Contracts for Java enables you to annotate your code with contracts in the form of preconditions, postconditions and invariants.

These contract annotations are

• easy to write and read,
• and checked at runtime.

Annotating code with contracts helps you:

• design,
• document,
• test, and
• debug

Here is a simple example of a Java class annotated with contracts:

interface Time {
  ...

  @Ensures({
    "result >= 0",
    "result <= 23"
  })
  int getHour();

  @Requires({
    "h >= 0",
    "h <= 23"
  })
  @Ensures("getHour() == h")
  void setHour(int h);

  ...
}

Here is what the contract for setHour regulates.

To use Cofoja you need to do the following two things:

• Download one of the JARs (Or grab an unofficial build; see the FAQ for more information)
• Read the README
• Add contracts to your classes

The following provide more information about Cofoja:

• Add support for contracts to your Ant project
• Quick reference
• Fine tune the runtime checking of contracts
• How to write good contracts
• Frequently asked questions
• Using Contracts with Eclipse (external link, thanks to Fabian Steeg)
• Using Contracts with IntelliJ IDEA (external link, java.dzone.com)