PreconditionsExplained

Preconditions

Guava provides a number of precondition checking utilities. We strongly recommend importing these statically. (How to do this easily in Eclipse.)

Each method has three variants:

• No extra arguments. Any exceptions are thrown without error messages.
• An extra Object argument. Any exception is thrown with the error message object.toString().
• An extra String argument, with an arbitrary number of additional Object arguments. This behaves something like printf, but for GWT compatibility and efficiency, it only allows %s indicators. Example:

checkArgument(i >= 0, "Argument was %s but expected nonnegative", i);
checkArgument(i < j, "Expected i < j, but %s > %s", i, j);

Signature (not including extra args)	Description	Exception thrown on failure
checkArgument(boolean)	Checks that the boolean is true. Use for validating arguments to methods.	IllegalArgumentException
checkNotNull(T)	Checks that the value is not null. Returns the value directly, so you can use checkNotNull(value) inline.	NullPointerException
checkState(boolean)	Checks some state of the object, not dependent on the method arguments. For example, an Iterator might use this to check that next has been called before any call to remove.	IllegalStateException
checkElementIndex(int index, int size)	Checks that index is a valid element index into a list, string, or array with the specified size. An element index may range from 0 inclusive to size exclusive. You don't pass the list, string, or array directly; you just pass its size. Returns index. IndexOutOfBoundsException
checkPositionIndex(int index, int size)	Checks that index is a valid position index into a list, string, or array with the specified size. A position index may range from 0 inclusive to size inclusive. You don't pass the list, string, or array directly; you just pass its size. Returns index.	IndexOutOfBoundsException
checkPositionIndexes(int start, int end, int size)	Checks that [start, end) is a valid sub range of a list, string, or array with the specified size. Comes with its own error message.	IndexOutOfBoundsException

We preferred rolling our own preconditions checks over e.g. the comparable utilities from Apache Commons for a few reasons. Piotr Jagielski discusses why he prefers our utilities, but briefly:

• After static imports, the Guava methods are clear and unambiguous. checkNotNull makes it clear what is being done, and what exception will be thrown.
  
• checkNotNull returns its argument after validation, allowing simple one-liners in constructors: this.field = checkNotNull(field).
  
• Simple, varargs "printf-style" exception messages. (This advantage is also why we recommend continuing to use checkNotNull over Objects.requireNonNull introduced in JDK 7.)

We recommend that you split up preconditions into distinct lines, which can help you figure out which precondition failed while debugging. Additionally, you should provide helpful error messages, which is easier when each check is on its own line.