<TextView
      android:layout_width="wrap_content"
      android:layout_height="0dp"
      android:paddingRight="30dp"
      android:layout_weight="1"
      android:textColor="@color/white"
      android:text=""
      android:layout_gravity="end"
      android:textSize="30sp"
      android:id="@+id/workingsTextView"/>
 
<Button
      android:layout_width="0dp"
      android:layout_height="match_parent"
      android:layout_weight="1"
      android:textSize="@dimen/TextBtnSize"
      android:background="@null"
      android:onClick="ClearOneOnClick"
      android:textColor="@color/orange"
      android:text="Del"/>

TextView textView;
TextView results;
String textButton = "";

@Override
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        dataTextView();
    }
 
private void dataTextView() {
        textView = (TextView) findViewById(R.id.workingsTextView);
        results = (TextView) findViewById(R.id.resultTextView);
    }
 
private void ClearOneOnClick(View view) {
        String str = (String) textView.getText();
 
        if(str.length()!=0) {
            str = str.substring(0, str.length() - 1);
            textView.setText(str);
        }
    }

 public void resultOnClick(View view) {
        Double result = null;
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("rhino");

        try
        {
            result = (Double) engine.eval(textButton);
        }
        catch (ScriptException e)
        {
            Toast.makeText(this, "Error!", Toast.LENGTH_SHORT).show();
        }

        if(result != null)
            results.setText(String.valueOf(result.doubleValue()));
    }

private void dataTextView() {
        textView = (TextView) findViewById(R.id.workingsTextView);
        results = (TextView) findViewById(R.id.resultTextView);
    }

private void setTextButton(String value) {
        textButton = textButton + value;
        textView.setText(textButton);
    }

public void sevenOnClick(View view) {
        setTextButton("7");
    }