Now I am able to resolve this issue by writing my own converter.
I have written one converter and given mapping of this converter into faces-config.xml. and putting converter with the given id for the attribute for which I was getting error.
In the converter I have overridden getAsObject and getAsString methods according to my need.
strange.. according to the code
public static final String CONVERTER_ID = "javax.faces.Integer";
you specified right id..
b.t.w. you could use f:convertNumber configuret to validate integer numbers instead of just f:converter
The problem is about the difference between list of select item, which has been rendered to the user interface and which is available during validate request phase.
This problem comes most frequently when we user <h:selectItem itemLabel="" ItemValue="" />.
So when we submit the form then this empty selection option is not available in the formbean's list of selectItem.
The better approach to have an empty select item in the list, to add this empty item in form bean itself either in getter method of whereever suitable for you.