Java is a programming language and computing platform first released by Sun Microsystems in 1995. It has evolved from humble beginnings to power a large share of today’s digital world, by providing the reliable platform upon which many services and applications are built. New, innovative products and digital services designed for the future continue to rely on Java, as well.

JavaARchive files are a big glob of Java classes, images, audio, etc., compressed to make one simple, smaller file to ease Applet downloading. Normally when a browser encounters an applet, it goes and downloads all the files, images, audio, used by the Applet separately. This can lead to slower downloads.

Null interfaces act as markers..they just tell the compiler that the objects of this class need to be treated differently..some marker interfaces are : Serializable, Remote, Cloneable

It's a modifier. Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

The smallest unit of source code that can be compiled, i.e. a .java file.

String is a class, but not a wrapper class. Wrapper classes like (Integer) exist for each primitive type. They can be used to convert a primitive data value into an object, and viceversa.