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

Question

So I have two questions about HashMaps in Java    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 lt something  something gt  map   new HashMap      Which is better  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

User · Answer

This is a change made with Java 1 5  What you list first is the old way  the second is the new way   By using HashMap you can do things like   HashMap lt String  Doohickey gt  ourMap   new HashMap lt String  Doohickey gt            Doohickey result   ourMap get  bob      If you didn t have the types on the map  you d have to do this   Doohickey result    Doohickey  ourMap get  bob      It s really very useful  It helps you catch bugs and avoid writing all sorts of extra casts  It was one of my favorite features of 1 5  and newer    You can still put multiple things in the map  just specify it as Map  then you can put any object in  a String  another Map  and Integer  and three MyObjects if you are so inclined

User · Answer

The way you re writing it is equivalent to   HashMap lt Object  Object gt  map   new HashMap lt Object  Object gt       What goes inside the brackets is you communicating to the compiler what you re going to put in the HashMap so that it can do error checking for you   If Object  Object is what you actually want  probably not  you should explicitly declare it   In general you should be as explicit as you can with the declaration to facilitate error checking by the compiler   What you ve described should probably be declared like this   HashMap lt String  Object gt  map   new HashMap lt String  Object gt       That way you at least declare that your keys are going to be strings  but your values can be anything   Just remember to use a cast when you get a value back out

User · Answer

Eclipse is recommending that you declare the type of the HashMap because that enforces some type safety   Of course  it sounds like you re trying to avoid type safety from your second part   If you want to do the latter  try declaring map as HashMap lt String Object gt

User · Answer

In answer to your second question  Yes a HashMap can hold different types of objects  Whether that s a good idea or not depends on the problem you re trying to solve   That said  your example won t work  The int value is not an Object  You have to use the Integer wrapper class to store an int value in a HashMap

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A HashMap can hold any object as a value  even if it is another HashMap  Eclipse is suggesting that you declare the types because that is the recommended practice for Collections  under Java 5  You are free to ignore Eclipse s suggestions   Under Java 5  an int  or any primitive type  will be autoboxed into an Integer  or other corresponding type  when you add it to a collection  Be careful with this though  as there are some catches to using autoboxing

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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 lt  String   Color  gt  preferences   Map of   favorite color    Color BLUE

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The 2nd one is using generics which came in with Java 1 5  It will reduce the number of casts in your code  amp  can help you catch errors at compiletime instead of runtime  That said  it depends on what you are coding  A quick  amp  dirty map to hold a few objects of various types doesn t need generics  But if the map is holding objects all descending from a type other than Object  it can be worth it   The prior poster is incorrect about the array in a map  An array is actually an object  so it is a valid value   Map lt String Object gt  map   new HashMap lt String Object gt     map put  one  1      autoboxed to an object map put  two   new int   1 2        array of ints is an object map put  three   hello       string is an object   Also  since HashMap is an object  it can also be a value in a HashMap

User · Answer

It really depends on what kind of type safety you need  The non-generic way of doing it is best done as    Map x   new HashMap      Note that x is typed as a Map  this makes it much easier to change implementations  to a TreeMap or a LinkedHashMap  in the future   You can use generics to ensure a certain level of type safety   Map lt String  Object gt  x   new HashMap lt String  Object gt       In Java 7 and later you can do  Map lt String  Object gt  x   new HashMap lt  gt       The above  while more verbose  avoids compiler warnings  In this case the content of the HashMap can be any Object  so that can be Integer  int    etc  which is what you are doing   If you are still using Java 6  Guava Libraries  although it is easy enough to do yourself  has a method called newHashMap   which avoids the need to duplicate the generic typing information when you do a new  It infers the type from the variable declaration  this is a Java feature not available on constructors prior to Java 7    By the way  when you add an int or other primitive  Java is autoboxing it  That means that the code is equivalent to    x put  one   Integer valueOf 1      You can certainly put a HashMap as a value in another HashMap  but I think there are issues if you do it recursively  that is put the HashMap as a value in itself

User · Answer

Eclipse is suggesting you to define generic type so that you can have type safety  You can write  Map m   new HashMap      which does not ensure type safety but following will ensure type safety  Map lt Object Object gt    new HashMap lt Object Object gt       The Object can be any type such as String  Integer etc

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

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