Blank final with Reference type

Question

why we can change a hashMap which is declared as blank final, but we cannot change a primitive type? for example if I create a map

final Map<String, String> someMap;

and initialize it in constructor, and still I can put values in this. But same is not the case with primitive

final int a;

I cant change the value of a in this case. can somebody explain this ?

This might be of interest: http://stackoverflow.com/questions/154314/when-should-one-use-final — Qben, Aug 29 '13 at 08:59
Are you saying you can't change 'a' in the constructor? Because that should be allowed. — Kayaman, Aug 29 '13 at 09:01

Juned Ahsan · Accepted Answer · 2013-08-29T08:59:35.763

3

final means it cannot be changed once initialized. You are just declaring the variable but not initializing it, hence it is allowed.

So doing this is valid

final Map<String, String> someMap;
someMap = new HashMap<String, String>();

But if you try to assign another value to it post initialization then compiler should throw an error that final variable is already intialized:

    final Map<String, String> someMap;
    someMap = new HashMap<String, String>();
    someMap = new TreeMap<String, String>(); //error here

Note: Also putting/removing values in hashmap does not change the reference of the final variable.

edited Aug 29 '13 at 08:59

answered Aug 29 '13 at 08:54

Juned Ahsan

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but same is not allowed in the case of primitives ? – Aug 29 '13 at 08:55
primitives are data types that hold the value, while references are like pointers, which do not hold their values but point to their values and are used on/with objects. If you understand this part, then you will understand also why you can do put on a final hashMap reference. – Juned Ahsan Aug 29 '13 at 09:08

score 1 · Answer 2 · answered Aug 29 '13 at 08:54

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It is just the reference to the map (i.e. the variable someMap) which cannot be changed. The map itself can be changed. You can for example insert values. But you cannot assign a new map to someMap.

answered Aug 29 '13 at 08:54

Henry

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score 1 · Answer 3 · answered Aug 29 '13 at 09:00

When using the final keyword on variables you are saying that the variable can be defined only once. In other words once a value has been assigned to the variable, it cannot be reassigned.

This yields obvious behavior with primitive types but is less obvious with objects. Importantly though when inserting values into a map, the object instance remains the same. This is important to remember when passing objects to methods, and really important when using get/set/clone methods as you may end up with multiple references to the same object, where a change in one place (insert entry into map) may have undefined effects in others.

If the Map in your question is important you can use java.util.Collections.unmodifiableMap(m); to stop people fiddling with it.

score 1 · Answer 4 · answered Aug 29 '13 at 09:14

*emphasized text*When you write:

final Map<String, String> someMap;

it's important to realise that someMap is a reference, and you're declaring the reference to be final. The actual object is not immutable, but the reference is. Hence you can't change the reference i.e. you can't do:

someMap = anotherMap;

later on.

Blank final with Reference type

4 Answers4