Why is JaCoCo not covering my String switch statements?

Question

I have a switch statement that extracts an addressing mode from a String and I've written unit tests to cover, what I thought was every eventuality but JaCoCo seems to skip my switch statements, resulting in lower coverage.

Why, if all my case statements, including a default are being executed in tests, would the switch statement not be counted as hit?

Answer 1

For the switch by String

class Fun  {
  static int fun(String s) {
    switch (s) {
      case "I":
        return 1;
      case "A":
        return 2;
      case "Z":
        return 3;
      case "ABS":
        return 4;
      case "IND":
        return 5;
      default:
        return 6;
    }
  }
}

Oracle Java compiler generates bytecode similar to the following code (Eclipse Compiler for Java generates slightly different bytecode)

    int c = -1;
    switch (s.hashCode()) {
      case 65: // +1 branch
        if (s.equals("I")) // +2 branches
          c = 0;
        break;
      case 73: // +1 branch
        if (s.equals("A")) // +2 branches
          c = 1;
        break;
      case 90: // +1 branch
        if (s.equals("Z")) // +2 branches
          c = 2;
        break;
      case 64594: // +1 branch
        if (s.equals("ABS")) // +2 branches
          c = 3;
        break;
      case 72639: // +1 branch
        if (s.equals("IND")) // +2 branches
          c = 4;
        break;
      default: // +1 branch
    }
    switch (c) {
      case 0: // +1 branch
        return 1;
      case 1: // +1 branch
        return 2;
      case 2: // +1 branch
        return 3;
      case 3: // +1 branch
        return 4;
      case 4: // +1 branch
        return 5;
      default: // +1 branch
        return 6;
    }

So that original switch-statement with 6 cases is represented in bytecode by a switch with 6 cases for hashCode of String plus 5 if-statements plus another switch with 6 cases. To see this bytecode you can use javap -c.

JaCoCo performs analysis of bytecode and in versions lower than 0.8.0 has no filter for switch by string. Your tests cover cases, where conditions in if-statements evaluate to true, but not the cases where they evaluate to false. Personally I would advise to simply ignore missing cases, because the goal is not to test that compiler generates proper code, but to test that your application behaves correctly. But for a sake of completeness of this answer - here is tests that cover all bytecode branches:

import org.junit.Test;
import static org.junit.Assert.*;

public class FunTest {
  @Test
  public void test() {
    // original strings:
    assertEquals(1, Fun.fun("I"));
    assertEquals(2, Fun.fun("A"));
    assertEquals(3, Fun.fun("Z"));
    assertEquals(4, Fun.fun("ABS"));
    assertEquals(5, Fun.fun("IND"));

    // same hash codes, but different strings:
    assertEquals(6, Fun.fun("\0I"));
    assertEquals(6, Fun.fun("\0A"));
    assertEquals(6, Fun.fun("\0Z"));
    assertEquals(6, Fun.fun("\0ABS"));
    assertEquals(6, Fun.fun("\0IND"));

    // distinct hash code to cover default cases of switches
    assertEquals(6, Fun.fun(""));
  }
}

And report generated by JaCoCo 0.7.9 as a proof:

JaCoCo version 0.8.0 provides filters, including filter for bytecode that javac produces for switch by string. And so generates following report even without additional tests: