Run Java App On Android

How Android Apps Run On Devices

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P. S. –

Update The Textview To Display The Header

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/HelloWorld.java

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

< 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.

It shows a list of colors defined in colors.xml. Click the Custom tab to choose a custom color with an interactive color chooser.

Change The Background Color Of The Layout

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

< color name="screenBackground2"> #26C6DA< /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 WelcomeMsgActivity.java.

Listing 3 Code in WelcomeMsgActivity.java file.

package com.bmharwani.welcomemsg import android.os.Bundle import android.app.Activity 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.

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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.

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.

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.

But Doesnt Android Run On Java

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 undesired.how 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

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

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

Time to push everything to the device:

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

Moment of truth :

adb shell /data/local/tmp/helloworld/helloworld.sh -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.

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.

Before adding another button, relabel this button so things are a little clearer about which button is which.

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Example: A Simple Java Program

The code shown in Listing 1 is a simple Java program called WelcomeMsg.java 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 WelcomeMsg.java.

import java.io.*                                                          #1class WelcomeMsg           catch       }}

In WelcomeMsg.java, 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