错误定位
当时并没有弄明白为什么会出错，一脸懵逼相
解决办法：
将source compatibility和target compatibility都改为1.7，重新build就ok了。
错误原因：
官方解释
You can replace the type arguments required to invoke the constructor of a generic class with an empty set of type parameters (<>) as long as the compiler can infer the type arguments from the context. This pair of angle brackets is informally called the diamond.
For example, consider the following variable declaration:
Map<String, List<String>> myMap = new HashMap<String, List<String>>();
In Java SE 7, you can substitute the parameterized type of the constructor with an empty set of type parameters (<>):
Map<String, List<String>> myMap = new HashMap<>();