Evaluate a Math Expression given in string form without using API

Evaluate a Math Expression given in string form without using API

I want to evaluate an expression like -4-12-2*12-3-4*5 given in a String
form without bothering about about BODMAS and without using API as I am a
beginner and want to grasp the logic.
Given below is my unsuccessful attempt to this problem which, if you may
like, ignore and suggest appropriate logic.And of course your codes are
also welcome :-)
public class SolveExpression3 {
static String testcase1 = "-4-12-2*12-3-4*5";
public static void main(String args[]){
SolveExpression3 testInstance= new SolveExpression3();
int result = testInstance.solve(testcase1);
System.out.println("Result is : "+result);
}
public int solve(String str){
int sum = 1;
int num1 = 0;
int num2 = 0;
String num = "";
int len = str.length();
System.out.println(str);
for (int i = len-1 ; i >= 0; i--)
{
char ch = str.charAt(i);
if(ch == '*')
{
String s = "";
num1 = num2 = 0;
//to get the number on left of *
for (int j = i; j >= 0; j--)
{
char c = str.charAt(j);
if(c == '+' || c == '-' || j == 1)
{
num1 = stringToInt(s);
s = "";
break;
}
else
{
s = c + s;
}
}
//to get the number on right of *
for (int j = i; j <= len; j++)
{
char c = str.charAt(j);
if(c == '+' || c == '-' || j == len-1)
{
num2 = stringToInt(s);
s = "";
break;
}
else
{
s = c + s;
}
}
sum = sum + num1*num2;
}
else
{
num = ch + num;
}
}
len = str.length();
for (int i = len-1; i >= 0; i--)
{
char ch = str.charAt(i);
if(ch==' ')
{}
else if(ch=='+')
{
sum = sum + stringToInt(num);
num = "";
}
else if(ch=='-')
{
sum = sum - stringToInt(num);
num = "";
}
else
{
num = ch + num;
}
}
return sum;
}
public int stringToInt(String str)
{
int number=0;
for(int i = 0; i < str.length(); i++)
{
int num = str.charAt(i) - 48;
number = number*10+num;
}
return number;
}
}