Java Fundamentals

Sun Microsystems

Simple, Object-Oriented, Robust & Architecture-Neutral

The source code of Java (.java file(s)) is compiled by the Java compiler into a bytecode (.class file(s)). This bytecode can run on any OS, but JRE(Java Runtime Environment) must be installed on the OS. This JRE is different for different OS such as Linux, Windows or Mac, but the bytecode generated will be same for all OS. So, JRE is architecture dependent but the bytecode is architecture neutral, i.e. will be same for every machine or OS.

 A Java compiler generated intermediate code, which can execute on any OS, regardless of its OS.

JVM: It is an abbreviation for Java Virtual Machine, it is an abstract machine (a software) which provides the runtime environment in which java bytecode can be executed. It is a specification. JVMs are available for many hardware and software platforms (so JVM is platform dependent/architecture dependent).
JRE: It stands for Java Runtime Environment. It is the implementation of JVM.
JDK: It is an acronym for Java Development Kit. It is basically a complete set of development tools necessary for developing & executing Java programs. It contains JRE in itself.

Just-In-Time(JIT) compiler: It is used to improve the performance. JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

JVM calls the main method in Java. Main method in Java is the starting point of standalone core Java Application. JVM starts executing main method and the thread which executes main is called main thread in Java.

public is an access modifier. static is a keyword which suggests only one copy will be maintained and the method would be called with class name and not with its object. void indicates return type, it returns nothing main is the name of the method String args[] represents an array of string type, named args

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

The program compiles properly but at runtime it will give "main() method not public." message.

Program compiles. But at runtime throws an error "NoSuchMethodError".

It is empty. But not null.

CLASSPATH and PATH are the two variables.

Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.

No the program fails to compile. The compiler says that the main() method is already defined in the class.

Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main() method.

No. It is by default loaded internally by the JVM. Though if you import, it doesn't result in any error.

Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serialized.

No, there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.

Object class, in package java.lang

Java uses primitive data types and hence is not a pure object oriented language.