用Java内置的JS引擎实现Infix Expression Evaluation

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class hello {
public static void main(String [] args) throws ScriptException {
ScriptEngineManager factory = new ScriptEngineManager();
ScriptEngine engine = factory.getEngineByName("JavaScript");
System.out.println(engine.eval("123+456"));
}
}

System.out.println(new ScriptEngineManager().getEngineByName("JavaScript").eval(scanner.nextLine()).toString());