Java related Questions for Interview #Part 1

JVM (Java Virtual Machine)

As we all aware when we compile a Java file, output is not an ‘exe’ but it’s a ‘.class’ file. ‘.class’ file consists of Java byte codes which are understandable by JVM. Java Virtual Machine interprets the byte code into the machine code depending upon the underlying operating system and hardware combination. It is responsible for all the things like garbage collection, array bounds checking, etc… JVM is platform dependent.

JRE (Java Runtime Environment)

Java Runtime Environment contains JVM, class libraries, and other supporting files. It does not contain any development tools such as compiler, debugger, etc. Actually JVM runs the program, and it uses the class libraries, and other supporting files provided in JRE. If you want to run any java program, you need to have JRE installed in the system.

JDK (Java Development Kit)

Java Development Kit is a superset of the JRE, and contains everything that is in the JRE, plus tools such as the compilers and debuggers necessary for developing applets and applications.

jdk2

Throws vs Throw

  1. Throws clause in used to declare an exception and throw keyword is used to throw an exception explicitly.
  2. If we see syntax wise then throw is followed by an instance variable and throws is followed by exception class names.
  3. The keyword throw is used inside method body to invoke an exception and throws clause is used in method declaration (signature).

List VS Set

1) Fundamental difference between List and Set in Java is allowing duplicate elements. List in Java allows duplicates while Set doesn’t allow any duplicate. If you insert duplicate in Set it will replace the older value. Any implementation of Set in Java will only contains unique elements.

2) Another significant difference between List and Set in Java is order. List is an Ordered Collection while Set is an unordered Collection. List maintains insertion order of elements, means any element which is inserted before will go on lower index than any element which is inserted after. Set in Java doesn’t  maintain any order. Though Set provide another alternative called SortedSet which can store Set elements in specific Sorting order defined by Comparable and Comparator methods of Objects stored in Set.

3) Set uses equals() method to check uniqueness of elements stored in Set, while SortedSet uses compareTo() method to implement natural sorting order of elements. In order for an element to behave properly in Set and SortedSet, equals and compareTo must be consistent to each other.

4) Popular implementation of List interface in Java includes ArrayList, Vector and LinkedList. While popular implementation of Set interface includes HashSet, TreeSet and LinkedHashSet.

Set doesn’t allow duplicates, but Why?

Internally SET store element using HASHTABLE, which is a structure of Key value pairs.Here what the values passed by the SET is treated as Keys of HASHTABLE Internally. Keys are unique cannot be duplicated. That is the reason if you pass any duplicate value it return false and does not added to the SET

String Pool

String Pool is a pool of Strings stored in Java heap memory. We know that String is special class in java and we can create String object using new operator as well as providing values in double quotes.
When we use double quotes to create a String, it first looks for String with same value in the String pool, if found it just returns the reference else it creates a new String in the pool and then returns the reference.
String Pool is possible only because String is immutable in Java.

Heap Memory

Heap Memory, which is the storage for Java objects. Non-Heap Memory, which is used by Java to store loaded classes and other meta-data. JVM code itself, JVM internal structures, loaded profiler agent code and data, etc.

References

http://www.javatpoint.com/difference-between-jdk-jre-and-jvm

Thanks 🙂

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