There is no difference between the objects; you have a HashMap<String, Object>
in both cases. There is a difference in the interface you have to the object. In the first case, the interface is HashMap<String, Object>
, whereas in the second it's Map<String, Object>
. But the underlying object is the same.
The advantage to using Map<String, Object>
is that you can change the underlying object to be a different kind of map without breaking your contract with any code that's using it. If you declare it as HashMap<String, Object>
, you have to change your contract if you want to change the underlying implementation.
Example: Let's say I write this class:
class Foo {
private HashMap<String, Object> things;
private HashMap<String, Object> moreThings;
protected HashMap<String, Object> getThings() {
return this.things;
}
protected HashMap<String, Object> getMoreThings() {
return this.moreThings;
}
public Foo() {
this.things = new HashMap<String, Object>();
this.moreThings = new HashMap<String, Object>();
}
// ...more...
}
The class has a couple of internal maps of string->object which it shares (via accessor methods) with subclasses. Let's say I write it with HashMap
s to start with because I think that's the appropriate structure to use when writing the class.
Later, Mary writes code subclassing it. She has something she needs to do with both things
and moreThings
, so naturally she puts that in a common method, and she uses the same type I used on getThings
/getMoreThings
when defining her method:
class SpecialFoo extends Foo {
private void doSomething(HashMap<String, Object> t) {
// ...
}
public void whatever() {
this.doSomething(this.getThings());
this.doSomething(this.getMoreThings());
}
// ...more...
}
Later, I decide that actually, it's better if I use TreeMap
instead of HashMap
in Foo
. I update Foo
, changing HashMap
to TreeMap
. Now, SpecialFoo
doesn't compile anymore, because I've broken the contract: Foo
used to say it provided HashMap
s, but now it's providing TreeMaps
instead. So we have to fix SpecialFoo
now (and this kind of thing can ripple through a codebase).
Unless I had a really good reason for sharing that my implementation was using a HashMap
(and that does happen), what I should have done was declare getThings
and getMoreThings
as just returning Map<String, Object>
without being any more specific than that. In fact, barring a good reason to do something else, even within Foo
I should probably declare things
and moreThings
as Map
, not HashMap
/TreeMap
:
class Foo {
private Map<String, Object> things; // <== Changed
private Map<String, Object> moreThings; // <== Changed
protected Map<String, Object> getThings() { // <== Changed
return this.things;
}
protected Map<String, Object> getMoreThings() { // <== Changed
return this.moreThings;
}
public Foo() {
this.things = new HashMap<String, Object>();
this.moreThings = new HashMap<String, Object>();
}
// ...more...
}
Note how I'm now using Map<String, Object>
everywhere I can, only being specific when I create the actual objects.
If I had done that, then Mary would have done this:
class SpecialFoo extends Foo {
private void doSomething(Map<String, Object> t) { // <== Changed
// ...
}
public void whatever() {
this.doSomething(this.getThings());
this.doSomething(this.getMoreThings());
}
}
...and changing Foo
wouldn't have made SpecialFoo
stop compiling.
Interfaces (and base classes) let us reveal only as much as is necessary, keeping our flexibility under the covers to make changes as appropriate. In general, we want to have our references be as basic as possible. If we don't need to know it's a HashMap
, just call it a Map
.
This isn't a blind rule, but in general, coding to the most general interface is going to be less brittle than coding to something more specific. If I'd remembered that, I wouldn't have created a Foo
that set Mary up for failure with SpecialFoo
. If Mary had remembered that, then even though I messed up Foo
, she would have declared her private method with Map
instead of HashMap
and my changing Foo
's contract wouldn't have impacted her code.
Sometimes you can't do that, sometimes you have to be specific. But unless you have a reason to be, err toward the least-specific interface.
Best Answer
This depends a lot on what you are trying to achieve: for flexibility, hash map is better. But the flexibility comes at a price: hash map is also larger and slower than a class with the identical number of strongly-typed fields.
There is also an impact on readability: when you business logic is specific to a class with a fixed number of fields, a special-purpose class clearly wins; when the fields are configured dynamically, hash table is your only option. You could also have a hybrid design, when an object uses a hash map for its storage internally, presents nicely named fields externally, and exposes semantics to add more "fields" as you go.
To summarize, before you decide to go with a hash map for its flexibility, you should decide if you really need all that flexibility in your design. Sometimes, the answer is "yes", and sometimes it is "no"; there is no "one size fits all" solution to this.