What is R.java in Android?

R.java is an automatically generated Java class file in Android that contains resource IDs for all the resources used in an Android application. It's generated by the Android build tools when you compile your app and it allows you to reference resources in your code by their IDs rather than by their filenames or paths.

The R.java file contains nested static classes that define the resource IDs for different types of resources such as layout files, strings, drawables, colors, and so on. For example, if you have a layout file named activity_main.xml, the R.java file will contain a nested class called 'layout' with a static field called 'activity_main' that contains the ID for that layout file.

You can use the IDs defined in R.java in your code to access resources programmatically. For example, if you want to set the text of a TextView to a string resource with ID R.string.hello_world, you can do so like this:

TextView textView = findViewById(R.id.myTextView);
    textView.setText(R.string.hello_world);
      

In this code, R.id.myTextView refers to the ID of a TextView in the layout file, and R.string.hello_world refers to the ID of a string resource.

Overall, R.java is an important part of the Android build process and allows you to easily reference resources in your code using their resource IDs.