A Google engineer, testifying in a high-stakes trial pitting business software company Oracle against search giant Google, denied that he referred to Oracle or any other company when he wrote in an email that Google should take a licence to use the Java programming language.

Another Google engineer specialising in the Java programming language said that he had copied from Java code into the Android system, and suggested that software interfaces known as APIs are designed – which, with Oracle trying to prove that they can be copyrighted, might be an important piece of evidence.

The trial entered its fourth day on Thursday, with Google engineer Timothy Lindholm – author of a key email whose admissibility the two sides fought over bitterly during the pre-trial phase – taking the stand to answer questions. The focus was on an email written in 2010 that has become a critical piece of evidence in the case.

Oracle sued Google in August 2010, saying Google's Android mobile operating system infringes its copyrights and patents for the Java programming language. Google countered that it does not violate Oracle's patents and that Oracle cannot copyright certain parts of Java, an "open-source," or publicly available, software language.

During opening statements, an Oracle attorney displayed several Google emails to the jury, calling them prime evidence that Google took its intellectual property.

One of them involved Lindholm, a former Sun Microsystems employee who began work at Google in 2005. Shortly before Oracle sued Google in 2010, Lindholm wrote an email to Android chief Andy Rubin, saying he had been asked by top Google executives to investigate technical alternatives to Java for Android.

"We've been over a bunch of these, and think they all suck," Lindholm wrote. "We conclude that we need to negotiate a license for Java under the terms we need."

In court on Thursday, Lindholm acknowledged that he wrote the email. However, he told Oracle attorney David Boies that in the phrases used he was not referring to a license from Sun. "It was not specifically a licence from anybody," Lindholm said.

Lindholm was also asked about the importance of APIs – application programming interfaces – which Oracle says are covered by copyright.

"As a software engineer, not a lawyer, it's always been my understanding the organisation of software APIs are free for use by other people," he said.

He then told Google attorney Christa Anderson during cross-examination that he understood the software over which Oracle is claiming copyright to be free for use by other people.

Earlier in the day, Google engineer Josh Bloch admitted to Oracle's lawyer that he had copied some Java code into Android while at Google. He also said that API design is a noble and rewarding craft - a statement that may become critical with Oracle attempting to prove that APIs attract copyright.

Early in the case, estimates of potential damages against Google ran as high as $6.1bn. But the company has narrowed Oracle's claims to only two patents from seven originally, reducing the possible award. Oracle is seeking roughly $1bn in copyright damages.