Write Java expression for: |a + b| / sqrt(a^2 + b^2)

Math.abs(a + b) / Math.sqrt(a * a + b * b)

Translate the mathematics piece by piece.

The modulus bars | a + b | become Math.abs(a + b).

The square root becomes Math.sqrt(...), and a^2 + b^2 is written a * a + b * b (Math.pow works too but returns a double and is clumsier here). The horizontal bar of the fraction becomes the / operator, and the whole denominator must sit inside its own brackets — writing Math.abs(a + b) / Math.sqrt(a * a) + b * b would divide by only part of it. Bracket the denominator whenever it is more than a single term.