Microsoft and Java are involved in a long-running legal dispute over Sun's Java Virtual Machine (JVM), a piece of software that is needed for a computer to run software written in Java.
The Java language, developed by Sun in 1995, is popular with internet-based applications and allows programmers to develop applications compatible with many types of computers, regardless of the operating system they run.
The dispute started when Microsoft, which has developed its own competing language and virtual machine, decided to drop JVM from its Windows XP product.
In its lawsuit, Sun claims that Microsoft has also forced developers to distribute products incompatible with Java. As a result, according to Sun, developers are now turning to Microsoft's .Net platform.
Together with the lawsuit, Sun asked the court to issue a temporary order requiring Microsoft to ship JVM with each copy of Windows XP and Internet Explorer, claiming that, if it waited until the case was heard, it would be left far behind in the competition. The court granted the order two days before Christmas, reasoning that Microsoft had gained an unfair advantage.
Microsoft appealed the order and earlier this month, a federal appeals court agreed to temporarily suspend the order, until the appeal itself is heard.
As part of its defence, Microsoft filed counterclaims on Friday in the US District Court for the District of Maryland, where the antitrust case is pending. In its defence, the software giant alleged that Sun had violated a settlement reached in 2001 between the two companies over Java.
That settlement ended a previous legal dispute dating back to 1997, when Sun accused Microsoft of violating its licensing agreement by distributing incompatible versions of Java whilst deceptively presenting them as compatible.
The two companies settled that lawsuit in January 2001, with Microsoft agreeing to pay Sun $20 million for the past use of the technology in dispute. Microsoft also agreed to stop using Sun's "Java Compatible" trade mark.
Sun, on the other hand, agreed to grant Microsoft a limited licence to continue shipping certain products that included Java for seven years.
Microsoft is making three counterclaims: breach of contract over the 2001 settlement, a breach of good faith and violation of California's competition laws.
The software giant claimed in its filing: "Sun did not seek to promote competition or consumer benefit, but to obtain a free ride on Microsoft's success." The company adds that Sun's actions "were undertaken wilfully and deliberately with an intent to cause competitive injury to Microsoft and to aid competitors."
Sun, on the other hand, has reportedly said that it will bring evidence proving its claims, which will "answer Microsoft's counterclaims in trial."
Microsoft's counterclaims filing is available here