hello friend iam beginner in java programming,
i want to know class name we always start with capital letter
why it is so? if it can be small then why we use this convention.
is there serious error can be generate when we give our class name
that start with small letter.

It is the accepted convention for class names to begin with a capital letter. The compiler won't choke or bark at you if you ignore this standard but your fellow developers might. Other conventions are that package names should all be lower case, constants (static final variables) should be in upper case and viarable names should use camel casing (myVariable, yourVariable, etc. Don't fight it accept it and go with the flow, having said that we live in a democracy and it's your right to make yourself unpopular ;)

Cheers

Martyn