how to create random UUID in Android when button click event happens?

You are explicitly avoiding the new UUID creation by:

if(uniqueId == null) {
   uniqueId = UUID.randomUUID().toString();
}

Remove the check.

First time it intialise the variable and next time when you click button it doesn't get null value

Remove if condition from this

if(uniqueId == null) { 
uniqueId = UUID.randomUUID().toString(); 
}

Use this

uniqueId = UUID.randomUUID().toString();

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