Question Java

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

public class Script
{
	private final static ScriptEngine JavaScript = new ScriptEngineManager().getEngineByName("JavaScript");
	
	public static double eval(String str,String[] var,double...val)
	{
		for (int i=0; i<var.length && i<val.length; i++)
			str = str.replaceAll(var[i], String.valueOf(val[i]));
		
		try 
		{
			return (Double) JavaScript.eval(str);
		} 
		catch (Exception e) 
		{
			System.err.println(e);
			return Double.NaN;
		}
	}
	
	public static void main(String...args)
	{
		String str = "5*x-2*y";
		String[] var = {"x","y"};
		
		System.out.println(eval(str,var,1,1)); // 3.0
		System.out.println(eval(str,var,1,2)); // 1.0
		System.out.println(eval(str,var,2,1)); // 8.0
		System.out.println(eval(str,var,2,2)); // 6.0
	}
}