I saw that interview question online and gave it a try:
Implement a function that gets a string which represents an arithmethic expression: contains only numbers or the the operators: '+', '-', '*', '/'.
The function returns the calculation result of the arithmethic expression which appears in the string.
This need to be done in O(n) run-time (n is the length of the string) and O(1) extra space.

Example:

"1+2*3" -> 7               

I succeed in implementing this (based mainly on some code I found online), but I just have one problem: logic is too complicated. So when reviewing the code, please tell me how can I simplify the logic here.

Here is the code:

class ArithmethicalStatments {

  private static int applyOperator(char operand, int firstNum, int secondNum){
      switch (operand) {
        case '*':
            firstNum *= secondNum;
            break;
        case '/':
            firstNum /= secondNum;
            break;
        case '+':
            firstNum = secondNum;
            break;
        case '-':
            firstNum = -secondNum;
            break;
      }

      return firstNum;
  }

  public static int calc(String str){
    int sum = 0;
    int lastApplyOpRes = 0;
    int lastNum = 0;
    int len = str.length();
    char prevOperator = '+';

    for (int i = 0; i < len; i++) {
        char c = str.charAt(i);

        if (c >= '0' && c <= '9') {
            lastNum = lastNum * 10 + c - '0';
            continue;
        }

        lastApplyOpRes = applyOperator(prevOperator, lastApplyOpRes , lastNum);
        lastNum = 0;

        switch (c) {
            case '+':
            case '-':
                sum += lastApplyOpRes;
                lastApplyOpRes = 0;
                break;
        }

        prevOperator = c;
    }

    lastApplyOpRes = applyOperator(prevOperator, lastApplyOpRes , lastNum);

    return sum + lastApplyOpRes;
  }
}              

In case you want to modify the code- some Junit tests I wrote for your convenience:

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

class ArithmethicalStatmentsTest {

    @Test
    void testEmpty() {
        assertEquals(0, ArithmethicalStatments.calc(""));
    }

    @Test
    void testOneNum() {
        assertEquals(5, ArithmethicalStatments.calc("5"));
    }

    @Test
    void testPlus() {
        assertEquals(8, ArithmethicalStatments.calc("5+3"));
    }

    @Test
    void testMul() {
        assertEquals(15, ArithmethicalStatments.calc("5*3"));
    }

    @Test
    void testMulPlus() {
        assertEquals(8, ArithmethicalStatments.calc("2*3+2"));
    }

    @Test
    void testPlusMul() {
        assertEquals(11, ArithmethicalStatments.calc("2+3*3"));
    }

    @Test
    void testPlusMulPlus() {
        assertEquals(16, ArithmethicalStatments.calc("2+3+3*3+2"));
    }

    @Test
    void testMinus() {
        assertEquals(2, ArithmethicalStatments.calc("5-3"));
    }

    @Test
    void testDivision() {
        assertEquals(2, ArithmethicalStatments.calc("6/3"));
    }

    @Test
    void testMinusPlus() {
        assertEquals(1, ArithmethicalStatments.calc("2-3+2"));
    }

    @Test
    void testPlusDivide() {
        assertEquals(5, ArithmethicalStatments.calc("3+4/2"));
    }

    @Test
    void testAll() {
        assertEquals(6, ArithmethicalStatments.calc("2-3+3/3+2*3"));
    }

}