Java Core

• How to make a class clonable
  • override clone method of Object.
  • implements Clonable interface.
• Immutability
  • the status of the object can not be changed once it has been initilized.
• How to make a class immutable
  • class should be final(no class can extend)
  • a field should mark with final, if field is public/have setter
  • if field
  • if there is collections, need to return new collections in getters
  • Make all mutable fields final so that it’s value can be assigned only once.
  • Initialize all the fields via a constructor performing deep copy.
• What is Garbage Collection? Explain how it is used. Is there a way to force garbage collection?
  • ref ++
  • java program stacks and one heap
  • garbage collection responsible for memory management
  • developer don’t need to explicitly allocate memory and dealocate space.
  • the memory space of objects that don’t refered by any varaible will be auto reclaim
  • eliminating memory leaks
• What is the difference between final, finally & finalize?
  • final is used to apply restrictions on class/method/variable. Final class can’t be inherited, final method can’t be overridden and final variable value can’t be changed.
  • finally in [try block], [close resources] code in the finally will be executed whether exception is handled or not.
  • finalize is used to perform clean up processing just before object is garbage collected. public void finalize(){ }
• What’s this and super?
  • super is used to access methods of the base class while this is used to access methods of the current class.
  • super(), it refers to constructor of the base class
  • this(), it refers to the constructor of the current class
• What’s a java package?
  • A package is a collection of related classes and interfaces providing access privileges and namespace management.
  • namespace management
  • diff class same purpose
• Explain the difference between the “equals ()” method and the “equals-equals (==) operator?
• When would you use an Interface instead of an abstract class?
• What does the “static” keyword do?
• What are exceptions? Discuss Exception Handling.
• What’s a constructor?
• What are annotations?

Java Core - Collections

• ?? how hashmap internally works??
  • hashmap use put and get
  • key object should be immutable
  • put: 1. hashcode on key object 2. hashcode passed hash function 3. according to hashvalue, the key value pairs will be stored in an entry.
• What is the difference between List, Set, & Map? ref
  • interfaces of Java collection framework
  • List provides ordered/indexed/duplicates collection of objs
  • Set provides unordered/unique collectin of objs
  • Map provides collection objs based on Key-Value pair and HASHing.
• Classes implements List interface. ??
• Classes implements Set interface. ??
• Classes implements Map interface. ??
  • HashTable, HashMap, TreeMap, LinkedHashMap, ConcurrentHashMap
• HashTable vs ConcurrentHashMap??
• What are generics/generic collections?
• HashMap: 2 object with same hashcode, how they stored in
  • https://stackoverflow.com/a/6493946

Java Core - Exception

• Explain try-catch, finally