immutable types like integers, floats and strings are hashable;
不可变类型比如整形、浮点数和字符串都是散列的;
if the internal state cannot change, there is no chance for different threads to see inconsistent views of the data. immutable types can be exported from your objects safely.
如果内部状态不能被改变,那么对于不同的线程来说,就没有机会看到这个数据的不同值。
the usually rules for immutable types were put into place such as no default constructors and only getters for properties.
通常这些稳定的类型规则被放在非默认的构造器(constructor)和只有getter的属性(property)里。