Question

I’m trying to create a simple thread that started from a Button but something went wrong.
Here is the code:

    b1.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) 
        {
            SendInfo si = new SendInfo();
            si.start();
            error.setText(si.getError());

        }
    });

And here is the the thread code:

package com.example.android.location;

public class SendInfo extends Thread
{
    private String error;

    public void run()
    {
        this.error = "working";
    }
    public String getError(){
        return this.error;
    }   
}

For some reason the error instance in the first code block stays empty.

Answer 1

You have a race.

After the start() call either:

1) your code continues and getError returns null and AFTER that the thread runs.

or

2) the thread runs AND finishes and getErrorwill return “working”.

In your case 1) is happening.

Update: actually in theory during option 2 you might not get “working” anyway as the return value. This is due to memory/thread visibility. Synchronizing access to the error variable or in this case just declaring that field volatile fixes that (but not your original problem).