logical database design and physical database design

hmmm,

I did say the physical comes from the logical;  which is, I think, slightwv's point;   but.  I don't think logical should be viewed as "requirements"

Requirements come first, then conceptual model / high level entity model preceding logical model.  Maybe just different SDLC in our respective environments but by the time you get down to a logical model, you're pretty much done.  The physical model just kinds of dumps out of it except where I noted above.  In fact, if you're using modelling tools, you can frequently generate a physical model from logical. Then tweak the ddl as needed based on the exceptions of the particular platform, but even then, some tools can attempt to generate platform specific physical models and associated ddl.

Being a java developed how far and depth i need to know this concept? please advice

If you don't have access to create objects, then creating a physical model might not be something you'll be involved with.    Permissions don't dictate this relationship but they often go together.

You may be the one to create a logical model though.  A lot of it depends on the size of your organization.  If you work in a large group you can have specialized positions for various tasks including dedicated data modelers.  In a smaller organization you may need to do it all yourself.

Regardless of what your actual responsibilities are you should be at least familiar with how to read the models even if you're not creating them.