How to start an activity in Android?

I am very new to android programming. I want to use a code that takes me to MainActivity from my current activity on a click of a Button.

Here is my current code:

package com.example.flashlightapp;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class Whitelight extends Activity implements OnClickListener {

    Button b1 = (Button) findViewById(R.id.b3);

    Intent i = new Intent(this, MainActivity.class);
    {
        this.startActivity(i);
    }

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

    }

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub

    }

}

What should I put in

@Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
        }

At first you must check if you declared all your activities in the manifest.xml file.

and in your Java code try this:

    package com.example.flashlightapp;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class Whitelight extends Activity  {
    

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_whitelight);
       Button b1 = (Button) findViewById(R.id.b3);
       b1.setOnClickListener(new OnClickListener() {
          @Override
          public void onClick(View v) {
        // TODO Auto-generated method stub
 
    Intent i = new Intent(Whitelight.this, MainActivity.class);
    
        this.startActivity(i);
    }
    });

   }

}

This tutorial explains how to use intents and listeners : https://houssem-lahiani.blogspot.com/2014/09/android-tutorial-using-intent-to-move.html