java HashMap String int not working

Question

HashMap lt String  int gt  doesn t seem to work but HashMap lt String  Integer gt  does work  Any ideas why

User · Answer

GNU Trove support this but not using generics  http   trove4j sourceforge net javadocs gnu trove TObjectIntHashMap html

User · Answer

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 to objects as input   public interface Map lt K extends Object  V extends Object gt    object means a class  and it means also that you can create an other class that exends from it  but you can not create a class that exends from int  So you can not use int variable as an object  I have tow solutions for your problem   Map lt String  Integer gt  map   new HashMap lt  gt       or  Map lt String  int   gt  map   new HashMap lt  gt     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

User · Answer

You can use reference type in generic arguments  not primitive type  So here you should use  Map lt String  Integer gt  myMap   new HashMap lt String  Integer gt       and store value as  myMap put  abc   5

User · Answer

You can t use primitive types as generic arguments in Java  Use instead   Map lt String  Integer gt  myMap   new HashMap lt String  Integer gt       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 lt String  Integer gt  myMap   new HashMap lt String  Integer gt     Map lt Integer  String gt  map2    Map lt Integer  String gt  myMap  map2 put 3   foo

User · Answer

You cannot use primitive types in HashMap  int  or double don t work  You have to use its enclosing type  for an example  Map lt String Integer gt  m   new HashMap lt String Integer gt       Now both are objects  so this will work

[java] java: HashMap<String, int> not working

Examples related to java

Examples related to generics