JDBC is an API for accessing data in relational databases such as Oracle, MySQL etc. from Java in a standard way using the SQL language. JDBC actually lets you access any tabular data sources like relational databases, spreadsheets or even flat files. You pass SQL to java methods in the JDBC classes and get back JDBC objects that represent the results of your query. JDBC is portable since Java is portable across platforms. JDBC also allow us to access an ODBC-based database using a JDBC-ODBC bridge.  

Inner class is a class within another class, method or block. An inner class can be one of the following four types: Anonymous, Local, Member and Nested top-level. 

Wrapper classes

The wrapper classes provide a mechanism to "wrap" primitive values in an object so that the primitives can be included in activities only for objects, like being added to Collections. There is a wrapper class for every primitive in Java. The wrapper class for int is Integer and the class for float is Float and so on. Wrapper classes also provide many utility functions for primitives like Integer.parseInt().

In object oriented programming or in Unified Modelling, relationships between objects can be classified as association, aggregation, composition and inheritance. 

Association

Association is a relationship between two objects. Here objects might not be completely dependent on each other.

Custom Exceptions are user defined Exception classes that extend either Exception class or RuntimeException class.