3.5. Adding Functionality to the App
Class MainActivity (Figs. 3.9–3.16) implements the Tip Calculator app’s functionality. It calculates the 15% and custom percentage tips and total bill amounts, and displays them in locale-specific currency format. To view the file, open src/com.deitel/tipcalculator and double clck MainActivity.java. You’ll need to enter most of the code in Figs. 3.9–3.16.
1 // MainActivity.java
2 // Calculates bills using 15% and custom percentage tips.
3 package com.deitel.tipcalculator;
4
5 import java.text.NumberFormat; // for currency formatting
6
7 import android.app.Activity; // base class for activities
8 import android.os.Bundle; // for saving state information
9 import android.text.Editable; // for EditText event handling
10 import android.text.TextWatcher; // EditText listener
11 import android.widget.EditText; // for bill amount input
12 import android.widget.SeekBar; // for changing custom tip percentage
13 import android.widget.SeekBar.OnSeekBarChangeListener; // SeekBar listener
14 import android.widget.TextView; // for displaying text
15
Fig. 3.9 | MainActivity’s package and import statements.
16 // MainActivity class for the Tip Calculator app
17 public class MainActivity extends Activity
18 {
Fig. 3.10 | Class MainActivity is a subclass of Activity.
19 // currency and percent formatters
20 private static final NumberFormat currencyFormat =
21 NumberFormat.getCurrencyInstance();
22 private static final NumberFormat percentFormat =
23 NumberFormat.getPercentInstance();
24
25 private double billAmount = 0.0; // bill amount entered by the user
26 private double customPercent = 0.18; // initial custom tip percentage
27 private TextView amountDisplayTextView; // shows formatted bill amount
28 private TextView percentCustomTextView; // shows custom tip percentage
29 private TextView tip15TextView; // shows 15% tip
30 private TextView total15TextView; // shows total with 15% tip
31 private TextView tipCustomTextView; // shows custom tip amount
32 private TextView totalCustomTextView; // shows total with custom tip
33
Fig. 3.11 | MainActivity class’s instance variables.
34 // called when the activity is first created
35 @Override
36 protected void onCreate(Bundle savedInstanceState)
37 {
38 super.onCreate(savedInstanceState); // call superclass's version
39 setContentView(R.layout.activity_main); // inflate the GUI
40
41 // get references to the TextViews
42 // that MainActivity interacts with programmatically
43 amountDisplayTextView =
44 (TextView) findViewById(R.id.amountDisplayTextView );
45 percentCustomTextView =
46 (TextView) findViewById(R.id.percentCustomTextView );
47 tip15TextView = (TextView) findViewById(R.id.tip15TextView);
48 total15TextView = (TextView) findViewById(R.id.total15TextView);
49 tipCustomTextView = (TextView) findViewById(R.id.tipCustomTextView);
50 totalCustomTextView =
51 (TextView) findViewById(R.id.totalCustomTextView);
52
53 // update GUI based on billAmount and customPercent
54 amountDisplayTextView.setText(
55 currencyFormat.format(billAmount));
56 updateStandard(); // update the 15% tip TextViews
57 updateCustom(); // update the custom tip TextViews
58
59 // set amountEditText's TextWatcher
60 EditText amountEditText =
61 (EditText) findViewById(R.id.amountEditText);
62 amountEditText.addTextChangedListener(amountEditTextWatcher);
63
64 // set customTipSeekBar's OnSeekBarChangeListener
65 SeekBar customTipSeekBar =
66 (SeekBar) findViewById(R.id.customTipSeekBar);
67 customTipSeekBar.setOnSeekBarChangeListener(customSeekBarListener);
68 } // end method onCreate
69
Fig. 3.12 | Overriding Activity method onCreate.
70 // updates 15% tip TextViews
71 private void updateStandard()
72 {
73 // calculate 15% tip and total
74 double fifteenPercentTip = billAmount * 0.15;
75 double fifteenPercentTotal = billAmount + fifteenPercentTip;
76
77 // display 15% tip and total formatted as currency
78 tip15TextView.setText(currencyFormat.format(fifteenPercentTip));
79 total15TextView.setText(currencyFormat.format(fifteenPercentTotal));
80 } // end method updateStandard
81
Fig. 3.13 | Method updateStandard calculates and displays the 15% tip and total.
82 // updates the custom tip and total TextViews
83 private void updateCustom()
84 {
85 // show customPercent in percentCustomTextView formatted as %
86 percentCustomTextView.setText(percentFormat.format(customPercent));
87
88 // calculate the custom tip and total
89 double customTip = billAmount * customPercent;
90 double customTotal = billAmount + customTip;
91
92 // display custom tip and total formatted as currency
93 tipCustomTextView.setText(currencyFormat.format(customTip));
94 totalCustomTextView.setText(currencyFormat.format(customTotal));
95 } // end method updateCustom
96
Fig. 3.14 | Method updateCustom calculates and displays the custom tip and total.
97 // called when the user changes the position of SeekBar
98 private OnSeekBarChangeListener customSeekBarListener =
99 new OnSeekBarChangeListener()
100 {
101 // update customPercent, then call updateCustom
102 @Override
103 public void onProgressChanged(SeekBar seekBar, int progress,
104 boolean fromUser)
105 {
106 // sets customPercent to position of the SeekBar's thumb
107 customPercent = progress / 100.0 ;
108 updateCustom(); // update the custom tip TextViews
109 } // end method onProgressChanged
110
111 @Override
112 public void onStartTrackingTouch(SeekBar seekBar)
113 {
114 } // end method onStartTrackingTouch
115
116 @Override
117 public void onStopTrackingTouch(SeekBar seekBar)
118 {
119 } // end method onStopTrackingTouch
120 }; // end OnSeekBarChangeListener
121
Fig. 3.15 | Anonymous inner class that implements interface OnSeekBarChangeListener to respond to the events of the customSeekBar.
122 // event-handling object that responds to amountEditText's events
123 private TextWatcher amountEditTextWatcher = new TextWatcher()
124 {
125 // called when the user enters a number
126 @Override
127 public void onTextChanged(CharSequence s, int start,
128 int before, int count)
129 {
130 // convert amountEditText's text to a double
131 try
132 {
133 billAmount = Double.parseDouble(s.toString()) / 100.0 ;
134 } // end try
135 catch (NumberFormatException e)
136 {
137 billAmount = 0.0; // default if an exception occurs
138 } // end catch
139
140 // display currency formatted bill amount
141 amountDisplayTextView.setText(currencyFormat.format(billAmount));
142 updateStandard(); // update the 15% tip TextViews
143 updateCustom(); // update the custom tip TextViews
144 } // end method onTextChanged
145
146 @Override
147 public void afterTextChanged(Editable s)
148 {
149 } // end method afterTextChanged
150
151 @Override
152 public void beforeTextChanged(CharSequence s, int start, int count,
153 int after)
154 {
155 } // end method beforeTextChanged
156 }; // end amountEditTextWatcher
157 } // end class MainActivity
Fig. 3.16 | Anonymous inner class that implements interface TextWatcher to respond to the events of the amountEditText.