What is difference between
JDK, JRE and JVM?
Java Development Kit (JDK) is for development purpose
and JVM is a part of it to execute the java programs.
JDK provides all the tools, executables and binaries
required to compile, debug and execute a Java Program. The execution part is
handled by JVM to provide machine independence.
JVM
is
responsible for converting byte code into machine readable code. JVM is not
platform independent, that’s why you have different JVM for different operating
systems
Java
Runtime Environment (JRE) is the implementation of JVM. JRE
consists of JVM and java binaries and other classes to execute any program
successfully. JRE doesn’t contain any development tools like java compiler,
debugger etc. If you want to execute any java program, you should have JRE
installed.
How many types of memory
areas are allocated by JVM?
Many
types:
- Class(Method) Area
- Heap
- Stack
- Program Counter
Register
- Native Method Stack
Internal Architecture of JVM
Let's understand
the internal architecture of JVM. It contains classloader, memory area,
execution engine etc.
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1) Classloader
Classloader is a subsystem of JVM that is used to load
class files.
2) Class(Method)
Area
Class(Method) Area stores per-class structures such as
the runtime constant pool, field and method data, the code for methods.
3) Heap
It is the runtime data area in which objects are
allocated.
4) Stack
Java Stack stores frames.It holds local variables
and partial results, and plays a part in method invocation and return.
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Each thread has a private JVM stack, created at the
same time as thread.
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A new frame is created each time a method is
invoked. A frame is destroyed when its method invocation completes.
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5) Program
Counter Register
PC (program counter) register. It contains the address
of the Java virtual machine instruction currently being executed.
6) Native
Method Stack
It contains all the native methods used in the
application.
7) Execution Engine
It contains:
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1) A virtual processor
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2) Interpreter: Read
bytecode stream then execute the instructions.
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3) 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.
What is classloader?
The classloader is a subsystem of JVM that is
used to load classes and interfaces.
Java
Classloader is the program that loads byte code program into memory when we
want to access any class.
There
are three types of built-in Class Loaders in Java:
· Bootstrap
Class Loader – It loads JDK internal classes,
typically loads rt.jar and other core classes.
·
Extensions
Class Loader – It loads classes from the JDK
extensions directory, usually $JAVA_HOME/lib/ext directory.
·
System
Class Loader – It loads classes from the current
classpath that can be set while invoking a program using -cp or -classpath
command line options
Oops Concepts
Oops
is an approach that provides a way of modularizing a program by creating
partitioned memory area for both data and methods that can be used as template
for creating copies of such modules on demand.
It simplifies the
software development and maintenance by providing some concepts:
Object
Any entity that has state and behavior
is known as an object. For example: chair, pen, table, keyboard, bike etc. It
can be physical and logical.
It is a bundle of related
variables and functions (also known methods).
Class
A
class is a prototype that defines the variables and the methods common to all
objects of a certain kind.
Inheritance
When
one object acquires all the properties and behaviours of parent object i.e.
known as inheritance. It provides code reusability. It is used to achieve
runtime polymorphism.
Polymorphism
When one
task is performed by different ways i.e. known as polymorphism.
In
java, we use method overloading and method overriding to achieve polymorphism.
Another
example can be to speak something e.g. cat speaks meaw, dog barks woof etc.
Abstraction
Hiding
internal details and showing functionality is known as
abstraction. For example: phone call, we don't know the internal processing.
In
java, we use abstract class and interface to achieve abstraction.
Encapsulation
Binding
(or wrapping) code and data together into a single unit is known as encapsulation. For example:
capsule, it is wrapped with different medicines.
A
java class is the example of encapsulation. Java bean is the fully encapsulated
class because all the data members are private here.
How to bring in
Encapsulation
1) Make the instance variables protected. 2) Create public accessor methods and use these methods from within the calling code. 3) Use the JavaBeans naming convention of getter and setter. Eg: getPropertyName, setPropertyName.
Difference Between Method
Overloading and Method Overriding in Java
1. When a class has more than one method with
same name but with different arguments, then we call it as method overloading. Whereas If subclass (child class) has the same
method as declared in the parent class, it is known as method
overriding.
Why Method Overloaing is not possible by changing the return type of
method?
In java, method overloading is not possible by changing the return type
of the method because there may occur ambiguity. Let's see how ambiguity may
occur:
because there was problem:
class Calculation3{
int sum(int a,int b){System.out.println(a+b);}
double sum(int a,int b){System.out.println(a+b);}
public static void main(String args[]){
Calculation3 obj=new Calculation3();
int result=obj.sum(20,20); //Compile Time Error
}
}
Output:
int result=obj.sum(20,20); //Here how can java determine which sum()
method should be called
Can
we overload main() method?
Yes, by method overloading. You can have any number of main methods in a
class by method overloading. Let's see the simple example:
class Overloading1{
public static void main(int a){
System.out.println(a);
}
public static void main(String args[]){
System.out.println("main() method invoked");
main(10);
}
}
Covariant Return Type
The covariant return type specifies that the return type may vary in the
same direction as the subclass.
