counter values (0,1,2,etc...) appearing twice. As for the order of appearance of the counter values, that depends on the internal implementation of TreeSet.
 
As for
 
 
 
Equal set of tree // is that correct output ?
 
 
Yes, both sets contain the same elements. The order doesn't matter. To clarify - the methods contains and containsAll of TreeSet consider an element x to be contained in the TreeSet if compare(x,y)==0 for some element y of the TreeSet, where compare is the compare method of the supplied Comparator. Therefore, in your example, only the id property determines if two elements are equal.
 
The following scenario explains the output :
 
0 // compare method of 1st CompareID object executed
1 // compare method of 1st CompareID object executed
2 // compare method of 1st CompareID object executed
3 // compare method of 1st CompareID object executed
0 // compare method of 2nd CompareID object executed
1 // compare method of 2nd CompareID object executed      
2 // compare method of 2nd CompareID object executed
3 // compare method of 2nd CompareID object executed
4 // compare method of 1st CompareID object executed
5 // compare method of 1st CompareID object executed
6 // compare method of 1st CompareID object executed
7 // compare method of 1st CompareID object executed
8 // compare method of 1st CompareID object executed
4 // compare method of 2nd CompareID object executed
5 // compare method of 2nd CompareID object executed
6 // compare method of 2nd CompareID object executed
7 // compare method of 2nd CompareID object executed
8 // compare method of 2nd CompareID object executed
 
EDIT : First you are adding elements to the first TreeSet (hence compare of the first Comparator is called several times in a row), then you are adding elements to the second TreeSet (hence compare of the second Comparator is called several times in a row), then you call treeSetA.containsAll(treeSetB) which causes compare of the first Comparator to be called several times in a row, and finally you call treeSetB.containsAll(treeSetA) which causes compare of the second Comparator to be called several times in a row.