This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
You're assuming that the tan funtion uses degrees. It doesn't - the API says it uses radians (pi=3.1419... radians equals 180 degrees). Most calculators have several possible modes - a "DRG" button toggles between degrees mode, radians mode, and grad mode (which has 100 grads in a 90 degree angle; seldom used). Your calculator is currently in degrees mode. Java's Math class doesn't have these modes; it always uses radians because that's the most mathematically natural mode to use. (Computations are actually simplest for the computer this way.) But you can use the toDegrees() and toRadians() methods to convert between radians and degrees.