difference between Thread and Handler

Threads are generic processing tasks that can do most things, but one thing they cannot do is update the UI.

Handlers on the other hand are background threads that allow you to communicate with the UI thread (update the UI).

So for example show a toast or a update a progress bar via a message (Runnable) posted to a handler but you can't if you start this runnable as a thread.

With handler you can also have things like MessageQueuing, scheduling and repeating.

I am yet to encounter a situation where I needed a thread in android.

I mostly use a combination of AsyncTasks and Handlers.

Handlers for the aforementioned tasks.

AsyncTasks for download/ data fetching and polling etc.

The same: you can both execute task asynchronously without blocking your current code,

The difference: Imagine you have a Runnable r = new Runnable{...}

• When you use new Thread(r).start(), you actually created a new thread and run task asynchronously.

• When you use new Handler().post(r) (or Message), you added the Runnable object to Looper and execute the code later in the same thread.

A Thread, generally MainThread or UIThread contains a Looper. When MainThread runs, it will loop the Looper and execute Runnable one by one.

When Thread is preferred:

When you're doing a heavy work like network communication, or decoding large bitmap files, a new thread is preferred. If a lot of thread is needed, maybe ExecutorService is preferred further. https://developer.android.com/reference/java/util/concurrent/ExecutorService.html

When Handler is preferred:

When you want to update UI objects (like TextView text) from other thread, it is necessary that UI objects could only be updated in UI Thread. Also, when you just want to run some light code later (like the delay for 300ms) you can use Handler because it's lighter and faster.

Please also refer to Handler vs AsyncTask vs Thread

Thread actually creates new thread - part of job running in background relatively to current thread.

Handler itself doesn't provide any mechanisms for background job - it is just a tool to access message queue (Looper) associated with some thread. UI thread have Looper attached by default, so it is common practice to update UI with Handler.post(Runable) which means execute some piece of code on thread which is associated with this Handler.
As soon as Handler serves Looper, it can't be created in a thread which have no associated Looper.