Android java get view by name

Access Android View in Activity

When programming Android apps the code will need to reference the screen elements to show data and read user actions. The screen elements are based on the Android View class and its sub-classes. This article covers the assignment of an identifier (id) to a screen item (View), and using that id to access the item in the app code. Example Android Studio code is provided.

The User Interface (UI) for an app, i.e. the app screens, can be designed in Android Studio. The screens, or parts of screens, are stored in Layouts as Extensible Markup Language (XML) files. Different layouts can be defined for different size screens, and different modes, e.g. portrait and landscape. This allows for support of multiple screen sizes and types, and eases software maintenance. (Screens can be constructed dynamically in code, but that is not covered in this article.)

In Android Get View Defined in XML in Activity Code

UI components made from Views include the widgets (components) the a users will work with in the app. These include the Button , Checkbox , EditText (a text box), ImageView , RadioButton , ProgressBar , TextView (a label) and many more. Several such widgets sit in a ViewGroup , another type of View, which provides a container for laying out the components. Different ViewGroups provide different types of layouts, including RelativeLayout , LinearLayout , ScrollView , WebView and others, including the recent ConstraintLayout . ViewGroups can contain other ViewGroups, therefore allowing complex displays by nesting different Views. Classes in the Android Software Development Kit (SDK) have methods used to access all the different types of Views.

First Create a Basic Android Screen

For this tutorial create a new, simple Android app project and call it Button Demo (if you don’t know how see Your First Android Hello World Java Program). A simple screen that just has a button on it was created using the starting layout, keeping the default layout name of activity_main.xml. With activity_main.xml open in the Design tab drag and drop a Button from the Palette onto the app layout screen.

To show how this view is accessed from the app’s code the text on the Button will be changed and it displays a message when it is clicked.

An Explanation of Android’s View Ids

The important part of a screen item, when accessing it in the code, is the android:id attribute. Here it is set to button. The id attribute is edited in the Attributes list in Studio. When looking at the screen in text mode (use the Text tab at the bottom of the edit area) then the Button XML shows android:id=»@+id/button». Here the plus sign (+) tells the Android SDK that it is a new id. When defining an id for a View it is in the form of @+id/name. Think of it as @ for address, the + for add new, the id for identifier, and /name for the uniqueness (it is not possible to have two Views with the same name). I.e. it is defining an id that uniquely addresses the screen element.

Having an id allows code to access that particular View. If a View does not need to be accessed it does not need an id. However, it does need an id for automatic state saving. If in doubt add an id, it usually helps with future app maintenance. It also helps to give the View a sensible id. For example a TextView showing a cell phone number could be called textCellPhone, which is better than textView24.

The build process for the Android app generates a Java public class called R for all the defined resources. The ids given to screen elements are resources, therefore they are added to the R class as integer values (the values are assigned during the build process). An id value can be accessed using R.id.name. In this example the Button id of button is used in the code with R.id.button. The id values are declared static final int, so they cannot be changed at execution time in the code. (Very occasionally the R class gets our of sync with the code. Choosing Clean Project from the Build menu often fixes it, or if it does not choose Invalidate Caches/Restart from the File menu.)

Using the View Id to Get a View Reference

When the app’s main Java class is opened (in this example using the default class name of MainActivity), it will be seen that it extends another class, the Activity class or AppCompatActivity class. This provides a method called findViewById, which takes the ids from the R class and returns a reference to the relevant View. This reference is then cast to the appropriate object, in this case a button. Therefore to get a reference to the Button use this code:

Button btn = (Button) findViewById(R.id.button);

The cast to the Button class will require import android.widget.Button; adding to the imports section of the MainActivity.java file. Studio can do this automatically. With the cursor on Button a prompt appears asking to press Alt-Enter to add the import. Pressing Alt-Enter adds the relevant import automatically.

