I’ve seen many people asking how to implement Listeners in their applications. Implementing a Listener is quite easy. There are 3 ways to implement an Listener and the have their advantages and disadvantages.

The tree way to implement Listeners are

• Inline Implementation
• Using the implements keyword
• By using variables

We’ll use our good old LoginExample application, created in previous tutorial which can be found at Android: Your first Android Application.

Inline Implementation

The first way, to implement an listener is by using Inline Implementation. In Inline Implementations we create an anonymous listener, define and pass it the the setLisener functions in the same step.

We did this already in our First Android Application Tutorial.

package com.tseng.examples;

...

public class LoginExample extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

	...

        // Set Click Listener
        btnLogin.setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(View v) {
				// Check Login
				String username = etUsername.getText().toString();
				String password = etPassword.getText().toString();

				if(username.equals("guest") && password.equals("guest")){
					lblResult.setText("Login successful.");
				} else {
					lblResult.setText("Login failed. Username and/or password doesn't match.");
				}
			}
		});
        btnCancel.setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(View v) {
				// Close the application
				finish();
			}
		});
    }
}

As we see, we create an anonymous class there by adding { … code … } behind the new OnClickListener interface and implementing the necessary onClick(View v) method.

Advantages

• Small and tidy
• Easy to implement
• Less overhead

Disadvantages

• Inflexible
• Can’t be reused
• Can be a bit harder to maintain

Usage

Inline implementations are usually used for short 1-time methods, for example if you have a button which closes the application or which displays, you don’t need to add an implementation to your class or create a variable, making your code less readable.

Using the “implements” keyword

The second method to implement an Listener is by adding an interface to your base class. In java you can do this by adding “implements Interfacename” to the class declaration.

package com.tseng.examples;

...

public class LoginExampleImplements extends Activity implements OnClickListener {

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

	...        

        // Set Click Listener
        btnLogin.setOnClickListener(this);
        btnCancel.setOnClickListener(this);
    }

	@Override
	public void onClick(View v) {
		if(v==btnLogin) {
			// Check Login
			String username = etUsername.getText().toString();
			String password = etPassword.getText().toString();

			if(username.equals("guest") && password.equals("guest")){
				lblResult.setText("Login successful.");
			} else {
				lblResult.setText("Login failed. Username and/or password doesn't match.");
			}
		} else if(v==btnCancel) {
			// Close the application
			finish();
		}
	}
}

As we can see, the “onClick(View v)” is being declared inside our LoginExample class and additionally we set the listener by passing a reference to our class to by using btnLogin.setOnClickListener(this);. This works, because we implemented this interface within our class public class LoginExampleImplements extends Activity implements OnClickListener. You may also have noticed, that we add the same listener to both buttons. Because both of the buttons use the same listener, we need to differentiate which one was clicked. This can be done by comparing the View v reference with the Button btnLogin reference as seen below:

	if(v==btnLogin) {
		// Check Login
		...
	} else if(v==btnCancel) {
		// Close the application
		...
	}

Advantages

• Methods/Listener can be reused in many different widgets
• Code of multiple Listeners is located in the same section of code
• Can be used to create one method for similar Listeners

Disadvantages

• Can contain much unnecessary and untidily code, if the actions executed are to different and you have to add an if / elseif / else blocks, making the code hard to read
• You can only have one implementation of this Listener per class

Usage

This method is best used, when you have multiple widgets/elements using same or similar listeners (i.E. doing a calculation or check on a click or key press). The example above is not the best example on the usage of the implement method. Let’s imagine, you have a calculator and have 14 buttons  and you want to update the formula you entered after every calculator button is pressed, you could implement it in the following way shown below.

package com.tseng.examples;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnKeyListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class CalculatorExample extends Activity implements OnClickListener {
	...

	@Override
	public void onClick(View v) {
		if(v==btnCalculate) {
			// Parse and calculate formula
			String formula = etFormula.getText().toString();
			Double result = performCalculation(formula);

			// Update the result TextView
			tvResult.setText(Strint.valueOf(result));

			// End it as we don't need or want to update the Formula field
			return;
		}

		// Get the button
		Button button = (Button)v;

		// Get the String/Button descritpion
		String strToAppend = button.getText().toString();

		// Update Formula
		etFormula.append(strToAppend);
	}
}

You could add this Listener to every of the calculators button and only need to define one Listener. When the buttons are clicked, the button text (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, +, –, /, * etc.) will be added to the TextView containing the formula. However, if you press the calculate button, it won’t add a = to the formula, but will instead perform the calculation.

By using Variables

This one is very similar to the previous one, with the difference that you don’t add the implementation to your class, but instead hold a reference to the Listener in a variable.

In our LoginExample it would look like this

package com.tseng.examples;

...

public class LoginExampleVariableImplementation extends Activity {
	...

	OnClickListener myClickListener = new OnClickListener() {
		@Override
		public void onClick(View v) {
			if(v==btnLogin) {
				// Check Login
				String username = etUsername.getText().toString();
				String password = etPassword.getText().toString();

				if(username.equals("guest") && password.equals("guest")){
					lblResult.setText("Login successful.");
				} else {
					lblResult.setText("Login failed. Username and/or password doesn't match.");
				}
			} else if(v==btnCancel) {
				// Close the application
				finish();
			}
		}
	};

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

	...

        // Set Click Listener
        btnLogin.setOnClickListener(myClickListener);
        btnCancel.setOnClickListener(myClickListener);
    }

}

Basically we create it anonymous Listener with the difference that we hold a reference to it. This allows us to add this Listener to more than only one widget. The main difference to the implements keyword method is, that we can have more than one Listener inside our class declared and use them more than once.

Advantages

• Can be reused
• You can have more than one Listener of the same kind in your class
• You can keep your listeners organized in one place, making your code easier to read

Disadvantages

• Too many listeners can make the code rather complicated to read

Usage

This is best to use if you have different Listeners for the same action i.e. 2 different OnClickListener which do a completely different task.

Another very important usage for this variant is if you’re implementing your own Listeners to your widgets, you could have a variable which can be assigned by the users of your widgets

package com.tseng.examples;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnKeyListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MyWidget extends View {
	...

	OnClickListener myClickListener = null;

	/** Called when the activity is first created. */
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);

		...

	}

	public void setOnClickListener(OnClickListener listener) {
		myClickListener = listener;
	}

	private onClick(View v) {
		// Check if Listener was set and call the onClick Method
		if(myClickListener!=null)
			myClickListener.onClick(v);
	}
	private void handleEventsMethod() {
		...
		// handle clicks
		onClick(this);
	}
}

This allows us to dynamically set the Listener to our widget without knowing what the listener will actually do with the click, as it can be implemented in any way the user or programmer wants it to be.

Summary

So there are no “right” ways to implement a Listener. It all depends on the situation and/or your personal preferences.

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