Nrthugu

Collection<String> a = new ArrayList<String>();



a.add("Hello");

a.add("World");



System.out.println(a.get(0));

Can anyone explain why i can't use get() method which pre-defined in ArrayList class?

Why I can't use get() method which pre-defined in ArrayList class?

In Java, reference variable type decides WHAT methods you can invoke on the object.

As your reference variable type is Collection to which you are assigning the ArrayList object, you can invoke only the methods declared by the Collection type which are specified (look here for API).

In simple words, get() is defined by ArrayList, but not by the Collection interface.

Now, if you want to invoke the get() method, you need to change the reference type from Collection to List as shown below:

List<String> a = new ArrayList<>();//No need to specify type for ArrayList

You can look at all of the methods declared by the List interface here and get(int i) is one of the methods.

Collection Interface don't have a get() method. List interface has get() method.

List<String> a = new ArrayList<String>();



a.add("Hello");

a.add("World");



System.out.println(a.get(0));

Now it will works fine.

Try this,

Print first item in the collection :

System.out.printf(a.iterator().next());

For java8 :

System.out.println(a.stream().findFirst().orElse("not found"));

Java — How to get first item from Collection?

If you still need a Collection Interface you can type caste and use it

Collection<String> a = new ArrayList<String>();



a.add("Hello");

a.add("World");



System.out.println(((ArrayList<String>)a).get(0));

The Collection Interface does not have a 'get(int index)' method.

You can either declare your a as a List, or just cast your Collection to a list:

List<String> a = new ArrayList<String>();

Also, < String > is redundant, so you can get rid of it.

List<String> a = new ArrayList<>();

If you want to cast your Collection to an ArrayList:

System.out.println(((ArrayList<String>)a).get(0));