Download com.r3.corda.notary.plugin.contractverifying.notary-plugin-contract-verifying-api : notary-plugin-contract-verifying-api.corda.cpk JAR file - All Versions:
Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window notary-plugin-contract-verifying-api.corda.cpk-5.2.0.0.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.