Assertions are often used to test the assumptions that developers have about a program. An assertion contains a boolean expression which developers believe to be true at a particular program point. It throws an error if the expression is not satisfied, which helps developers to detect and correct bugs. Since assertions make developer assumptions explicit, assertions are also believed to improve under-standability of code. Recently, Casalnuovo et al. analyse C and C++ programs to understand the relationship between assertion usage and defect occurrence. Their results show that asserts have a small effect on reducing the density of bugs and developers often add asserts to methods they have prior knowledge of and larger ownership. In this study, we perform a partial replication of the above study on a large dataset of Java projects from GitHub (185 projects, 20 million LOC, 4 million commits, 0.2 million files and 1 million methods). We collect metrics such as number of asserts, number of defects, number of developers and number of lines changed to a method, and examine the relationship between asserts and defect occurrence. We also analyse relationship between developer experience and ownership and the number of asserts. Furthermore, we perform a study of what are different types of asserts added and why they are added by developers. We find that asserts have a small yet significant relationship with defect occurrence and developers who have added asserts to methods often have higher ownership of and experience with the methods than developers who did not add asserts.