如何计算字符串“3+6 = ”的值？

import javax.script.ScriptException;
import java.io.ByteArrayOutputStream;
import java.io.PrintStream;

public class Solution {
    public static TestString testString = new TestString();

    public static void main(String[] args) throws ScriptException {

        PrintStream consoleStream = System.out;
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        PrintStream stream = new PrintStream(outputStream);
        System.setOut(stream);

        testString.printSomething();
        String result = stream.toString();
        System.setOut(consoleStream);
         //需要计算result中3+6=的值，该如何计算
        System.out.println(result);
    }

    public static class TestString {
        public void printSomething() {
            System.out.println("3 + 6 =");
        }
    }
}