HashMap<String, int> doesn't seem to work but HashMap<String, Integer> does work.
Any ideas why?
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your choice of words for your question is confusing, can you clarify? And exactly what is not working, can you post the code?Anthony Forloney– Anthony Forloney2009-11-22 23:23:00 +00:00Commented Nov 22, 2009 at 23:23
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5 Answers
You can't use primitive types as generic arguments in Java. Use instead:
Map<String, Integer> myMap = new HashMap<String, Integer>();
With auto-boxing/unboxing there is little difference in the code. Auto-boxing means you can write:
myMap.put("foo", 3);
instead of:
myMap.put("foo", new Integer(3));
Auto-boxing means the first version is implicitly converted to the second. Auto-unboxing means you can write:
int i = myMap.get("foo");
instead of:
int i = myMap.get("foo").intValue();
The implicit call to intValue() means if the key isn't found it will generate a NullPointerException, for example:
int i = myMap.get("bar"); // NullPointerException
The reason is type erasure. Unlike, say, in C# generic types aren't retained at runtime. They are just "syntactic sugar" for explicit casting to save you doing this:
Integer i = (Integer)myMap.get("foo");
To give you an example, this code is perfectly legal:
Map<String, Integer> myMap = new HashMap<String, Integer>();
Map<Integer, String> map2 = (Map<Integer, String>)myMap;
map2.put(3, "foo");
2 Comments
T3rm1
Your last example doesn't work: Cannot cast from Map<String,Integer> to Map<Integer,String>
fresh learner
considering each separate code in a new line, the code on line 5 must be first casted to Integer before using the intValue() method because it is considered as an object when you use the get() method.
GNU Trove support this but not using generics. http://trove4j.sourceforge.net/javadocs/gnu/trove/TObjectIntHashMap.html
Comments
int is a primitive type, you can read what does mean a primitive type in java here, and a Map is an interface that has two objects as input:
public interface Map<K extends Object, V extends Object>
Object means a class, and it also means that you can create another class that extends from it, but you can not create a class that extends from int.
So you can not use int variable as an object. I have two solutions for your problem:
Map<String, Integer> map = new HashMap<>();
or
Map<String, int[]> map = new HashMap<>();
int x = 1;
//put x in map
int[] x_ = new int[]{x};
map.put("x", x_);
//get the value of x
int y = map.get("x")[0];
Comments
You can use reference type in generic arguments, not primitive type. So here you should use
Map<String, Integer> myMap = new HashMap<String, Integer>();
and store value as
myMap.put("abc", 5);
1 Comment
smac89
This does not answer the question