Run Java App On Android


How Android Apps Run On Devices

Run a Java application in Android Studio

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P. S. –

Update The Textview To Display The Header

  • In fragment_second.xml, select textview_second, which currently has the text “Hello second fragment. Arg: %1$s” in the hello_second_fragment string resource.
  • If android:text isn’t set, set it to the hello_second_fragment string resource.
  • Change the id to textview_header in the Attributes panel.
  • Set the width to match_constraint, but set the height to wrap_content, so the height will change as needed to match the height of the content.
  • Set top, left and right margins to 24dp. Left and right margins may also be referred to as “start” and “end” to support localization for right to left languages.
  • Remove any bottom constraint.
  • In strings.xml, change hello_second_fragment to “Here is a random number between 0 and %d.”
  • Use Refactor > Rename… to change the name of hello_second_fragment to random_heading.
  • Here is the XML code for the TextView that displays the heading:

    Compiling And Dexing The Java Class

    Next step is to compile the java class . In case you are not used to doing this outside of an IDE, heres the command:

    javac -source 1.7 -target 1.7 -d bin -cp lib/commons-cli-1.3.1.jar src/com/example/

    Make sure the program compiled properly:

    java -cp lib/commons-cli-1.3.1.jar:bin com.example.HelloWorld -husage: Hello world -h   Print help -v   Print version

    Android cannot run Java class files directly. They have to be converted to Dalviks DEX format first :

    ./android-sdk-linux/build-tools/23.0.2/dx --output=helloworld.jar --dex ./bin lib/commons-cli-1.3.1.jar

    NOTE: Android Build Tools v28.0.2 and later contain a dx upgrade, called d8. The d8 tool can process Java 8 class files. Ill stick with dx for backwards compatibility reasons here.

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    Add A New Color To Use As The Screen Background Color

  • Back in colors.xml, create a new color resource called screenBackground:
  • < color name="screenBackground"> #FFEE58< /color> 

    A Color can be defined as 3 hexadecimal numbers representing the red, blue, and green components. The color you just added is yellow. Notice that the colors corresponding to the code are displayed in the left margin of the editor.

    Note that a color can also be defined including an alpha value which represents the transparency . When included, the alpha value is the first of 4 hexadecimal numbers .

    The alpha value is a measure of transparency. For example, #88FFEE58 makes the color semi-transparent, and if you use #00FFEE58, it’s fully transparent and disappears from the left-hand bar.

  • Go back to fragment_first.xml.
  • In the Component Tree, select the ConstraintLayout.
  • In the Attributes panel, select the background property and press Enter. Type “c” in the field that appears.
  • In the menu of colors that appears, select . Press Enter to complete the selection.
  • Click on the yellow patch to the left of the color value in the background field.
  • It shows a list of colors defined in colors.xml. Click the Custom tab to choose a custom color with an interactive color chooser.

  • Feel free to change the value of the screenBackground color, but make sure that the final color is noticeably different from the colorPrimary and colorPrimaryDark colors.
  • Change The Background Color Of The Layout

    EASY CODER : Learn to develop &  run java programs for Android

    Give your new activity a different background color than the first activity:

  • In colors.xml, add a new color resource:
  • < color name="screenBackground2"> #26C6DA< /color> 
  • In the layout for the second activity, fragment_second.xml, set the background of the ConstraintLayout to the new color.
  • In the Attributes panel:

    Or in XML:

    Your app now has a completed layout for the second fragment. But if you run your app and press the Random button, it may crash. The click handler that Android Studio set up for that button needs some changes. In the next task, you will explore and fix this error.

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    Writing Java Code To Load The Layout File

    The controls defined in the activity layout file need to be loaded to display them on the screen. In addition to loading the controls, we want to perform certain tasks such as associating events to controls, listening for events, and taking action when events occur. To do all these tasks, write the Java code shown in Listing 3 in the Java activity file

    Listing 3 Code in file.

    package com.bmharwani.welcomemsg import android.os.Bundle import import android.view.Menu import android.widget.TextView import android.widget.EditText import android.widget.Button import android.view.View public class WelcomeMsgActivity extends Activity         })     }}

    In this Java activity file, the following tasks are performed:

    We can run the Android project on a physical device as well on a virtual device. For this example, we’ll run the Android project by creating an Android Virtual Device as described in the next section.

    How To Get Started With Java Programming

    If you plan on developing Java apps on your desktop computer, then you will need to download and install the JDK.

    You can get the latest version of the JDK directly from Oracle. Once youve installed this, your computer will have the ability to understand and run Java code. However, you will still need an additional piece of software in order to actually write the code. This is the Integrated Development Environment or IDE: the interface used by developers to enter their code and call upon the JDK.

