Simple arithmetic expression parser, can do 4 basic arithmetic actions +-*/

ExpressionNode.java

package com.company;

/**
 * Created by Сергій Шнуренко on 18.11.2015.
 */
public class ExpressionNode {
    private ExpressionNode arg1;
    private ExpressionNode arg2;
    private String value;

    public ExpressionNode(String value){

        int currentPriority=3;
        int lowestPriorityIndex=0;
        char[] charArray = value.toCharArray();
        for(int i=0;i<charArray.length;i++){
            switch (charArray[i]){
                case '+':
                    currentPriority = 1;
                    lowestPriorityIndex = i;
                    break;
                case '-':
                    currentPriority = 1;
                    lowestPriorityIndex = i;
                    break;
                case '*':
                    if(currentPriority>2) {
                        currentPriority = 2;
                        lowestPriorityIndex = i;
                    }
                    break;
                case '/':
                    if(currentPriority>2) {
                        currentPriority = 2;
                        lowestPriorityIndex = i;
                    }
                    break;
            }
        }
        if(lowestPriorityIndex==0){
            this.value = value;
            arg1 = null;
            arg2 = null;
        }else{
            this.value =String.valueOf(charArray[lowestPriorityIndex]);
            this.arg1 = new ExpressionNode(value.substring(0,lowestPriorityIndex));
            this.arg2 = new ExpressionNode(value.substring(lowestPriorityIndex+1,value.length()));
        }
    }

    public double getNumValue(){
        if (value.equals("+")){
            return arg1.getNumValue()+arg2.getNumValue();
        }else if(value.equals("-")){
            return arg1.getNumValue()-arg2.getNumValue();
        }else if(value.equals("*")){
            return arg1.getNumValue()*arg2.getNumValue();
        }else if(value.equals("/")){
            return arg1.getNumValue()/arg2.getNumValue();
        }else {
            return Double.parseDouble(value);
        }
    }

    public String toString(){
        if(arg1==null&&arg2==null)
            return value;
        else
            return arg1.toString()+value+arg2.toString();
    }


}

Main.java

package com.company;

public class Main {

    public static void main(String[] args) {
	    ExpressionNode head = new ExpressionNode("99*99-1");
        System.out.println(head.getNumValue());
    }
}