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

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So I have two questions about HashMaps in Java:

  1. What is the correct way to initialize a HashMap? I think it might be best in my situation to use:

    HashMap x = new HashMap();

    But Eclipse keeps suggesting that I use:

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

    Which is better?

  2. Can a HashMap hold different types of objects/data types as values? For example, would this work and be OK:

    map.put("one", 1);
map.put("two", {1, 2});
map.put("three", "hello");

    In the first put(), I want an int as a value, in the second an int[], and third a string. Is this okay to do in Java with HashMaps? Also, is it okay to store a HashMap as a value within a HashMap?

hashmap standards java
9 Answers

Map.of literals

As of Java 9, there is yet another way to instantiate a Map. You can create an unmodifiable map from zero, one, or several pairs of objects in a single-line of code. This is quite convenient in many situations.

For an empty Map that cannot be modified, call Map.of(). Why would you want an empty set that cannot be changed? One common case is to avoid returning a NULL where you have no valid content.

For a single key-value pair, call Map.of( myKey , myValue ). For example, Map.of( "favorite_color" , "purple" ).

For multiple key-value pairs, use a series of key-value pairs. ``Map.of( "favorite_foreground_color" , "purple" , "favorite_background_color" , "cream" )`.

If those pairs are difficult to read, you may want to use Map.of and pass Map.Entry objects.

Note that we get back an object of the Map interface. We do not know the underlying concrete class used to make our object. Indeed, the Java team is free to used different concrete classes for different data, or to vary the class in future releases of Java.

The rules discussed in other Answers still apply here, with regard to type-safety. You declare your intended types, and your passed objects must comply. If you want values of various types, use Object. 

Map< String , Color > preferences = Map.of( "favorite_color" , Color.BLUE ) ;
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