public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button redoBtn = (Button) findViewById(R.id.redo);
        Button undoBtn = (Button) findViewById(R.id.undo);
        EditText editText = (EditText) findViewById(R.id.edittext);
      
        // pass edittext object to TextViewUndoRedo class
        TextViewUndoRedo helper = new TextViewUndoRedo(edittext);
        
        // call the method from TextViewUndoRedo class
        redoBtn.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View v) {
            helper.redo(); // perform redo
          }
        });
        undoBtn.setOnClickLisener(new View.OnClickListener() {
          @Override
          public void onClick(View v) {
            helper.undo(); // perform undo
          }
        });
    }
}