Features Of java : Some features include Object Oriented, Platform Independent, Robust, Interpreted, Multi-threaded
JVM : JVM is an acronym for Java Virtual Machine, it is an abstract machine which provides the run time environment in which java byte code can be executed. It is a specification. JVMs are available for many hardware and software platforms (so JVM is platform dependent).
JRE : JRE stands for Java Run time Environment. It is the implementation of JVM.
JDK : JDK is an acronym for Java Development Kit. It physically exists. It contains JRE + development tools.
Memory Areas by JVM :
- Class(Method) Area
- Heap
- Stack
- Program Counter Register
- Native Method Stack
Architectural Neutral : It’s compiler generates an architecture-neutral object file format, which makes the compiled code to be executable on many processors, with the presence of Java run time system.
Just-In-Time(JIT) compiler : It is used to improve the performance. JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.
Object : Object is a run time entity and it’s state is stored in fields and behavior is shown via methods. Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication.
Class : A class is a blue print from which individual objects are created. A class can contain fields and methods to describe the behavior of an object.
platform : A platform is basically the hardware or software environment in which a program runs. There are two types of platforms software-based and hardware-based. Java provides software-based platform.
classloader : The classloader is a subsystem of JVM that is used to load classes and interfaces.There are many types of classloaders e.g. Bootstrap classloader, Extension classloader, System classloader, Plugin classloader etc.
Local Variable : Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and it will be destroyed when the method has completed.
Instance Variable : Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded.
Class Variable : These are variables declared with in a class, outside any method, with the static keyword.
Singleton class : Singleton class control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes.
constructor : Constructor is just like a method that is used to initialize the state of an object. It is invoked at the time of object creation.
purpose of default constructor : The default constructor provides the default values to the objects. The java compiler creates a default constructor only if there is no constructor in the class.
static variable : static variable is used to refer the common property of all objects (that is not unique for each object) e.g. company name of employees,college name of students etc.
static variable gets memory only once in class area at the time of class loading.
static method : A static method belongs to the class rather than object of a class.
A static method can be invoked without the need for creating an instance of a class.
static method can access static data member and can change the value of it.
static block : Is used to initialize the static data member. It is executed before main method at the time of classloading.
this in java : It is a keyword that that refers to the current object.
Inheritance : Inheritance is a mechanism in which one object acquires all the properties and behaviour of another object of another class. It represents IS-A relationship. It is used for Code Resusability and Method Overriding.
composition : Holding the reference of the other class within some other class is known as composition.
super in java : It is a keyword that refers to the immediate parent class object.
protected access modifier : Variables, methods and constructors which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected members' class.
synchronized Non Access Modifier : Java provides these modifiers for providing functionalities other than Access Modifiers, synchronized used to indicate that a method can be accessed by only one thread at a time.
Why is String class considered immutable?
The String class is immutable, so that once it is created a String object cannot be changed. Since String is immutable it can safely be shared between many threads ,which is considered very important for multithreaded programming.
finalize() method : It is possible to define a method that will be called just before an object's final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.
Exception : An exception is a problem that arises during the execution of a program. Exceptions are caught by handlers positioned along the thread's method invocation stack.
Checked Exceptions : It is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
Runtime Exceptions : It is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation.
Polymorphism : Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.
method overloading : If a class have multiple methods by same name but different parameters, it is known as Method Overloading. It increases the readability of the program.
method overriding : If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding. It is used for runtime polymorphism and to provide the specific implementation of the method.
final variable : If you make any variable as final, you cannot change the value of final variable(It will be constant).
abstraction : Abstraction is a process of hiding the implementation details and showing only functionality to the user.
abstraction and encapsulation : Abstraction hides the implementation details whereas encapsulation wraps code and data into a single unit.
abstract class : A class that is declared as abstract is known as abstract class. It needs to be extended and its method implemented. It cannot be instantiated.
interface : Interface is a blueprint of a class that have static constants and abstract methods.It can be used to achieve fully abstraction and multiple inheritance.
package : A package is a group of similar type of classes interfaces and sub-packages. It provides access protection and removes naming collision.
