Why is this an issue?

When the same code is duplicated in two or more separate branches of a conditional, it can make the code harder to understand, maintain, and can potentially introduce bugs if one instance of the code is changed but others are not.

Having two clauses in a when statement or two branches in an if chain with the same implementation is at best duplicate code, and at worst a coding error.

if (a >= 0 && a < 10) {
  doFirstThing()
  doTheThing()
}
else if (a >= 10 && a < 20) {
  doTheOtherThing()
}
else if (a >= 20 && a < 50) { // Noncompliant; duplicates first condition
  doFirstThing()
  doTheThing()
}
else {
  doTheRest()
}

when (x) {
  1 -> {
    doFirstThing()
    doSomething()
  }
  2 -> doSomethingDifferent()
  3 -> { // Noncompliant; duplicates case 1's implementation
    doFirstThing()
    doSomething()
  }
  else -> doTheRest()
}

If the same logic is needed for both instances, then:

• in an if structure they should be combined

if ((a >= 0 && a < 10) || (a >= 20 && a < 50)) {
  doFirstThing()
  doTheThing()
}
else if (a >= 10 && a < 20) {
  doTheOtherThing()
}
else {
  doTheRest()
}

• for a when, the values should be put together in the branch expression list.

when (x) {
  1, 3 -> {
    doFirstThing()
    doSomething()
  }
  2 -> doSomethingDifferent()
  else -> doTheRest()
}

Exceptions

Blocks in an if chain or in a when branch that contain a single line of code are ignored.

if (a == 1) {
    doSomething()  //no issue, usually this is done on purpose to increase the readability
} else if (a == 2) {
    doSomethingElse()
} else {
    doSomething()
}

But this exception does not apply to if chains without else-s, or to when-es without else clauses when all branches have the same single line of code. In case of if chains with else-s, or of when-es with default clauses, rule {rule:kotlin:S3923} raises a bug.

if (a == 1) {
  doSomething()  // Noncompliant, this might have been done on purpose but probably not
} else if (a == 2) {
  doSomething()
}

Resources

Related rules

• {rule:kotlin:S3923} - All branches in a conditional structure should not have exactly the same implementation