    When developing for Android, you will use the Android Studio IDE. This not only serves as an interface for your Java code, but also acts as a bridge for accessing Android-specific code from the SDK. For more on that, check out our guide to Android development for beginners.

    For the purposes of this Java tutorial, it may be easier to write your code directly into a Java compiler app. You can download these for Android and iOS, or even find web apps that run in your browser. These tools provide everything you need in one place and let you start testing code.

    Also Check: How To Implement Referral Program In Mobile Apps For Android

    Explore Width And Height Properties

    Now that you have a new screen background color, you will use it to explore the effects of changing the width and height properties of views.

  • In fragment_first.xml, in the Component Tree, select the ConstraintLayout.
  • In the Attributes panel, find and expand the Layout section.
  • The layout_width and layout_height properties are both set to match_parent. The ConstraintLayout is the root view of this Fragment, so the “parent” layout size is effectively the size of your screen.

    Tip: All views must have layout_width and layout_height properties.

  • Notice that the entire background of the screen uses the screenBackground color.
  • Select textview_first. Currently the layout width and height are wrap_content, which tells the view to be just big enough to enclose its content
  • Change both the layout width and layout height to match_constraint, which tells the view to be as big as whatever it’s constrained to.
  • The width and height show 0dp, and the text moves to the upper left, while the TextView expands to match the ConstraintLayout except for the button. The button and the text view are at the same level in the view hierarchy inside the constraint layout, so they share space.

  • Explore what happens if the width is match_constraint and the height is wrap_content and vice versa. You can also change the width and height of the button_first.
  • Set both the width and height of the TextView and the Button back to wrap_content.
  • But Doesnt Android Run On Java

    How To Run Java (JavaME) Apps & Games on Android

    Yes. Apps are written in Java and the kernel of the Android OS includes Java compatible libraries. This doesnt have anything to do with Java Applets running in the web browser though. Its kind of like asking My application was written in C++, why cant it run another C++ application?. It just doesnt work that way.

    Hopefully this answers many questions for Android users confused about Java. If you have anything to add to this post or a question, feel free to leave comments below.

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    Whats This Javascript Setting In My Android Browser For Then

    Users often meet confusion when they discover a JavaScript setting within the web browser settings and cant run Java Applets. JavaScript is a totally different thing than Java though. JavaScript can run native within a web browser and doesnt use many resources. Running a Java applet on an Android device would hog resources and run very slow even on the fastest Android device.

    How To Launch An Activity From Another Application In Android

    I want to launch an installed package from my Android application. I assume that it is possible using intents, but I didn’t find a way of doing it. Is there a link, where to find the information?

    • 2what happen if I open second app from first one and then click directly the icon of second app, I get two instances of the app, which is to manage it ??

    If you don’t know the main activity, then the package name can be used to launch the application.

    Intent launchIntent = getPackageManager.getLaunchIntentForPackage if  
    • 5Any reason as to why this would not work? I didn’t get it to work at least.Dec 21, 2012 at 20:18
    • 29Jul 29, 2013 at 10:02
    • 2Yes, this can return null. “The current implementation looks first for a main activity in the category CATEGORY_INFO, and next for a main activity in the category CATEGORY_LAUNCHER. Returns null if neither are found.

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    Using Java For Building Mobile Applications

    The mobile edition of Java is called Java ME. Java ME is based on Java SE and is supported by most smartphones and tablets. The Java Platform Micro Edition provides a flexible, secure environment for building and executing applications that are targeted at embedded and mobile devices. The applications that are built using Java ME are portable, secure, and can take advantage of the native capabilities of the device. Java ME addresses the constraints that are involved in building applications that are targeted at mobile devices. In essence, Java ME addresses the challenge of executing applications on devices that are low on available memory, display, and power.

    There are various ways to build applications for Android devices, but the recommended approach is to leverage the Java programming language and the Android SDK. You can explore more about the Android SDK Manager from here.

    The Dalvik Virtual Machine

    Best way to run java on Android

    The Android platform leverages the Dalvik Virtual machine for memory, security, device, and process management. Although the internal intricacies of how the Dalvik Virtual Machine works is not that important to an average developer, you can think of the Dalvik VM as a box that provides the necessary environment for you to execute an Android application sans the need of having to worry about the target device.

    To get started using Java for Android, you should first download and install Android Studio. You then may want to take advantage of the SDK Manager to download and install the latest SDK tools and platforms.

