Calculator: can’t have two operators in a row

I can’t seem to figure out how to stop a user from typing two operators in a row. For example: 4+5**, the user shouldn’t be able to write two operators in a single row. I tried setEnabled to disable the operators when an operator is used but this does not work. Can someone give me tips on how I can fix this issue. Thanks.

https://i.stack.imgur.com/xRA84.png

public class Rekenmachine extends AppCompatActivity {

/**
 * Declare variabels
 */
ImageButton ibtnDelete;

TextView tvResult, tvCalculation;

Button btnOne, btnTwo, btnThree, btnFour, btnFive, btnSix, btnSeven, btnEight, btnNine, btnZero,
        btnPlus, btnMinus, btnMultiply, btnDivide, btnPoint, btnEvaluate;

String calculation;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_rekenmachine);
    Toast.makeText(this, "Hold cross to clear", LENGTH_SHORT).show();

    /**
     * Assign variables
     */

    ibtnDelete = (ImageButton) findViewById(R.id.imb_verwijderen);

    tvCalculation = (TextView) findViewById(R.id.tv_calculation);
    tvResult = (TextView) findViewById(R.id.tv_result);

    btnOne = (Button) findViewById(R.id.btn_een);
    btnTwo = (Button) findViewById(R.id.btn_twee);
    btnThree = (Button) findViewById(R.id.btn_drie);
    btnFour = (Button) findViewById(R.id.btn_vier);
    btnFive = (Button) findViewById(R.id.btn_vijf);
    btnSix = (Button) findViewById(R.id.btn_zes);
    btnSeven = (Button) findViewById(R.id.btn_zeven);
    btnEight = (Button) findViewById(R.id.btn_acht);
    btnNine = (Button) findViewById(R.id.btn_negen);
    btnZero = (Button) findViewById(R.id.btn_nul);

    btnPlus = (Button) findViewById(R.id.btn_operator_plus);
    btnPlus.setEnabled(true);
    btnMinus = (Button) findViewById(R.id.btn_operator_min);
    btnMultiply = (Button) findViewById(R.id.btn_operator_maal);
    btnDivide = (Button) findViewById(R.id.btn_operator_delen);

    btnPoint = (Button) findViewById(R.id.btn_punt);

    btnEvaluate = (Button) findViewById(R.id.btn_operator_gelijkaan);


    /**
     * Deze is voor clear
     */
    ibtnDelete.setOnLongClickListener(new View.OnLongClickListener() {
        @Override
        public boolean onLongClick(View view) {
            //When clicked put calculation tv and result tv to " "
            tvCalculation.setText("");
            tvResult.setText("");
            return true;
        }
    });

    /**
     * Dit is voor 1 per 1 te verwijderen
     */
    ibtnDelete.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            //In calculation worden alle ingevoerde dingen bewaard
            calculation = tvCalculation.getText().toString();
            //Alleen als calculation langer is dan 0 tekens
            //Zonder if statement crasht de app als je <leeg> verwijdert
            if (calculation.length() > 0) {
                //Met substring worden alle getallen "geselecteerd" en de laatste getal wordt met "-1" verwijdert
                calculation = calculation.substring(0, calculation.length() - 1);
                //Hier wordt "calculation" als text gezet met 1 getal of teken minder
                tvCalculation.setText(calculation);
            } else {
                //als tvCalculation 1 teken heeft maak tvCalculation leeg
                tvCalculation.setText("");
            }
        }
    });

    /**
     * Set onClick on the number buttons
     *
     * Men drukt de toetsen "12546" in. Alles wat dan in tvCalculation staat
     * wordt bewaard in de variable "calculation". Daarna wordt als je "btnOne" drukt "calculation + 1" gedaan
     */
    btnOne.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "1");
        }
    });

    btnTwo.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "2");
        }
    });

    btnThree.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "3");
        }
    });

    btnFour.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "4");
        }
    });

    btnFive.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "5");
        }
    });

    btnSix.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "6");
        }
    });

    btnSeven.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "7");
        }
    });

    btnEight.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "8");
        }
    });

    btnNine.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "9");
        }
    });

    btnZero.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "0");
        }
    });

    /**
     * Set onClick on the operator buttons
     */
    btnPlus.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            //This should disable the plus button when clicked
            Character lastChar = calculation.charAt(calculation.length() - 1);
            if (lastChar.toString() == "+") {
                btnPlus.setEnabled(false);
            } else {
                //when the last character of calculation isn't an operator, add "+"
                tvCalculation.setText(calculation + "+");
            }
        }
    });

    btnMinus.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "-");
        }
    });

    btnMultiply.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "*");
        }
    });

    btnDivide.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + "/");
        }
    });

    btnPoint.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation = tvCalculation.getText().toString();
            tvCalculation.setText(calculation + ".");
        }
    });

    /**
     * Set onClick on the evaluate button
     */
    btnEvaluate.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            //calculation = calculation.replaceAll("X", "*");
            calculation = tvCalculation.getText().toString();

            Context rhino = Context.enter();
            rhino.setOptimizationLevel(-1);

            String result = "";

            try {
                Scriptable scope = rhino.initStandardObjects();
                result = rhino.evaluateString(scope, calculation, "JavaScript", 1, null).toString();
            } catch (Exception e) {
                result = "Error";
            }

            tvResult.setText(result);
        }
    });
}

}