Class Variables and Instance Variables

Lines 20–32 of Fig. 3.11 declare class MainActivity’s variables. The NumberFormat objects (lines 20–23) are used to format currency values and percentages, respectively. NumberFormat static method getCurrencyInstance returns a NumberFormat object that formats values as currency using the device’s default locale. Similarly, static method getPercentInstance formats values as percentages using the device’s default locale.

The bill amount entered by the user into amountEditText will be read and stored as a double in billAmount (line 25). The custom tip percentage (an integer in the range 0–30) that the user sets by moving the Seekbar thumb will be multiplied by 0.01 to create a double for use in calculations, then stored in customPercent (line 26). For example, if you select 25 with the SeekBar, customPercent will store 0.25, so the app will multiply the bill amount by 0.25 to calculate the 25% tip.

Line 27 declares the TextView that displays the currency-formatted bill amount. Line 28 declares the TextView that displays the custom tip percentage based on the SeekBar thumb’s position (see the 18% in Fig. 3.1(a)). The variables in line 29–32 will refer to the TextViews in which the app displays the calculated tips and totals.