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    How To Use Android Studio To Write Pure Java

    For those looking to sharpen skills using Data Structures and building Algorithms youre most likely reading Cracking The Code Interview as well doing practice problems on a number of the sites out there

    I prefer using one compiler to do everything, and since Im developing apps in Android Studio Id like to also construct my coding challenge here because

  • Some coding challenge sites / classes pick a specific language and I want to use Java or Im doing questions out of a book.
  • I can automatically save my solution for referring back in a Google Drive folder as well as on GitHub through the Android Studio UI.
  • Before I show you how, if you wantto get running right away you can download or fork this example Repo and you wont need to read further. If you plan to upload it to a type of Version Control just make sure to remove the origin and replace it with your Repo using this command in the terminal:

    Howto: Running Java Code Directly On Android

    A step by step instruction for compiling a Java program into an Android executable and using ADB to run it.

    When you want to create a system / commandline tool for Android, you have to write it in C or do you?

    TLDR heres the final proof of concept.

    Sticking with Java would have the benefit of avoiding all of the native ABI hassle and also being able to call into the Android runtime. So how do we do that?

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    Comparison Of Java And Android Api

    This article compares the application programming interfaces and virtual machines of the programming language Java and operating system Android.

    While most Android applications are written in Java-like language, there are some differences between the Java API and the Android API, and Android does not run Java bytecode by a traditional Java virtual machine , but instead by a Dalvik virtual machine in older versions of Android, and an Android Runtime in newer versions, that compile the same code that Dalvik runs to Executable and Linkable Format executables containing machine code.

    Java bytecode in Java Archive ” rel=”nofollow”> JAR) files is not executed by Android devices. Instead, Java classes are compiled into a proprietary bytecode format and run on Dalvik , a specialized virtual machine designed for Android. Unlike Java VMs, which are stack machines , the Dalvik VM is a register machine .

    Dalvik has some traits that differentiate it from other standard VMs:

    • The VM was designed to use less space.
    • The constant pool has been modified to use only 32-bit indexes to simplify the interpreter.
    • Standard Java bytecode executes 8-bit stack instructions. Local variables must be copied to or from the operand stack by separate instructions. Dalvik instead uses its own 16-bit instruction set that works directly on local variables. The local variable is commonly picked by a 4-bit virtual register field.

    Installing And Running The App

    Run java console application using Android Studio(2019)

    Time to push everything to the device:

    adb shell mkdir -p /data/local/tmp/helloworldadb push helloworld.jar /data/local/tmp/helloworldadb push /data/local/tmp/helloworldadb shell chmod 777 /data/local/tmp/helloworld/

    Moment of truth :

    adb shell /data/local/tmp/helloworld/ -vHello World  v0.1

    NOTE: Since nothing was installed into the system, getting rid of the program is simply done by deleting the directory again.

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    How Java Is Used In Android App Development

    Java is the technology of choice for building applications using managed code that can execute on mobile devices.

    Android is an open source software platform and Linux-based operating system for mobile devices. The Android platform allows developers to write managed code using Java to manage and control the Android device. Android applications can be developed by using the Java programming language and the Android SDK. So, familiarity with the basics of the Java programming language is a prerequisite for programming on the Android platform. This article discusses where Java fits in mobile application development and how we can use Java and Android SDK to write applications that can work on Android devices.

    Add A Constraint To The New Button

    You will now constrain the top of the button to the bottom of the TextView.

  • Move the cursor over the circle at the top of the Button.
  • The Button moves up to sit just below the TextView because the top of the button is now constrained to the bottom of the TextView.

  • Take a look at the Constraint Widget in the Layout pane of the Attributes panel. It shows some constraints for the Button, including Top -> BottomOf textView.
  • Take a look at the XML code for the button. It now includes the attribute that constrains the top of the button to the bottom of the TextView.
  • You may see a warning, “Not Horizontally Constrained“. To fix this, add a constraint from the left side of the button to the left side of the screen.
  • Also add a constraint to constrain the bottom of the button to the bottom of the screen.
  • Before adding another button, relabel this button so things are a little clearer about which button is which.

  • Look at the Attributes panel on the right, and notice the id field.
  • Change the id from button to toast_button.
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    Example: A Simple Java Program

    The code shown in Listing 1 is a simple Java program called that asks the user to enter a name. After entering the name, when the user presses Enter, a welcome message appears, including the name the user typed. For example, if the user enters the name “Kelly,” the message, “Welcome Kelly!” appears on the screen.

    Listing 1 Code in

    import*                                                          #1class WelcomeMsg           catch       }}

    In, statement #1 imports the packages required for input/output operations. Statement #2 creates a reader called istream. The reader will read from the standard input stream that is, from the keyboard. The entered text is stored in the form of Unicode characters. Statement #3 uses the istream reader to convert the data into string form. Statement #4 defines a try..catch block to check for any errors that might have occurred while the reader entered the text. Statements #5 and #6 ask the user to enter a name that is then assigned to the string name. The entered name is then displayed on the screen by statement #8.

    When the program is run, it asks the user to enter a name. After the user types a name and presses Enter, the welcome message is displayed with the entered name, as shown in Figure 1.

    Output of the Java program

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