Java Basics

Varargs (Variable number of arguments)

  • Method that took arbitrary number of argument, varargs features automate and hide the process
  • It is upward compatible with preexisting APIs
  • String.format(String format, Object… args)


  • Automating process to cast between primitive value and appropriate wrapper class

Final (JAVA)

  • Final Classes
    • Final class can not be subclassed
    • Grant security and efficiency benefits
    • Many of JAVA standard libraries are final
    • Method inside final class implicitly work like final method
  • Final Methods
    • Cannot be overridden or hidden by subclasses
    • Prevent unexpected behaviour of subclass
    • Final method has nothing to do with efficiency or performance issue
  • Final Variables
    • Can only be initialized once
      • via initializer
      • via constructor
    • Blank final instance variable must be definitely assigned at the end of every constructor

Controlling Access to Members of a Class



  • Form of syntactic metadata that can be added java code.
    • Classes
    • Methods
    • Variables
    • Parameters
    • Packages


  • Document generator from Oracle

Thread Safety

  • A code is thread safe if it only manipulates shared data structure in a manner that guarantees safe execution by multiple thread at the same time
  • Thread Safe
    • Implementation is guaranteed to be free of race conditions when accessed by multiple thread simultaneously
  • Conditionally Safe
    • Different thread can access different objects simultaneously, and access of shared data is protected from race condition
  • Not Thread Safe
    • Code should not accessed simultaneously by different thread

Race Condition

  • A behaviour of electronic, software or other system where the output is dependent on the sequence or timing of other uncontrollable events
  • Two signal racing each other to influence the output first
  • Critical race condition often happens when processes or threads depend on the shared state

Dead Lock

  • A situation in which two or more competing actions are each waiting for the other to finish, and thus neither ever does

Difference Between Atomic, Volatile, Synchronize


Synchronized Method

  • It is not possible for two invocations of synchronized methods on the same object to interleave.
  • Changes of the state of the objects are visible to all threads

Thread Pooling

  • Creating number of threads to perform number of tasks.
  • When a thread done a task, it request next task until all the tasks are done.
  • After all the tasks are done, thread can be either terminated or wait for new tasks.


  • When variable's value will modified by different threads
  • Access of the variable acts as though it is enclosed in synchronized block (synchronised on itself)

Volatile in Java 5

  • Accessing volatile variable creates memory barrier
  • Synchronize all cached copies of variables with main memory


  • int, long, double is not atomic and all other primitive variables are already atomic

The volatile keyword is used:

  • to make non atomic 64-bit operations atomic: long and double. (all other, primitive accesses are already guaranteed to be atomic!)
  • to make variable updates guaranteed to be seen by other threads + visibility effects: after writing to a volatile variable, all the variables that where visible before writing that variable become visible to another thread after reading the same volatile variable (happen-before ordering).


  • member variable not to be serialized when it persist to stream of bytes
  • Not transferred over network, lost intentionally


  • The native keyword is applied to a method to indicate that the method is implemented in native code using JNI(Java Native Interface).

Generic programming

  • Style of computer programming in which algorithms are written in terms of types to-be-specified-later that are then instantiated when needed for specific types provided as parameters

Covariant return type

  • Covariant return type of a method is one that can be replaced by a "narrower" type when the method is overridden in a subclass

Private Class v/s Package-Private Class


If we can have inner private class then why can't we have outer private class...?

You can, the distinction is that the inner class is at the "class" access level, whereas the "outer" class is at the "package" access level. From the [Oracle Tutorials][1]:

If a class has no modifier (the default, also known as package-private), it is visible only within its own package (packages are named groups of related classes — you will learn about them in a later lesson.)

Thus, package-private (declaring no modifier) is the effect you would expect from declaring an "outer" class private, the syntax is just different.


Name     : Leonardo Taehwan Kim
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