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Traditional code metrics (product and process metrics) have been widely used in defect prediction. However, these metrics have an inherent limitation: they do not reveal system traits that are tied to certain building blocks of a given programming language. Taking these building blocks of a programming language into account can lead to further insights about a software system and improve defect prediction. To fill this gap, this paper reports an empirical study on the usage of knowledge units (KUs) of the Java programming language. A KU is a cohesive set of key capabilities that are offered by one or more building blocks of a given programming language. This study aims to understand whether we can obtain richer results in defect prediction when using KUs in combination with traditional code metrics. Using a defect dataset covering 28 releases of 8 Java systems, we analyze source code to extract both traditional code metrics and KU incidences. We find empirical evidence that KUs are different and complementary to traditional metrics, thus indeed offering a new lens through which software systems can be analyzed. We build a defect prediction model called KUCLS, which leverages the KU-based features. Our KUCLS achieves a median AUC of 0.82 and significantly outperforms the CC_PROD (model built with product metrics). The normalized AUC improvement of the KUCLS over CC_PROD ranges from 5.1% to 28.9% across the studied releases. Combining KUs with traditional metrics in KUCLS_CC further improves performance, with AUC gains of 4.9% to 33.3% over CC and 5.6% to 59.9% over KUCLS. Finally, we develop a cost-effective model that significantly outperforms the CC. These encouraging results can be helpful to researchers who wish to further study the aspect of feature engineering and building models for defect prediction.