How to start Intent when either actionbar or builtin back button is pressed

So I have an intent that I want to start when either of 2 back buttons is pressed. Either this back button "getSupportActionBar().setDisplayHomeAsUpEnabled(true);" or the one that's on all android phones that's built in to the device. I've searched around and could not find a solution.

Here's my code where you can see in the if statement I want the "if back button is pressed" code to be. I appreciate any input!

    CheckBox routeCheckBox;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_route_details);

    //back button for route details view
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    //sets actionbar title
    routeName = getIntent().getExtras().getString("routeName");
    getSupportActionBar().setTitle(routeName);


    ///////checkbox///////
    routeCheckBox = (CheckBox) findViewById(R.id.routeCheckBox);

    routeCheckBox.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view)
        {
          if (routeCheckBox.isChecked())
          {
              Intent check = new     Intent(RouteDetails.this,MainActivity.class);
              check.putExtra("checkImageResource", R.drawable.checkmark);

  /////////////////// ////////////     
              if ( actionbar back button or the built in backbutton on all   android devices is pressed)
              {
                  startActivity(check);
              } 
     ////////////////////////////////
          }
        }
    });