Trows : If a method does not handle a checked exception, the method must declare it using the throwskeyword. The throws keyword appears at the end of a method's signature.
Throw : An exception can be thrown, either a newly instantiated one or an exception that you just caught, by using throw keyword.
Encapsulation : It is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. Therefore encapsulation is also referred to as data hiding.
applet : An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.
immutable object : An immutable object can’t be changed once it is created.
JAR file : JAR files is Java Archive fles and it aggregates many files into one. It holds Java classes in a library. JAR files are built on ZIP file format and have .jar file extension.
WAR file : This is Web Archive File and used to store XML, java classes, and JavaServer pages. which is used to distribute a collection of JavaServer Pages, Java Servlets, Java classes, XML files, static Web pages etc.
JIT compiler : It improves the runtime performance of computer programs based on bytecode.
NullPointerException : A NullPointerException is thrown when calling the instance method of a null object, accessing or modifying the field of a null object etc.
Serialization and deserialization : Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
synchronization : Synchronization is the capability to control the access of multiple threads to shared resources. synchronized keyword in java provides locking which ensures mutual exclusive access of shared resource and prevent data race.
primitive Java types : The eight primitive types are byte, char, short, int, long, float, double, and boolean.
dot operator : The dot operator(.) is used to access the instance variables and methods of class objects.It is also used to access classes and sub-packages from a package.
enumeration : An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It allows sequential access to all the elements stored in the collection.
Exception Handling : Exception Handling is a mechanism to handle runtime errors.It is mainly used to handle checked exceptions.
finally block : finally block is a block that is always executed.
nested class : A class which is declared inside another class is known as nested class. There are 4 types of nested class member inner class, local inner class, annonymous inner class and static nested class.
I/O filter : An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
wrapper classes : Wrapper classes are classes that allow primitive types to be accessed as objects.
multithreading : Multithreading is a process of executing multiple threads simultaneously. Its main advantage is:
Threads share the same address space.
Thread is lightweight.
Cost of communication between process is low.
thread : A thread is a lightweight subprocess.It is a separate path of execution.It is called separate path of execution because each thread runs in a separate stack frame.
deadlock : Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource which is held by the other waiting thread.
Iterator : Iterator can traverse legacy and non-legacy elements. Iterator is fail-fast. Iterator is slower than Enumeration.
ListIterator : ListIterator traverses the elements in backward and forward directions both. ListIterator can be used in List only.
ArrayList : ArrayList uses a dynamic array. ArrayList is not efficient for manipulation because a lot of shifting is required. ArrayList is better to store and fetch data. ArrayList is not synchronized. ArrayList is not a legacy class. ArrayList increases its size by 50% of the array size.
Vector : Vector is synchronized. Vector is a legacy class. Vector increases its size by doubling the array size.
LinkedList : LinkedList uses doubly linked list. LinkedList is efficient for manipulation. LinkedList is better to manipulate data.
difference between HashSet and TreeSet: HashSet maintains no order whereas TreeSet maintains ascending order.
difference between Set and Map : Set contains values only whereas Map contains key and values both.
difference between HashSet and HashMap : HashSet contains only values whereas HashMap contains entry(key,value). HashSet can be iterated but HashMap need to convert into Set to be iterated.
difference between HashMap and TreeMap : HashMap maintains no order but TreeMap maintains ascending order.
HashMap : HashMap is not synchronized. HashMap can contain one null key and multiple null values.
Hashtable :Hashtable is synchronized. Hashtable cannot contain any null key or null value.
Downcasting : It is the casting from a general to a more specific type, i.e. casting down the hierarchy.
difference between Path and Classpath : Path and Classpath are operating system level environment variales. Path is defines where the system can find the executables(.exe) files and classpath is used to specify the location of .class files.
Externalizable interface : Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.
Socket : Sockets provide the communication mechanism between two computers using TCP. A client program creates a socket on its end of the communication and attempts to connect that socket to a server.
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