Using the Reference Returned by findViewById

With the reference returned by findViewById cast to the appropriate object the interface elements can be manipulated using their class methods. For example the text for the Button is changed using the button’s setText method:

To handle the button being pressed an instance of the OnClickListener class must be attached to the button reference, using the well named setOnClickListener method (which is provided by the View class). The implementation of OnClickListener takes an onClick function, which is called when the View’s click occurs. Here is a definition of an onClick function, being passed into the constructor of an OnClickListener implementation, it simply displays a message. Again add any required imports using Alt-Enter:

class HandleClick implements OnClickListener < public void onClick(View arg0) < Toast.makeText(getApplicationContext(), "Hello world!", Toast.LENGTH_SHORT ).show(); >>

Notice that the onClick function takes a View object as a parameter, here called arg0. This parameter is a reference to the pressed View. Again this can be cast to the screen element pressed, here a Button, so that the clicked item can be accessed in the onClick function:

Button btn = (Button) arg0; btn.setText("Thankyou");

Using an Anonymous Class with findViewById

If you need to access a reference to a View only once it saves code to use an anonymous class. This is where you use the return value of findViewById but do not assign it to a declared class variable. Behind the scenes Java still creates a class but it cannot be seen, hence it is anonymous. This is done by wrapping the cast and call to findViewById in brackets and using it like it is a class itself, so this:

Button btn = (Button) findViewById(R.id.button); btn.setText("Say Hello");

((Button) findViewById(R.id.button)).setText("Say Hello"); 

Using Module Level View References

If a View is going be accessed in several place in a Java file, then declare the View variable at the module level. The following source code is the final MainActivity.java class for this demo. Notice how the btn variable is declared at the start of the class definition. It is assigned in the onCreate method (after the call to setContentView). It is then used in the HandleClick class with out needing to cast the arg0 parameter to its own Button reference:

package com.example.buttontest; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.Toast; public class MainActivity extends AppCompatActivity < Button btn; //stores reference to button for the whole class @Override protected void onCreate(Bundle savedInstanceState) < super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); //get reference to button btn = (Button) findViewById((R.id.button)); //change the buttons caption //(usually done in the XML or via a string resource) btn.setText("Say Hello"); //attach the click listener btn.setOnClickListener(new HandleClick()); >class HandleClick implements View.OnClickListener < public void onClick(View arg0) < Toast.makeText(getApplicationContext(), "Hello world!", Toast.LENGTH_SHORT ).show(); btn.setText("Thankyou"); >> >

For completeness here is the activity_main.xml that was used:

Download some example code in view-id.zip from this article ready for importing into Android Studio. See the instructions in the zip file, alternatively the code can also be accessed via the Android Example Projects page.

See Also

• Different Ways to Code Android Event Listeners
• Saving Activity State in an App when it’s Interrupted
• See the Android Example Projects page for lots of Android sample projects with source code.
• For a full list of all the articles in Tek Eye see the full site alphabetical Index.

Author: Daniel S. Fowler Published: 2013-06-12 Updated: 2017-12-15

Do you have a question or comment about this article?

(Alternatively, use the email address at the bottom of the web page.)

↓markdown↓ CMS is fast and simple. Build websites quickly and publish easily. For beginner to expert.

Free Android Projects and Samples:

Источник

The Android findViewById() method explained with examples

The findViewById() method is a method of Android’s View and Activity classes.

The method is used to find an existing view in your XML layout by its android:id attribute.

For example, suppose you have a TextView in your XML layout with the following definition:

 You can get the TextView above in your Activity class with the following code:

The same can be done for any valid Android View object, such as a Button or a CheckBox view.

When you call the findViewById() method, the method is usually called from the AppCompatActivity class, which is a subclass of the Activity class.

The findViewById() method accepts only one parameter, which is the id of the view with int type reference:

The Android framework generates an internal reference of the id you specified in the XML layout. The internal reference is of an int type.

You need to refer to this generated int value by calling R.id in your code.

Using findViewById in Kotlin

In Kotlin, you can use the syntax to use the findViewById() method is as follows:

You need to specify the generic type of the view inside the angle brackets <> as shown above.

And that’s how you use the findViewById() method when developing an Android project. 👍

Take your skills to the next level ⚡️

I’m sending out an occasional email with the latest tutorials on programming, web development, and statistics. Drop your email in the box below and I’ll send new stuff straight into your inbox!

About

Hello! This website is dedicated to help you learn tech and data science skills with its step-by-step, beginner-friendly tutorials.
Learn statistics, JavaScript and other programming languages using clear examples written for people.

Search

Type the keyword below and hit enter

Tags

Click to see all tutorials tagged with:

Источник