From Java 7, you can have underscores in between digits, to improve readability:
From JLS - Section 3.10.1 and JLS Section 3.10.2:
Underscores are allowed as separators between digits that denote the integer.
For floating point literal also:
Underscores are allowed as separators between digits that denote the whole-number part, and between digits that denote the fraction part, and between digits that denote the exponent.
For e.g., 1000000000 can now be written as 1_000_000_000. So, it is better for eyes, isn't it.
Similarly you can write - 0x7fff_ffff, 0b0111_1111.
And regarding variable name, _ is a valid variable name. As per Java standard, a variable name should start with a $, _ or a letter.
