public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.materi);

    ((ImageView) findViewById(R.id.button_semester1)).setOnClickListener(this);
    ((ImageView) findViewById(R.id.button_semester2)).setOnClickListener(this);
  }
 @Override
 public void onClick(View v) {
   Intent intent;
   switch (v.getId()) {
     case R.id.button_semester1:
       intent = new Intent(this, BabSelectionActivity.class);
       intent.putExtra("semester", 1);
       startActivity(intent);
       break;
     case R.id.button_semester2:
       intent = new Intent(this, BabSelectionActivity.class);
       intent.putExtra("semester", 2);
       startActivity(intent);
       break;
     default:
       super.onClick(v);
       break;
   }
 }