Java Questions

There will always be a default constructor in every class 

The synchronized keyword indicates that a method can be accessed by only one thread at a time

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

Platform independence means that we can write and compile the java code in one platform (Eg Windows) and can execute the class in any other supported platform (Eg Linux,Solaris,etc).

Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. For example, converting an int to an Integer, a double to a Double, and so on. If the conversion goes the other way, this is called unboxing. 

PATH and CLASSPATH are operating system level environment variables. PATH is used to define where the system can find the executables (.exe) files whereas CLASSPATH is used to specify the location of .class files.

No. The JDBC-ODBC Bridge does not support multi threading. The JDBC-ODBC Bridge uses synchronized methods to serialize all of the calls that it makes to ODBC. Multi-threaded Java programs may use the Bridge, but they won't get the advantages of multi-threading.