# Listener

A **listener** (or **observer**) is an [abstract class](https://www.educative.io/answers/what-is-a-cpp-abstract-class) or [interface](https://www.educative.io/answers/what-is-a-java-interface) that is used to provide functionality for an interactable UI component (e.g., a button). It is a part of the [**observer design pattern**](https://www.educative.io/answers/what-is-the-observer-design-pattern) that detects any events related to the UI; for example, clicking or tapping a part of the screen executes a certain action. Every listener has a function (*callback* method) that is [overridden](https://www.educative.io/answers/overloading-vs-overriding) when implementing a concrete listener.

> A UI component is known as a **view** in Android.

## `onClick(View v)`

In Android, the **OnClickListener()** interface has an `onClick(View v)` method that is called when the view (component) is clicked. The code for a component's functionality is written inside this method, and the listener is set using the **`setOnClickListener()`** method.

# Code

Since the functionality of a component is usually different from other components, a listener (in Java) is commonly implemented using an *anonymous class*. Declaring a separate listener for each component also works, but it ​is not a good practice.

// Get button using its ID:
Button btn = findViewById(R.id.my_btn);

// Set OnCLickListener() using anonymous class:
btn.setOnClickListener(new View.OnClickListener(){
    @Override
    public void onClick(View v){
        // Functionality for the button...
    }
});

Anonymously

// Declaring a class that implements OnClickListener interface:
class MyFunctionality implements View.OnClickListener(){
    @Override
    public void onClick(View v){
        // Functionality for the button...
    }
}
// Get button using its ID:
Button btn = findViewById(R.id.my_btn);

// Set OnCLickListener() using declared class:
btn.setOnClickListener(new MyFunctionality());

Declaratively

What is an OnClickListener?

An OnClickListener is an interface in Android used to detect and handle click events on UI components (views) through the onClick(View v) method.