SolveQuads

solves quadratic equations.  It demonstrates the use of functions (methods with parameters and return values).

Important Commands

public double toDouble(String s)
Changes a String to a double value.  If the String contains a non-numeric value (or something else fails), the function returns NaN (Not-a-Number).

public boolean bad(double num)
Decides whether a number is NaN (e.g. bad) or normal (okay).

public double root(double num)
Safely calculates the square-root of a number - returns the square-root or NaN (if num is negative).

Math.sqrt(num)
Calculates the square-root of a number.

Double.parseDouble(s)
Changes a String to a double value.

Math.pow(b,2)
Calculates a power = b^2

Font bigFont = new Font("Arial",0,24);
lTitle.setFont(bigFont)
Changes the Font in the lTitle label to a larger font.

Comments

Java includes a variety of standard functions, but the programmer will want to make their own.  For example, Double.parseDouble is used so often, that it is useful to "wrap" it inside a user-defined function (e.g. toDouble) to simplify the coding of the rest of the program.  The programmer can also add error-handling code, making a "safe" version of a function.

The Math class contains lots of standard functions - absolute value, power, square-root, sin, cos, etc.  These are static, so they can be used without instantiating a Math object.

The Double object is not the same as a double variable.  The Double object contains a single double value, along with a variety of methods for manipulating double values.  Many of these methods are static and can be used without instantiating an object.