Why do people call a cup of coffee a cup of Joe?

The strongest drink available on board navy ships was coffee. It was dubbed "a cup of Joe" after the secretary. It also refers to the GIs' favorite drink. During World War II, the US defense workers were supplied with as much coffee as they wanted. Coffee was a source of warmth and comfort for troops.
A variation is that "a cup of Joe" is the average drink of the average man. There was a New York company named Martinson's Coffee owned by a man named Joe Martinson. The neighborhood of the company would be saturated by the aroma of roasting coffee, and coffee became known as "a cup of Joe."