Java Mappings and Primitives

Question

I want to create a mapping that takes a String as the key and a primitive as the value. I was looking at the Java docs and did not see that Primitive was a class type, or that they shared some kind of wrapping class.

How can I constrain the value to be a primitive?

Map<String, Primitive> map = new HashMap<String, Primitive>();

Answer 1

Java Autoboxing allows to create maps on Long, Integer, Double and then operate them using primitive values. For example:

java.util.HashMap<String, Integer> map = new java.util.HashMap<String, Integer>();
map.put("one", 1); // 1 is an integer, not an instance of Integer

If you want to store in one map different primitive types, you can to it by making a Map<String, Number>. Allows to store values of BigDecimal, BigInteger, Byte, Double, Float, Integer, Long, Short (and AtomicLong, AtomicInteger).

Here is an example:

Map<String, Number> map = new HashMap<String, Number>();

map.put("one", 1);
map.put("two", 2.0);
map.put("three", 1L);

for(String k:map.keySet()) {
  Number v = map.get(k);
  System.err.println(v + " is instance of " + v.getClass().getName() + ": " + v);
}