rspec/rules/S2251/java/rule.adoc

== Why is this an issue?

A `for` loop with a counter moving away from the end of the specified range is likely a programming mistake.

If the intention is to iterate over the specified range, this differs from what the loop does because the counter moves in the wrong direction.

If the intention is to have an infinite loop or a loop terminated only by a break statement, there are two problems:

1. The loop condition is not infinite because the counter variable will eventually overflow and fulfill the condition. This can take a long time, depending on the data type of the counter.
2. An infinite loop terminated by a `break` statement should be implemented using a `while` or `do` `while` loop to make the developer's intention clear to the reader.

== How to fix it

=== Code examples

==== Noncompliant code example

Change the direction of the counter.

[source,java,diff-id=1,diff-type=noncompliant]
----
for (int i = 10; i > 0; i++) { // Noncompliant, wrong direction
  System.out.println("Hello, world!") // executed ca. 2 billion times
}
----

[source,java,diff-id=2,diff-type=noncompliant]
----
public void doSomething(String [] strings) {
  for (int i = 0; i < strings.length; i--) { // Noncompliant, wrong direction
    String string = strings[i];  // ArrayIndexOutOfBoundsException when i reaches -1
    // ...
  }
}
----

==== Compliant solution

[source,java,diff-id=1,diff-type=compliant]
----
for (int i = 10; i > 0; i--) { // Compliant
  System.out.println("Hello, world!") // executed 10 times
}
----

[source,java,diff-id=2,diff-type=compliant]
----
public void doSomething(String [] strings) {
  for (int i = 0; i < strings.length; i++) { // Compliant
    String string = strings[i];
    // ...
  }
}
----

==== Noncompliant code example

If the intention is to have an infinite loop or a loop terminated only by a break statement, use a `while` or a `do` `while` statement instead.

[source,java,diff-id=3,diff-type=noncompliant]
----
for (int i = 0; i < 0; i++) { // Noncompliant, loop is not infinite
  String event = waitForNextEvent();
  if (event == "terminate") break;
  processEvent(event);
}
----

==== Compliant solution

[source,java,diff-id=3,diff-type=compliant]
----
while (true) { // Compliant
  String event = waitForNextEvent();
  if (event == "terminate") break;
  processEvent(event);
}
----






== Resources

=== Documentation

* https://wiki.sei.cmu.edu/confluence/x/aTdGBQ[CERT, MSC54-J.] - Avoid inadvertent wrapping of loop counters

=== Articles & blog posts

* https://en.wikipedia.org/wiki/Integer_overflow[Wikipedia - Integer overflow]


include::../rspecator.adoc[]