In Android we can set click listener by three ways

1. Setting individual click listener for each element.
2. Implementing OnClickListener in activity level.
3. Creating a separate function for handling button click and add this function in xml

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.bm.buttonclickeg.MainActivity">

    <Button
        android:id="@+id/btn_one"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button 1" />

    <Button
        android:id="@+id/btn_two"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button 2" />

    <Button
        android:id="@+id/btn_three"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="myBtnClick"
        android:text="Button 3" />

</LinearLayout>

In the above example btn_three I have added a property android:onClick=”myBtnClick” myBtnClick is a function I have defined in activiy

MainActivity.java

package com.example.bm.buttonclickeg;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //option 1
        
        Button btn1 = (Button) findViewById(R.id.btn_one);

        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Toast.makeText(MainActivity.this, "Created individual click listener", Toast.LENGTH_SHORT).show();
            }
        });


        //option 2
        Button btn2 = (Button) findViewById(R.id.btn_two);

        btn2.setOnClickListener(this);

    }


    //option 2
    @Override
    public void onClick(View v) {

        switch (v.getId()) {
            case R.id.btn_two:
                Toast.makeText(MainActivity.this, "Implemented click listener in activity", Toast.LENGTH_SHORT).show();
                break;
        }

    }


    //option 3
    // This function I have added in the btn_tree xml
    public void myBtnClick(View v) {
        Toast.makeText(MainActivity.this, "Created a separate function for handling button click and added this function in button xml", Toast.LENGTH_SHORT).show();
    }
}