Why is this an issue?

Nested code - blocks of code inside blocks of code - is eventually necessary, but increases complexity. This is why keeping the code as flat as possible, by avoiding unnecessary nesting, is considered a good practice.

Merging if statements when possible will decrease the nesting of the code and improve its readability.

Code like

if (condition1) {
  if (condition2) {             // Noncompliant
    /* ... */
  }
}

Will be more readable as

if (condition1 && condition2) { // Compliant
  /* ... */
}

How to fix it

If merging the conditions seems to result in a more complex code, extracting the condition or part of it in a named function or variable is a better approach to fix readability.

Code examples

Noncompliant code example

if (file != null) {
  if (file.isFile() || file.isDirectory()) {  // Noncompliant
    /* ... */
  }
}

Compliant solution

if (file != null && isFileOrDirectory(file)) { // Compliant
  /* ... */
}

private static boolean isFileOrDirectory(File file) {
  return file.isFile() || file.isDirectory();
}