执行sqrt函数时强制关闭

Question

我正在开发一个计算器应用程序，我已经完成了所有基本功能。唯一的问题是，当我使用平方根函数时，它会返回错误Unexpected: √。请帮忙。我附上了我认为导致问题的代码部分。

Calculator.java

public static double eval(final String str) {
    return new Object() {
        int pos = -1, ch;

        void nextChar() {
            ch = (++pos < str.length()) ? str.charAt(pos) : -1;
        }

        boolean eat(int charToEat) {
            while (ch == ' ') nextChar();
            if (ch == charToEat) {
                nextChar();
                return true;
            }
            return false;
        }

        double parse() {
            nextChar();
            double x = parseExpression();
            if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
            return x;
        }

        // Grammar:
        // expression = term | expression `+` term | expression `-` term
        // term = factor | term `*` factor | term `/` factor
        // factor = `+` factor | `-` factor | `(` expression `)`
        //        | number | functionName factor | factor `^` factor

        double parseExpression() {
            double x = parseTerm();
            for (;;) {
                if      (eat('+')) x += parseTerm(); // addition
                else if (eat('-')) x -= parseTerm(); // subtraction
                else return x;
            }
        }

        double parseTerm() {
            double x = parseFactor();
            for (;;) {
                if      (eat('x')) x *= parseFactor(); // multiplication
                else if (eat('÷')) x /= parseFactor(); // division
                else return x;
            }
        }

        double parseFactor() {
            if (eat('+')) return parseFactor(); // unary plus
            if (eat('-')) return -parseFactor(); // unary minus

            double x;
            int startPos = this.pos;
            if (eat('(')) { // parentheses
                x = parseExpression();
                eat(')');
            } else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
                while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
                x = Double.parseDouble(str.substring(startPos, this.pos));
            } else if (ch >= 'a' && ch <= 'z') { // functions
                while (ch >= 'a' && ch <= 'z') nextChar();
                String func = str.substring(startPos, this.pos);
                x = parseFactor();
                if (func.equals("√")) x = Math.sqrt(x);
                else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
                else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
                else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));
                else throw new RuntimeException("Unknown function: " + func);
            } else {
                throw new RuntimeException("Unexpected: " + (char)ch);
            }

            if (eat('^')) x = Math.pow(x, parseFactor()); // exponentiation

            return x;
        }
    }.parse();
}

logcat的

06-09 00:51:17.906 8128-8128/com.arc.lumen.calculator E/AndroidRuntime: FATAL EXCEPTION: main 
Process: com.arc.lumen.calculator, PID: 8128 java.lang.RuntimeException: Unexpected: √
at com.arc.lumen.calculator.ScientificCalculator$20.parseFactor(ScientificCalculator.java:287)
at com.arc.lumen.calculator.ScientificCalculator$20.parseTerm(ScientificCalculator.java:257)
at com.arc.lumen.calculator.ScientificCalculator$20.parseExpression(ScientificCalculator.java:248)
at com.arc.lumen.calculator.ScientificCalculator$20.parse(ScientificCalculator.java:236)
at com.arc.lumen.calculator.ScientificCalculator.eval(ScientificCalculator.java:294)
at com.arc.lumen.calculator.ScientificCalculator.getResult(ScientificCalculator.java:206)
at com.arc.lumen.calculator.ScientificCalculator.onClickEquals(ScientificCalculator.java:182)
at com.arc.lumen.calculator.ScientificCalculator$18.onClick(ScientificCalculator.java:166)
at android.view.View.performClick(View.java:4461)
at android.view.View$PerformClick.run(View.java:18525)
at android.os.Handler.handleCallback(Handler.java:733)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5118)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:794)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:610)
at dalvik.system.NativeStart.main(Native Method)