This codelab is written for programmers and assumes you know Java. If you are an experienced programmer and you are adept at reading code, you will likely be able to follow this codelab even if you don't have much experience with Java. To work through this codelab, you will need a computer that can run Android Studio or already has Android Studio installed. You use Android Studio to build Android apps.
Android Studio is available for Windows, Mac, and Linux computers. The installation is similar for all platforms. Any differences are noted in the sections below.
If you are using a computer that already has Android Studio installed, go to the next task, Create your first project. In this step, you will create a new Android project for your first app. This simple app displays the string "Hello World" on the screen of the Android virtual or physical device.
In this task, you will use the Android Virtual Device AVD manager to create a virtual device also known as an emulator that simulates the configuration for a particular type of Android device.
The emulator starts and boots just like a physical device. Depending on the speed of your computer, this may take a while. You can look in the small horizontal status bar at the very bottom of Android Studio for messages Download hook up messenger for java see the progress. Once your app builds and the emulator is ready, Android Studio uploads the app to the emulator and runs it. You should see your app as shown in the following screenshot.
You may need to install the appropriate USB driver for your device.
See the Using Hardware Devices documentation. Check the latest Android Studio documentation. Generally each screen in your Android app is associated with one Java class known as an activity. The single screen with "Hello World" displayed is created by one activity, called MainActivity. This was generated for you when you created your new project. Each visible activity in an Android app has a layout that defines the user interface for the activity.
Android Studio has a layout editor where you can create and define layouts. Layouts are defined in XML. The layout editor lets you define and modify your layout either by coding XML or by Download hook up messenger for java the interactive visual editor. You can switch between editing your layout in XML and in the visual design editor as often as you like.
Every element in a layout is a view. In this task, you will explore some of the panels in the layout editor, and you will learn how to change property values for views. So far you have learned how to change property values.
Next, you will learn how to create resources. Using resources enables you to use the same values in multiple places, or to define values and have the UI update automatically whenever the value is changed.
You now know how to create new string resources by extracting them from existing field values. You can also add new resources to the strings. Now that you have a new screen background color, you will use it to explore the effects of changing width and height properties of views. To learn how to use constraints to connect the positions of views to each other, you will add three buttons to the layout.
Here is the XML code for the layout. Your layout might have different margins and perhaps some different vertical or horizontal bias constraints. The exact values of the attributes for the appearance of the TextView might be different for you. The next task is to make the buttons do something when they are pressed. Before learning how to make your buttons interactive, finish getting the UI ready.
Also the layout is messed up in the Design tab. Your app's layout is now basically complete, but it's appearance can be improved by a few small changes. If you implemented all the updates, your app Download hook up messenger for java look like the following figure. If you used different colors and fonts, then your app will look a bit different. You have added buttons Download hook up messenger for java your app's main screen, but currently the buttons do nothing.
You'll fix that in the next task. In this task, you will make your button respond when the user presses the buttons. First you will make the Toast button show a pop-up message called a toast.
Next you will make the Count button update the number that is displayed in the TextView. Before adding behavior to your buttons, you will enable auto-imports so that Android Studio automatically imports any classes that are needed by the Java code. In this step, you will add a method to the Toast button to show a toast when the user presses it.
A toast is a short message that appears briefly at the bottom of the screen.
The method that shows the toast is very simple; it does not interact with any other views in the layout. The next step is to add behavior to your layout to find and update other views.
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You have learned that you can get a View by using findViewById. Typically you need to cast the result to the relevant subclass of View. You can get and set values on the view using methods such as getText and setText.
There are other methods for other properties. So far, your app has a single screen. Now you will add a second activity that has its own screen. You will update the Random button to display a second activity that shows a random number between 0 and the current count. You will implement methods in this Activity to calculate and display a random number. First, though, you need to create the views in the layout to show the random number.
The screen for the new activity will display a heading title and the random number. Here is what the screen will look like in the Design tab: Your app now has a completed layout for the second activity.
You can view it in the Design panel, but if you run your app there is no way yet to switch to the second activity. To start a new activity, you create an Intent, then call startActivity with that Intent. You will do that now.