Getting start Java8 OOP with Calculator

Calculator.java

/*
Getting start Java8 OOP with Calculator
The MIT License (MIT)

Copyright (c) Bsidesoft co.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
 */
package lesson1;

import java.io.InputStream;
import java.util.*;
import java.util.function.BiFunction;
import java.util.stream.Stream;

public class Calculator {
    static class Expression {
        static private Map<String, BiFunction<Integer, Integer, Integer>> op = new HashMap<>();
        static {
            op.put("+", (l, r) -> l + r);
            op.put("-", (l, r) -> l - r);
            op.put("*", (l, r) -> l * r);
            op.put("/", (l, r) -> l / r);
        }

        public Expression() {}
        public String calc(List<String> que) {
            final List<String> ex = new ArrayList<>();
            Scanner scanner = new Scanner(
                    String.join("", que.toArray(new String[0]))
                            .replaceAll("([+*/\\-])", " $1 ")
            );
            while (scanner.hasNext()) ex.add(scanner.next());
            Stream.of("[*/],[+\\-]".split(",")).forEach(rex -> {
                int i = 0;
                while (i < ex.size()) {
                    String c = ex.get(i);
                    if (c.matches(rex)) {
                        int ret = op.get(c).apply(
                                Integer.parseInt(ex.get(i - 1)),
                                Integer.parseInt(ex.get(i + 1))
                        );
                        ex.set(i - 1, Integer.toString(ret));
                        ex.remove(i);
                        ex.remove(i);
                        i--;
                    }
                    i++;
                }
            });
            return ex.get(0);
        }
    }

    static class Panel {
        private ArrayList<String> que = new ArrayList<>();
        private Expression ex;

        public Panel(Expression ex) {this.ex = ex;}
        public void add(String v) {
            que.add(v);
            render();
        }
        public void result() {
            String result = ex.calc(que);
            que.clear();
            add(result);
        }
        public void clear() {
            que.clear();
            render();
        }
        private void render() {
            System.out.println(String.join("", que.toArray(new String[0])));
        }
    }

    static abstract class KeyPad {
        protected Panel panel;
        protected String label;

        public KeyPad(Panel p, String l) {
            panel = p;
            label = l;
        }
        abstract public void click();
    }

    static class InputKey extends KeyPad {
        public InputKey(Panel panel, String label) {
            super(panel, label);
        }
        @Override
        public void click() {
            panel.add(label);
        }
    }

    static class ResultKey extends KeyPad {
        public ResultKey(Panel panel, String label) {
            super(panel, label);
        }
        @Override
        public void click() {
            panel.result();
        }
    }

    static class ClearKey extends KeyPad {
        public ClearKey(Panel panel, String label) {
            super(panel, label);
        }
        @Override
        public void click() {
            panel.clear();
        }
    }

    public static void main(String[] a) {
        new Calculator().on(System.in);
    }
    private Panel panel = new Panel(new Expression());
    private Map<String, KeyPad> keys = new HashMap<>();

    public Calculator() {
        Stream.of("0123456789+-/*".split("")).forEach(
                s -> keys.put(s, new InputKey(panel, s))
        );
        keys.put("=", new ResultKey(panel, "="));
        keys.put("c", new ClearKey(panel, "c"));
    }
    public void on(InputStream src) {
        Scanner in = new Scanner(src).useDelimiter("\\s*");
        exit:
        while (true) {
            in.reset();
            while (in.hasNext()) {
                String c = in.next();
                if (!keys.containsKey(c) || "exit".equals(c)) break exit;
                keys.get(c).click();
            }
        }
    }
}