corejava5

21. What is classloader?

The classloader is a subsystem of JVM which is used to load classes and interfaces.

 Types :-                                                                                                           

Bootstrap classloader                                                                                         

Extension classloader                                                                                        

System classloader                                                                                                    

Plugin classloader  etc.

22. How do you find memory usage from Java program and How much percent of the heap is used?

java.lang.Runtime class has all memory related methods to get free memory,total memory and maximum heap memory in Java.From that you can find out how many % of heap is used and how much is remaining.

Following methods can be used:                                             

Runtime.freeMemory() -returns amount of free memory (in bytes)     

Runtime.totalMemory()- returns total memory in (bytes)   

Runtime.maxMemory() -returns maximum memory (in bytes)

23. What's the difference between "a == b" and ‘a.equals(b)’?

The ‘a == b’ is used for object reference matching. If both a and b are an object then ‘a==b’ only return true if both are pointing to the same object in the heap memory.

The ‘a.equals(b)’ is used for logical mapping and to provide logical equality, an object has to override this method. It actually does object content matching.

24. Can you guarantee the garbage collection process? 
No, you cannot , but you can make a request using System.gc() or Runtime.gc() method.

25. What is the difference between Serial and Parallel Garbage Collector? 

  A serial collector is a default copying collector which uses only one GC thread for garbage collection while a parallel collector uses multiple GC threads for garbage collection.