Unity3d: OnCollisionEnter issue - unity3d

Question

I have came across similar problem which few people already posted on the forum. I checked the exisiting posts but still not couldn't rectify the issue. I have cube in my program which hits the floor when program starts. However, I found that onCollisionEnter is not being called when cube hits the floor. Rigidbody is attached to the cube. Could anyone guide me where am I making mistake?
Code of cube is:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class cube : MonoBehaviour
{
public float speed = 3.5f;
public float jumpingforce = 10f;
private bool canjump = false;
// Start is called before the first frame update
void Start()
{
}
// Update is called once per frame
void Update()
{
if(Input.GetKey("right"))
{
transform.position += Vector3.right * speed * Time.deltaTime;
}
if(Input.GetKey("left"))
{
transform.position += Vector3.left * speed * Time.deltaTime;
}
if(Input.GetKeyDown("space"))
{
GetComponent<Rigidbody>().AddForce(0,jumpingforce,0);
}
}
void onCollisionEnter(Collision collision)
{
Debug.Log("checking collision....");
}
}

Answer 1

First of all OnCollisionEnter needs to start (as btw any method and class name should in c#) with a capital O .. otherwise Unity will not recognize it and never call it. Unity calls these methods (like also Awake, Start, Update) as "Messages". If they do not exactly match the name they are never found and never called.
Some more things:
Do not call GetComponent repeatedly. It is quite expensive! Rather store the reference once and re-use it.
Then whenever a Rigidbody is involved you should not update it's transformations using the Transform component! This breaks the Physics! Rather use Rigidbody.MovePosition within FixedUpdate
Small Sidenote: Remove any empty Unity message method like Start, Awake, Update. Even if they are empty they are called by Unity as message only causing unnecessary overhead.
So you should change your code to something like
public class cube : MonoBehaviour
{
public float speed = 3.5f;
public float jumpingforce = 10f;
private bool canjump = false;
// you can also already reference this via the Inspector
// then it skips the GetComponnet call
[SerializeField] privte Rigidbody rigidbody;
private void Awake()
{
if(!rigidbody) rigidbody = GetComponent<Rigidbody>()
}
private void FixedUpdate()
{
// Getting this input in FixedUpdate is ok since it is a continues call
if(Input.GetKey("right"))
{
rigidbdy.MovePosition(rigidbody.position + Vector3.right * speed * Time.deltaTime);
}
if(Input.GetKey("left"))
{
rigidbdy.MovePosition(rigidbody.position + Vector3.left * speed * Time.deltaTime);
}
}
private void Update()
{
// this one-time input event has to be checked in Update / frame-based
// since it is true only for one frame and FixedUpdate might not be called during that frame
if(Input.GetKeyDown("space"))
{
// also it's ok calling this in Update since it is a one-time event
// and unlikely to happen twice until FixedUpdate is called again
rigidbody.AddForce(0,jumpingforce,0);
}
}
private void OnCollisionEnter(Collision collision)
{
Debug.Log("checking collision....");
}
}