19. We write a simple calculator

In this lesson we:

- we write application - calculator

Let's try to write a simple calculator that takes two numbers and performs addition, subtraction, multiplication, or division operations with them. The result displays as a complete expression.

Create a project:

Project name : P0191_SimpleCalculator
Build Target : Android 2.3.3
Application name : SimpleCalculator
Package name : ru.startandroid.develop.simplecalculator
Create Activity : MainActivity

Open main.xml and draw a screen:

<? xml version = "1.0" encoding = "utf-8"?>
<LinearLayout
xmlns: android = "http://schemas.android.com/apk/res/android"
android: orientation = "vertical"
android: layout_width = "fill_parent"
android: layout_height = "fill_parent">
<LinearLayout
android: layout_width = "match_parent"
android: layout_height = "wrap_content"
android: id = "@ + id / linearLayout1"
android: layout_marginLeft = "10pt"
android: layout_marginRight = "10pt"
android: layout_marginTop = "3pt">
<Edittext
android: layout_weight = "1"
android: layout_height = "wrap_content"
android: layout_marginRight = "5pt"
android: id = "@ + id / etNum1"
android: layout_width = "match_parent"
android: inputType = "numberDecimal">
</ Edittext>
<Edittext
android: layout_height = "wrap_content"
android: layout_weight = "1"
android: layout_marginLeft = "5pt"
android: id = "@ + id / etNum2"
android: layout_width = "match_parent"
android: inputType = "numberDecimal">
</ Edittext>
</ LinearLayout>
<LinearLayout
android: layout_width = "match_parent"
android: layout_height = "wrap_content"
android: id = "@ + id / linearLayout2"
android: layout_marginTop = "3pt"
android: layout_marginLeft = "5pt"
android: layout_marginRight = "5pt">
<Button
android: layout_height = "wrap_content"
android: layout_width = "match_parent"
android: layout_weight = "1"
android: text = "+"
android: textSize = "8pt"
android: id = "@ + id / btnAdd">
</ Button>
<Button
android: layout_height = "wrap_content"
android: layout_width = "match_parent"
android: layout_weight = "1"
android: text = "-"
android: textSize = "8pt"
android: id = "@ + id / btnSub">
</ Button>
<Button
android: layout_height = "wrap_content"
android: layout_width = "match_parent"
android: layout_weight = "1"
android: text = "*"
android: textSize = "8pt"
android: id = "@ + id / btnMult">
</ Button>
<Button
android: layout_height = "wrap_content"
android: layout_width = "match_parent"
android: layout_weight = "1"
android: text = "/"
android: textSize = "8pt"
android: id = "@ + id / btnDiv">
</ Button>
</ LinearLayout>
<Textview
android: layout_height = "wrap_content"
android: layout_width = "match_parent"
android: layout_marginLeft = "5pt"
android: layout_marginRight = "5pt"
android: textSize = "12pt"
android: layout_marginTop = "3pt"
android: id = "@ + id / tvResult"
android: gravity = "center_horizontal">
</ TextView>
</ LinearLayout>

There are two input fields , 4 buttons and a text field for output . Notice the inputType attribute for EditText . It sets the content type . I specified numberDecimal - i.e. In the field you will be able to enter only numbers and a comma , he will not miss the letters. It is convenient, it is not necessary to code various checks.

The gravity attribute is specified for TextView . It indicates how the text will be located in TextView. Do not confuse with layout_gravity, which is responsible for placing a TextView in a ViewGroup.

Now we need to read the contents of the fields, determine which button was pressed and display the desired result. Open MainActivity.java and write the code.

I think everything is clear by kamentam. We read the values, define the button, perform the operation and print it in the text field. The button handler is the Activity .

All save and run.

Let's make a menu with more items for clearing fields and exiting the application for more functionality. Items will be called Reset and Quit .

Add two constants - these will be the ID of the menu items.

(add only underlined code)

And we will write the code for creating and processing the menu:

Let's save everything, run. There are two menu items:
Reset - clears all fields
Quit - closes the application

As an independent work, you can implement a check division by zero . And display a thread message using Toast or directly in the result field.

In the next lesson:

- consider the animation of view components