Generics Implementation in Java SE 5

[Sriram Kannan]

Hi Everybody,

I have been going through Joshua Bloch's "Effective Java". In the second chapter I found the following:

Using Static Factory methods to reduce the verbosity of creating parameterized instances.

Instead of,

Map<String, List<String>> testMap = new HashMap<String, List<String>>();

the suggestion is a following Static Factory Method:

public static <K, V> HashMap<K, V> newInstance() {
      return new HashMap<K, V>();
 }

Therefore, the change would be

Map<String, List<String>> testMap = testClass.newInstance();

In the above example, can anyone tell me how this is done ?

Because until the assignment to "testMap", the Map object has no way of knowing what type of key-value instances are to be assigned to it.

This is different from a generic class.

public class Test<V> {

 }

 Test<Integer> s = new Test<Integer>();

Here, V can be considered a placeholder and we know it is replaced with "Integer".

But I am not seeing anything of that sort with the static factory method.

Can anybody explain how ?

Thanks in advance :-)

[Rob Spoor]

It's called type inference. It retrieves the V from the left-hand side. So assigning to a Test<String> will replace V with String, assigning to a Test<Integer> with Integer, etc.