Before Java5, it was not possible to override any method by changing the
return type. But now, since Java5, it is possible to override method by
changing the return type if subclass overrides any method whose return type is
Non-Primitive but it changes its return type to subclass type. Let's take a
simple example:
class A{
A get(){return this;}
}
class B1 extends A{
B1 get(){return this;}
void message(){System.out.println("welcome to covariant return type");}
public static void main(String args[]){
new B1().get().message();
}
}
As you can see in the
above example, the return type of the get() method of A class is A but the
return type of the get() method of B class is B. Both methods have different
return type but it is method overriding. This is known as covariant return
type.
Binding refers to the link between method call and method
definition. This picture clearly shows what is binding.
Static
Binding in Java:
Static binding is a binding which happens during compilation.
It is also called early binding because binding happens before
a program actually runs.
Method overloading is the best example of
static binding.
Private, static and final methods
show static binding. Because, they cannot be overridden.
Dynamic
Binding in Java:
Dynamic binding is a binding which happens during run
time. It is also called late binding because binding
happens when program actually is running.
Method overriding is the best example of
dynamic binding.
Other than private, static and final
methods show dynamic binding. Because, they can be overridden.
What is constructor?
Rules for
creating java constructor
There are basically two rules defined for the
constructor.
Every class
should have at least one constructor. If you don’t
write constructor for your class, compiler will give
default constructor. Default constructor is always public and it has no
arguments (No-Arg Constructor)
What is constructor
chaining?
Constructor
Chaining is a technique of calling another constructor from one
constructor. this() is used to call same
class constructor whereas super() is used to call super
class constructor.
class
SuperClass
{
public SuperClass(int i)
{
System.out.println("Super Class
Constructor");
}
}
class
SubClass extends SuperClass
{
public SubClass()
{
this(10); //Calling same class constructor
}
public SubClass(int i)
{
super(i); //Calling super class constructor
}
Difference between constructor
and method in java
There are many differences between constructors and
methods. They are given below.
Java static keyword
The static keyword in java
is used for memory management mainly. We can apply java static keyword with
variables, methods, blocks and nested class. The static keyword belongs to the
class than instance of the class.
The static can be:
What is static variable?
Advantage of static variable
It makes your program memory
efficient (i.e it saves memory).
Java static method
If you apply static keyword with any method,
it is known as static method.
Restrictions for static method
Why java main method is
static?
Because object is not required to call static
method if it were non-static method, jvm create object first then call main()
method that will lead the problem of extra memory allocation.
Java static block
Example:
class StaticBlock{
static{System.out.println("static block is invoked");}
public static void main(String args[]){
System.out.println("Hello main");
}
}
Can we execute a program
without main() method?
Yes, one
of the way is static block but in previous version of JDK not in JDK 1.7.
class A3{
static{
System.out.println("static block is invoked");
System.exit(0);
}
}
In JDK7
and above, output will be:
Output: Error:
Main method not found in class A3, please define the main method as:
public
static void main(String[] args)
What is difference between
static (class) method and instance method?
this keyword in java
There
can be a lot of usage of java this keyword. In java, this is a reference
variable that refers to the current object.
Usage of java this keyword
Here
is given the 6 usage of java this keyword.
super keyword in java
The super keyword
in java is a reference variable that is used to refer immediate parent class
object.
Whenever
you create the instance of subclass, an instance of parent class is created
implicitly i.e. referred by super reference variable.
Usage
of java super Keyword
Inheritance in Java
Inheritance
in java is a mechanism in which one object acquires all
the properties and behaviours of parent object.
Inheritance
represents the IS-A relationship, also known as parent-child relationship.
Why
use inheritance in java
On the basis of class, there can be three types of
inheritance in java: single, multilevel and hierarchical.
Why multiple inheritance is not supported in java?
To reduce the complexity and simplify the language,
multiple inheritance is not supported in java.
Consider a scenario where A, B and C are three
classes. The C class inherits A and B classes. If A and B classes have same
method and you call it from child class object, there will be ambiguity to call
method of A or B class.
Since compile time errors are better than runtime
errors, java renders compile time error if you inherit 2 classes. So whether
you have same method or different, there will be compile time error now.
class A{
void msg(){System.out.println("Hello");}
}
class B{
void msg(){System.out.println("Welcome");}
}
class C extends A,B{//suppose if it were
Public Static void main(String args[]){
C obj=new C();
obj.msg();//Now which msg() method would be invoked?
}
}
Output: Compile
Time Error
What is composition?
Holding
the reference of the other class within some other class is known as
composition.
Aggregation in Java
If a class have an entity reference, it is known as
Aggregation. Aggregation represents HAS-A relationship.
Consider a situation, Employee object contains many information’s
such as id, name, emailId etc. It contains one more object named address, which
contains its own information’s such as city, state, country, zipcode etc. as
given below.
class Employee{
int id;
String name;
Address address;//Address is a class
...
}
In such case, Employee has an entity reference
address, so relationship is Employee HAS-A address.
Object Cloning in Java
The object
cloning is a way to create exact copy of an object. For this purpose,
clone() method of Object class is used to clone an object.
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