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    private boolean mDoublePressBackToExit = false;
    private boolean mIsBackPressedByTheUser = false;

    @Override
    public void onBackPressed()
    {
        if (mDoublePressBackToExit)
        {
            // process the back button as per normal
            mIsBackPressedByTheUser = true;
            super.onBackPressed();
            return;
        }
        
        mDoublePressBackToExit = true;  // set flag
        Toast.makeText(this, "Press back again to exit...", Toast.LENGTH_SHORT).show();

        // reset after time delay
        new Handler().postDelayed(
            new Runnable() {
                @Override
                public void run() {
                    mDoublePressBackToExit = false;
                }
            }, BACK_PRESS_DELAY
        );
    }