## 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 `cases` in a `switch` 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) {
      doFirstThing();
      doTheThing();  // Noncompliant; duplicates first condition
    }
    else {
      doTheRest();
    }

    switch (i) {
      case 1:
        doFirstThing();
        doSomething();
        break;
      case 2:
        doSomethingDifferent();
        break;
      case 3:  // Noncompliant; duplicates case 1's implementation
        doFirstThing();
        doSomething();
        break;
      default:
        doTheRest();
    }

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

- in an `if` chain they should be combined

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

- for a `switch`, one should fall through to the other

    switch (i) {
      case 1:
      case 3: // Compliant
        doFirstThing();
        doSomething();
        break;
      case 2:
        doSomethingDifferent();
        break;
      default:
        doTheRest();
    }

When all blocks are identical, either this rule will trigger if there is no default clause or rule {rule:java:S3923} will raise if there is a default clause.

### Exceptions

Unless all blocks are identical, blocks in an `if` chain that contain a single line of code are ignored. The same applies to blocks in a `switch` statement that contains a single line of code with or without a following `break`.

    if (a == 1) {
      doSomething();  // Compliant, usually this is done on purpose to increase the readability
    } else if (a == 2) {
      doSomethingElse();
    } else {
      doSomething();
    }

## Resources

### Related rules

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