Design by Contract (DbC) is a software development methodology that focuses on clearly defining the interfaces between components to produce better quality object-oriented software. The idea behind DbC is that a method defines a contract stating the requirements a client needs to fulfill to use it, the precondition, and the properties it ensures after its execution, the postcondition. Though there exists ample support for DbC for sequential programs, applying DbC to concurrent programs presents several challenges. Using Java as the target programming language, this paper tackles such challenges by augmenting the Java Modelling Language (JML) and modifying the JML compiler to generate Runtime Assertion Checking (RAC) code to support DbC in concurrent programs. We applied our solution in a carefully designed case study on a highly concurrent industrial software system from the telecommunications domain to assess the effectiveness of contracts as test oracles in detecting and diagnosing functional faults in concurrent software. Based on these results, clear and objective requirements are defined for contracts to be effective test oracles for concurrent programs whilst balancing the effort to design them. Main results include that contracts of a realistic level of completeness and complexity can detect around 76% of faults and reduce the diagnosis effort for such faults by at least ten times. We, therefore, show that DbC can not only be applied to concurrent software but can also be a valuable tool to improve the economics of software engineering.