Map put方法抛出空指针异常的问题
我在使用put方法的时候有空指针异常,看javadoc说nullPointerException this map does not permit null keys or values, and the specified key or value is
null.A null return can also indicate that the map previously associated null with the specified key, if the implementation supports
null values.那什么样的实现才可以允许value为null?