MainActivity

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;


public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private EditText editTextInput;
    private TextView textViewOutput,textViewLog;
    private final String  sAdd = "+",sSub="-",sDiv="/",sMul="X";
    private Button calcButton,clearButton;
    private Button addButton,subButton,divButton,mulButton;
    Boolean bOperSel = false;
    Boolean bJustCalc = false; //Set to true just after a calculation has occurred
    enum currentOperSelected
    {
        NONE,
        DIV,
        MUL,
        ADD,
        SUB
    }
    currentOperSelected operator = currentOperSelected.NONE;
    List<Button> buttonList = new ArrayList<>(4);
    List<Double> calcList = new ArrayList<>();
    double dTotal =0.0;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //Input box
        editTextInput = findViewById(R.id.input_left);
        editTextInput.addTextChangedListener(inputTextWatcher);
        //Buttons
        calcButton = findViewById((R.id.calculate_button));
        calcButton.setOnClickListener(this);

        clearButton = findViewById(R.id.clear_button);
        clearButton.setOnClickListener(this);

        addButton = findViewById(R.id.addition_button);
        addButton.setOnClickListener((this));
        buttonList.add(addButton);

        subButton = findViewById(R.id.subtraction_button);
        subButton.setOnClickListener(this);
        buttonList.add(subButton);

        divButton = findViewById((R.id.division_button));
        divButton.setOnClickListener(this);
        buttonList.add(divButton);

        mulButton = findViewById((R.id.multiplication_button));
        mulButton.setOnClickListener(this);
        buttonList.add(mulButton);

        //Text View
        textViewOutput = findViewById(R.id.output);
        textViewLog = findViewById(R.id.calculate_log);
       setOperators(false);
       calcButton.setEnabled(false);
       clearButton.setEnabled(false);
    }
    public void onClick(View v) {
        switch (v.getId()) {
            case  R.id.addition_button: {
                add();
                break;
            }
            case R.id.subtraction_button: {
               subtract();
                break;
            }
            case R.id.division_button:{
                divide();
                break;
            }
            case R.id.multiplication_button:{
                multiply();
                break;
            }
            case R.id.calculate_button:{
                calculate();
                break;
            }
            case R.id.clear_button: {
                clear();
                break;
            }
        }
    }

    private TextWatcher inputTextWatcher = new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after)
        {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count)
        {
            if (count > 0 && !bOperSel) { setOperators(true); }
        }

        @Override
        public void afterTextChanged(Editable s) {

        }
    };
    void add()
    {
       operator = currentOperSelected.ADD;
       getInputFromUser(sAdd);
    }

    void subtract()
    {
        operator = currentOperSelected.SUB;
        getInputFromUser(sSub);
    }

    void multiply()
    {
       operator = currentOperSelected.MUL;
       getInputFromUser(sMul);
    }

    void divide()
    {
       operator = currentOperSelected.DIV;

       getInputFromUser(sDiv);
    }
    void calculate()
    {
        getInputFromUser("");
        if (operator == currentOperSelected.DIV)
        {
            if (calcList.size() > 2)
            {
                dTotal /=  calcList.get(calcList.size()-1);
            }
            else {
                dTotal = calcList.get(0) / calcList.get(1);
            }
        }
        else if (operator == currentOperSelected.MUL)
        {
            if (calcList.size() > 2)
            {
                dTotal *=  calcList.get(calcList.size()-1);
            }
            else {
                dTotal = calcList.get(0) * calcList.get(1);
            }
        }
        else if (operator == currentOperSelected.ADD)
        {
            if (calcList.size() > 2)
            {
                dTotal +=  calcList.get(calcList.size()-1);
            }
            else {
                dTotal = calcList.get(0) + calcList.get(1);
            }
        }
        else if (operator == currentOperSelected.SUB)
        {
            if (calcList.size() > 2)
            {
                dTotal -=  calcList.get(calcList.size()-1);
            }
            else {
                dTotal = calcList.get(0) - calcList.get(1);
            }
        }
        if (calcList.size() > 2)
        {
          clearButton.setEnabled(true);
        }
        setOperators(true);
        bJustCalc = true;
        editTextInput.setEnabled(false);
        String sTotal = Double.toString(dTotal);
        textViewOutput.setText(sTotal);
        operator = currentOperSelected.NONE;
    }

    void setOperators(Boolean bVal)
    {
      for (Button b: buttonList)
      {
          b.setEnabled(bVal);
      }
      if (bVal)
      {
        calcButton.setEnabled(false);
      }
      else {
          calcButton.setEnabled(true);
      }

    }
    void clear()
    {
      setOperators(false);
      dTotal = 0.0;
      operator = currentOperSelected.NONE;
      calcButton.setEnabled(false);
      calcList.clear();
      bOperSel = false;
      bJustCalc = false;
      textViewOutput.setText("");
      textViewLog.setText("");
      editTextInput.setText("");
      if (!editTextInput.isEnabled()) {
            editTextInput.setEnabled(true);
        }
    }

  void getInputFromUser(String sOperator)
  {
      if (!editTextInput.isEnabled()){
          editTextInput.setEnabled(true);
      }
      if (!bJustCalc)
      {
          String sUserInput = editTextInput.getText().toString();
          double val = Double.parseDouble(sUserInput);
          textViewLog.setText(textViewLog.getText()+sUserInput);
          calcList.add(val);
          editTextInput.setText("");
      }
      textViewLog.setText(textViewLog.getText()+sOperator);
      bOperSel = true;
      setOperators(false);
      bJustCalc = false;

  }



}