Correct way to initialize HashMap and can HashMap hold different value types?

Solution 1:

It really depends on what kind of type safety you need. The non-generic way of doing it is best done as:

 Map x = new HashMap();

Note that x is typed as a Map. this makes it much easier to change implementations (to a TreeMap or a LinkedHashMap) in the future.

You can use generics to ensure a certain level of type safety:

Map<String, Object> x = new HashMap<String, Object>();

In Java 7 and later you can do

Map<String, Object> x = new HashMap<>();

The above, while more verbose, avoids compiler warnings. In this case the content of the HashMap can be any Object, so that can be Integer, int[], etc. which is what you are doing.

If you are still using Java 6, Guava Libraries (although it is easy enough to do yourself) has a method called newHashMap() which avoids the need to duplicate the generic typing information when you do a new. It infers the type from the variable declaration (this is a Java feature not available on constructors prior to Java 7).

By the way, when you add an int or other primitive, Java is autoboxing it. That means that the code is equivalent to:

 x.put("one", Integer.valueOf(1));

You can certainly put a HashMap as a value in another HashMap, but I think there are issues if you do it recursively (that is put the HashMap as a value in itself).

Solution 2:

This is a change made with Java 1.5. What you list first is the old way, the second is the new way.

By using HashMap you can do things like:

HashMap<String, Doohickey> ourMap = new HashMap<String, Doohickey>();

....

Doohickey result = ourMap.get("bob");

If you didn't have the types on the map, you'd have to do this:

Doohickey result = (Doohickey) ourMap.get("bob");

It's really very useful. It helps you catch bugs and avoid writing all sorts of extra casts. It was one of my favorite features of 1.5 (and newer).

You can still put multiple things in the map, just specify it as Map, then you can put any object in (a String, another Map, and Integer, and three MyObjects if you are so inclined).

Solution 3:

Eclipse is recommending that you declare the type of the HashMap because that enforces some type safety. Of course, it sounds like you're trying to avoid type safety from your second part.

If you want to do the latter, try declaring map as HashMap<String,Object>.

Solution 4:

The way you're writing it is equivalent to

HashMap<Object, Object> map = new HashMap<Object, Object>();

What goes inside the brackets is you communicating to the compiler what you're going to put in the HashMap so that it can do error checking for you. If Object, Object is what you actually want (probably not) you should explicitly declare it. In general you should be as explicit as you can with the declaration to facilitate error checking by the compiler. What you've described should probably be declared like this:

HashMap<String, Object> map = new HashMap<String, Object>();

That way you at least declare that your keys are going to be strings, but your values can be anything. Just remember to use a cast when you get a value back out.