Java Scientific calculator GUI

I am working on a calculator with a GUI. I'm not sure how I should gather all the numbers from the user and do the math for them. I tried storing them in variables, but that doesn't really work since I need to be able to have multiple numbers and operators at one time. Any input would be greatly appreciated. Thanks!

import java.util.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class GUI extends JFrame implements ActionListener {
    private JFrame frame;
    private JPanel buttons;

    //
    private JPanel text;
    private JTextField field;

    //
    private ImageIcon WCU;
    private JLabel labelWCU;

    ///
    JButton[] nums = new JButton[10];
    JButton[] functions = new JButton[15];

    ArrayList<Double> Addition = new ArrayList<>();

    double A;
    double S;
    double M;
    double D;
    double Answer;

    private JButton num1, num2, num3, num4, num5, num6, num7, num8, num9, num0;
    private JButton decimal, add, subtract, multiply, divide, equals, squareRoot;
    private JButton sin, cos, tan, exclamation, oneOverX, xSquared, log;
    private JButton clear;

    double input1 = 0, input2 = 0, answer = 0;
    char op;

    public GUI() {
        frame = new JFrame();
        buttons = new JPanel();

        //
        text = new JPanel();
        field = new JTextField(30);

        //
        WCU = new ImageIcon("WCU.png");
        labelWCU = new JLabel(WCU);
        

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setTitle("Calculator"); // make sure to center
        frame.setSize(525,750);
        frame.setLocationRelativeTo(null);
        frame.setResizable(false);
        frame.setLayout(new BorderLayout());
        frame.add(text, BorderLayout.NORTH);
        frame.add(buttons, BorderLayout.CENTER);

        text.add(field);
        field.setEditable(false);
        text.setBackground(Color.RED);
        

        buttons.setBackground(Color.red);
        buttons.setBorder(BorderFactory.createEmptyBorder(20,50,300,50)); 
        buttons.setLayout(new GridLayout(6,4,20,20));

        //  
        buttons.add(decimal = new JButton ("."));
        functions[0] = decimal;

        buttons.add(add = new JButton ("+"));
        functions[1] = add;

        buttons.add(subtract = new JButton ("-"));
        functions[2] = subtract;

        buttons.add(multiply = new JButton ("*"));
        functions[3] = multiply;

        buttons.add(divide = new JButton ("/"));
        functions[4] = divide;

        buttons.add(equals = new JButton ("="));
        functions[5] = equals;

        buttons.add(squareRoot = new JButton ("√"));
        functions[6] = squareRoot;

        buttons.add(num1 = new JButton ("1"));
        nums[1] = num1;

        buttons.add(num2 = new JButton ("2"));
        nums[2] = num2;
        
        buttons.add(num3 = new JButton ("3"));
        nums[3] = num3;

        buttons.add(num4 = new JButton ("4"));
        nums[4] = num4;

        buttons.add(num5 = new JButton ("5"));
        nums[5] = num5;

        buttons.add(num6 = new JButton ("6"));
        nums[6] = num6;

        buttons.add(num7 = new JButton ("7"));
        nums[7] = num7;

        buttons.add(num8 = new JButton ("8"));
        nums[8] = num8;

        buttons.add(num9 = new JButton ("9"));
        nums[9] = num9;

        buttons.add(num0 = new JButton ("0"));
        nums[0] = num0;

        buttons.add(sin = new JButton ("sin"));
        functions[7] = sin;

        buttons.add(cos = new JButton ("cos"));
        functions[8] = cos;

        buttons.add(tan = new JButton ("tan"));
        functions[9] = tan;

        buttons.add(exclamation = new JButton ("!"));
        functions[10] = exclamation;

        buttons.add(oneOverX = new JButton ("1/x"));
        functions[11] = oneOverX;

        buttons.add(xSquared = new JButton ("x^2"));
        functions[12] = xSquared;

        buttons.add(log = new JButton ("log"));
        functions[13] = log;

        text.add(clear = new JButton("Clear"));
        functions[14] = clear;

        labelWCU.setIcon(WCU);

        frame.setVisible(true);

        //
        for (int i = 0; i < 15; i++){
            functions[i].addActionListener(this);
            functions[i].setFocusable(false);
        }

        for (int i = 0; i < 10; i++){
            nums[i].addActionListener(this);
            nums[i].setFocusable(false);
        }

    }
    
    public void actionPerformed(ActionEvent e){

        for (int i = 0; i < 10; i++){
            if(e.getSource() == nums[i]){
                field.setText(field.getText().concat(String.valueOf(i)));
            }
        }

        if(e.getSource() == decimal){
            field.setText(field.getText().concat("."));
        }

        if(e.getSource() == add){
            A = Double.parseDouble(field.getText());
            op = '+';
            field.setText("");
        }

        if(e.getSource() == subtract){
            input1 = Double.parseDouble(field.getText());
            op = '-';
            field.setText("");
        }

        if(e.getSource() == multiply){
            input1 = Double.parseDouble(field.getText());
            op = '*';
            field.setText("");
        }

        if(e.getSource() == divide){
            input1 = Double.parseDouble(field.getText());
            op = '/';
            field.setText("");
        }

        if(e.getSource() == equals){
            
            field.setText(String.valueOf(Addition));
        }

        if(e.getSource() == clear){
            field.setText("");
            A = 0;
        }

    }
}