JAVA

• JVM- It stands for Java Virtual Machine. JVM is a kind of abstract machine that gives an idea of the time during which the byte code of java can be carried out. Loading a code, verifying the code, then executing the code and finally providing the runtime environment for the code are the four main tasks that JVM performs. JVM is dependent on many software and hardware platforms.

• JDK- It stands for Java Development Kit. It has physical existence and it contains JRE (Java Runtime Environment) and development tools like javac etc.

• JRE- It stands for Java Runtime Environment. JRE usually gives an idea of the java runtime environment. Some set of libraries (rt.jar) and some files that are used by JVM during the run time usually make up a JRE. JRE is basically an application of JVM and it also has physical existence.

Modifiers

JVM

Interpreter: takes only one instruction at a time for executionJust-in-time: takes a block of code at once and compile it before execute. so has plenty room for optimization

Rules for Method Overriding

• The argument list should be exactly the same as that of the overridden method.The return type should be the same or a subtype of the return type declared in the original overridden method in the superclass.The access level cannot be more restrictive than the overridden method's access level. For example: If the superclass method is declared public then the overridding method in the sub class cannot be either private or protected.Instance methods can be overridden only if they are inherited by the subclass.A method declared final cannot be overridden.A method declared static cannot be overridden but can be re-declared.If a method cannot be inherited, then it cannot be overridden.A subclass within the same package as the instance's superclass can override any superclass method that is not declared private or final.A subclass in a different package can only override the non-final methods declared public or protected.An overriding method can throw any uncheck exceptions, regardless of whether the overridden method throws exceptions or not. However, the overriding method should not throw checked exceptions that are new or broader than the ones declared by the overridden method. The overriding method can throw narrower or fewer exceptions than the overridden method.Constructors cannot be overridden.

For overloaded

1. Same Method Name

2. Same Return Type 

3. Different patameters

4. Different access modifiers

5. Different Exceptions

For overriding

1. Same Method Name

2. Same Return Type   if different then "The return type is incompatible with <ParentClass>.<ParentMethod>method()"

3. Same parameters  if different then "The method bmethod(int) of type A must override or implement a supertype method"

4. same or greater access modifiers then "Cannot reduce the visibility of the inherited method from B"

5. same or Different Exceptions means child Exception


1. Opps
Encapsulation
Abstraction
Polymorphism
overriding
overloading
Inheritance
2. JVM
GIT vs Interpiter
Memory Locations
Class Loaders
Execution Engine
3. GC
Ways to eligible for GC
4. Collections
Queues vs Topic
Concurent Map / list/ set

5. Threads
Thread Local
Thread Pool
How many ways to create thread
Syncronized thread
Syncronized static thread
callable
Thread LifeCycle
wait
notify
notifyAll
Sleep
yeild
6. Exception Handling
7. Java version topics
java 1.5
Enum
Generic
ForEach
Queue
Annotations
Autoboxing Unboxing
Var args
Static imports
Covatiant Return types
java 1.6
Navigable Set/Map
java 1.7
Enhancements in HashMap
try with resources
Try with multiple catch
Switch can take string args
java 1.8
Lambda Expressions
Streams
Functional Interface
Interface changes with default / static methods
Java DateTime API(joda.org ==> joda api)
Method Reference / constructor refence by :: operator
predicate,  function, consumer ==> predefined functional interfaces

+------------------------+-------------------------------+----------------+
| Feature | Lombok | Records |
+------------------------+-------------------------------+----------------+
| Immutability | No | Yes |
| Extensibility | Yes | No |
| Boilerplate code | Reduces | Reduces |
| Readability | Can be more difficult to read | Easier to read |
| Robustness | Less robust | More robust |
| Third-Party dependency | Yes | No |
| IDE compatibility | Not easy | Easy |
+------------------------+-------------------------------+----------------+

Important JVM Parameters
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Enabling JMX

-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=9999 -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false

Control Memory Usage

-Xms512m -Xmx1200m -Xss256k -XX:PermSize=256m -XX:MaxPermSize=512m

Large heaps

if 4G is max RAM then reduce the heap so with assumption that u have 4G RAM set the following:
-verbose:gc -Xss256k -Xmn512m -Xms3g -Xmx3g -XX:TargetSurviorRatio=80 -XX:+PrintGCTimeStamps -XX:+PrintGCDetails -XX:+PrintGCApplicationStoppedTime -XX:+HeapDumpOnOutOfMemoryError -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.port=3002

256K thread stack size optimization is related to the L2 processor cache size of 256K

ms : Initial heap memory ,  mx : max heap memory ,   ss: thread stack size (useful when you have lots of threads) , permsize : heap memory used for short lived objects , it is better to collect objects in the young generation instead of making them go through a old generation collection

Enable GC Statistics

-verbose:gc
-XX:-HeapDumpOnOutOfMemoryError
-XX:-PrintGCDetails
-XX:-PrintGCTimeStamps
-XX:-TraceClassUnloading   <-   useful if u need to trace classes getting unloaded due to memeory pressures

Other GC Optimizations

-Dsun.rmi.dgc.server.gcInterval=900000 -Dsun.rmi.dgc.client.gcInterval=900000	Trigger a GC every 15 minutes instead of the default  1 hour in  JDK 6
-server	JVM in server mode
-XX:+UseCompressedOops	on 64 bit JVMs memory is managed through 32 bit offsets.
-XX:+OptimizeStringConcat	Useful in tomcat which has lots of string manipulations
-XX:+DoEscapeAnalysis	Helps compiler figuring out if synchonized block can be eliminate as in local variables. Also figures out if object can be allocated on stack