在Java中将基元或对象放入对象映射之间的区别

Question

I have a simple map:

Map<String, Object> myMap;

And I am just wondering, what is the difference between these two solutions: First, when I put my double value as a Double object:

myMap.put("object", Double.valueOf(myPrimitiveDouble));

Second when I put my double value as a primitive type:

myMap.put("primitive", myPrimitiveDouble);

I think the first one is unnecessary, am I right? Thanks for the answers!

Answer 1

When you put your primitive double as a value into your Map<String, Object> , it will automatically be boxed into a Double type, as generic collections (that includes maps) do not use primitives.

This is called autoboxing .

Therefore, the Double.valueOf invocation is indeed redundant in this case.

Example

Map<String, Object> myMap = new HashMap<>();
double foo = 5;
myMap.put("", foo);
System.out.println(myMap.get("").getClass());

Output

class java.lang.Double

Note

Java will auto-box your primitive double to Double when the collection is parametrized with either Object , Number or Double .

Your code would not compile, however, if your map values were parametrized with an incompatible type, eg Integer .