Core Java Interview Question part-2

Q) Access Specifiers & Access modifiers?

Access Specifiers à A.S gives access privileges to outside of application (or) others, they are Public, Protected, Private, Defaults. Access Modifiers  à A.M which gives additional meaning to data, methods and classes, final cannot be modified at any point of time.

	Private	Public	Protected	No modifier
Same class	No	Yes	Yes	Yes
Same package Subclass	No	Yes	Yes	Yes
Same package non-subclass	No	Yes	Yes	Yes
Different package subclass	No	Yes	Yes	No
Different package non-subclass	No	Yes	No	No

Q) Default Values

           

long	-2^63 to 2^63 –1 à 0L	double	0.0d
Int	-2^31 to 2^31 –1 à 0	float	0.0f
Short	-2^15 to 2^15 –1 à 0	Boolean	false
Byte	-2^7 to 2^7 –1     à 0	char	0 to 2^7 –1 à null character (or) ‘\u 0000’

    

Q) Byte code  & JIT compiler & JVM & JRE & JDK

à Byte code is a highly optimized set of instructions. JVM is an interpreter for byte code. Translating a java   program into byte code helps makes it much easier to run a program in a wide variety of environment.

à JVM is an interpreter for byte code

à JIT (Just In Time) is a part of JVM, it compiles byte code into executable code in real time, will increase the performance of the interpretations.

à JRE is an implementation of the Java Virtual Machine, which actually executes Java programs.

à JDK is bundle of software that you can use to develop Java based software, Tools provided by JDK is

(i) javac – compiler        (ii) java – interpretor       (iii) jdb – debugger        (iv) javap - Disassembles

(v) appletviewer – Applets          (vi) javadoc - documentation generator   (vii) javah - 'C' header file generator

Q) Wrapper classes

Primitive data types can be converted into objects by using wrapper classes. These are java.lang.package.

Q) Does Java pass method arguments by value or by reference?

Java passes all arguments by value, not by reference

Q) Arguments & Parameters

While defining method, variable passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.

Q) Public static void main (String [] args)

à We can overLoad the main () method.

à What if the main method is declared as “Private”?

    The program compiles properly but at runtime it will give "Main method not public." Message

à What if the static modifier is removed from the signature of the main method?

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

à We can write “static public void” instead of “public static void” but not “public void static”.

à Protected static void main (), static void main (), private static void main () are also valid.

à If I do not provide the String array as the argument to the method?

    Program compiles but throws a runtime error "NoSuchMethodError".

à If no arguments on the command line, String array of Main method will be empty or null?

    It is empty. But not null.

à Variables can have the same name as a method or a class

Q) Can an application have multiple classes having main() method?

A) 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.