用Java初始化地图的地图

Question

If I have a Map of Map in Java and I initialized the outer map, then what will be the value of the inner map (null or new Map()) ?

For example, I have:

Map<User, Map<Component, Float>> summaryByEmp = new TreeMap<>();

Now, what will be the value of the inner map? Do I need to initialize too?

Answer 1

There is something wrong with your understanding of Map here. Doing

Map<User, Map<Component, Float>> outerMap = new TreeMap<User, Map<Component, Float>>()

initializes a TreeMap referenced by outerMap . The type parameter only says that, the key to be put needs to be of type User and the value to the key to be of type Map<Component, Float> . Nothing more.

So when you insert a key-value pair of type mentioned, you insert it to the TreeMap .

Answer 2

Your map summaryByEmp will be empty utill you add some key value pair in your map. So question

what will be the value of inner map

does not arises at very first place

Map<User, Map<Component, Float>> summaryByEmp = new TreeMap<>();

If you want to add it

Map<Component, Float> innerMap = new TreeMap();
summaryByEmp.put(new User(),innerMap  )

Answer 3

It will have no value, just like the type User , Component and Float . You will need to add data to the map like:

Map<Component, Float> mapVal = new TreeMap<>();
/// Fill mapVal
User u = new User();
summaryByEmp.put(u, mapVal);