Is there an beautiful way to assert pre-conditions in Java methods?

Question

A lot of my functions have a whole load of validation code just below the declarations:

if ( ! (start < end) ) {
    throw new IllegalStateException( "Start must be before end." );
    }

I'd like to precisly specify the valid ranges of certain inputs - for example a A > B, C => 1 or str_d.length() > 0.

Given that some of my functions have quite a lot of arguments which must be validated I can end up writing a lot of boiler-plate just to validate the pre-conditions. I'm writing a library which is mainly going to be used by non-technical developers, we've found that validating function inputs is the best way to help our users operate our API correctly. The sooner we raise an error the less work our customers will have to do.

Is there a more elegant method to specify the pre-conditions, post-condtions (and possibly the invariant conditions) in my methods.

A colleague told me about a feature of the Eiffel programming language which allows pre/post/invariant conditions to be described in very natural ways without repeating a lot of boilerplate code. Is there an add-on to the Java language which will allow me to use some of this magic?

Answer 1

Guava's Preconditions class is just for this. You typically use it with static imports, so your example would look like:

checkArgument(start < end, "Start must be before end");

It makes it easy to add more information to the message as well, without paying the cost of String concatenation if the check passes.

checkArgument(start < end, "Start (%s) must be before end (%s)", start, end);

Unlike assert statements, these can't be disabled.

Answer 2

Check out the Cofoja project which provides contracts for Java through annotations. It provides Pre-/Postconditions and Invariants. Also in contrast to other Java implementations it correctly handles contracts defined in parent classes/interfaces. Contract evaluation can be enabled/disabled at runtime.

Here is a code snippet from their tutorial:

import com.google.java.contract.Invariant;
import com.google.java.contract.Requires;

@Invariant("size() >= 0")
interface Stack<T> {
  public int size();

  @Requires("size() >= 1")
  public T pop();

  public void push(T obj);
}

Answer 3

How about assert start < end. Have a look at the documentation.