You need more than one piece of the puzzle.

My bet is that currently you don't specify a "package" in either of
your java files.  That means they're both in the "default package,"
and I don't think you'll be able to use import successfully.

Package statements go on the very first line of a file.  So you could
add 'package src;' to the top of Pair.java and 'package tests;' to the
top of Tests.java.  Then you would add 'import src.Pair;' to Tests.java
between the package statement and the class definition.

Strictly speaking, packages should follow a naming convention that is
unique to you.  The preferred convention is to use a reversed domain
name.  So, since my domain is bounceswoosh.org, a proper package name
might be 'org.bounceswoosh.grokulator' (assuming I have a grokulator
application) or 'org.bounceswoosh.lib' (for common stuff).

Hope that's slightly clearer than mud.