button text to be changed each time it’s clicked/tapped

  Boolean isStart = true; initialize a global variable


  protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        stop = (Button) findViewById(R.id.start);
        stop.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        if(v.getId()==R.id.start){ 
            if(isStart){
                stop.setText("Stop")
            }else{
                stop.setText("Start");  
            }
            isStart = !isStart;
        }
    }

v.getId()==R.id.start – In this code I am checking if you have clicked that button or not. If you have only 1 view which uses this as onClickListener, then there is no need for that check. If you have multiple items, you should check for which one is clicked (preferably with switch-case).