I started using Java a while ago so this is probably a silly question for most of you, I want to use Set in my code (assume I have a class T),
Set<T> mySet;
Eclipse gives my an error : The local variable mySet may not have been initialized. Than I tried to initialize it:
Set<T> mySet = new Set<T>();
but than Eclipse gives the error : "Cannot instantiate the type Set".
What am I doing wrong here ?
Set<T> is an interface and cannot be instantiated. You could use HashSet<T>:
Set<T> set = new HashSet<T>();
List<T> list = new ArrayList<T>();.
Set<T> set the set variable sees only the methods that are declared on the interface, whereas if you declare it as a concrete class HashSet<T> set it can use all methods on this concrete class. In general it is considered best practice to work with the highest possible in the hierarchy type which in this case is Set<T>. This allows you to more easily switch between different implementations without this having an impact on the client code.Set is an interface and cannot be instantiated, you have to chose an implementation of Set, like:
Set<T> mySet = new TreeSet<T>();
Set is an Interface available in java.util. You cannot instantiate an interface. You should use an implementation of set like HashSet, TreeSet etc.
so the declaration should be something like this.
Set<T> set = new HashSet<T>();
or
Set<T> set = new TreeSet<T>();
In Java, an object is not created on the stack. Instead you have a just a reference which has to be initialised. To create a new object you have to explicitly specify which concrete class is to be used.
T. There is an interface Set and a class HashSet. T is a constraint.