Java Notes:

JDK,JRE,JVM

diff b/w jdk,jvm,jre

JDK: Java Development Kit

- Compiler: To create .class(byte codes) from the .java file
- Debugger:
package of Tools to develop java programs

JRE: Java Runtime Environment

- Java class libraries
- ClassLoader and Byte code verification
package of Tools to run java programs
loads compiled code to the memory and connects the code to the appropriate java class libraries

JVM: Java Virtual Machine

Java Interpreter : To run java programs
JVM converts the bytecode into machine code.
JVM is platform independent as JVM doesn’t depend on the hardware and operating system of the machine.

Serialization:

Marker Interface:

Serializable is a marker interface (has no data member and method).
It is used to "mark" Java classes so that the objects of these classes may get a certain capability.
The Cloneable and Remote are also Marker interfaces.
It is used to deliver type information at runtime to the JVM so that it can take some action based on the information received

Serialization and DeSerialization:

The Serializable interface must be implemented by the class whose object needs to be persisted.
The String class and all the wrapper classes implement the java.io.Serializable interface by default.

ObjectOutputStream class

The ObjectOutputStream class is used to write primitive data types, and Java objects to an OutputStream. Only objects that support the java.io.Serializable interface can be written to streams.

ObjectInputStream class

An ObjectInputStream deserializes objects and primitive data written using an ObjectOutputStream.

serialization java program example

String in Java

String

String is immutable and so you have to assign again to store the result back to string variable after performing operations like strcmp,strcpy etc.,
Literals saved in StringPool in java heap memory.
JVM checks for each time when creating String object in memory(string pool).
String literal is available in (string pool)heap or not.If it is available it just returns object reference to increase performance.

String uses StringBuffer or StringBuilder to be mutable.

StringBuffer

StringBuffer is mutable,thread safe,poor performance.

StringBuilder

StringBuilder is mutable,good performance but not thread safe

string example program

Thread and Multithreading

Thread

Thread is a path for execution.

Thread States

Thread states: New,Runnable,Wait/Timely-wait/blocked,Terminated.

Multithreading

Multithreading refers to a process of executing two or more threads simultaneously for maximum utilization of the CPU.
A thread in Java is a lightweight process requiring fewer resources to create and share the process resources.

Synchronization

Synchronization is the process of controlling other thread access when execution of current thread.

thread example program

OOPS Concepts

JAVA

Java is a high-level, class-based,platform independent, object-oriented programming language that is designed to have as few implementation dependencies as possible.

Object

A Java object is a member (also called an instance) of a Java class.
Each object has an identity, a behavior and a state.
he state of an object is stored in fields (variables), while methods (functions) display the object's behavior.
Objects are created at runtime from templates, which are also known as classes

Class

A class — in the context of Java — is a template used to create objects and to define object data types and methods.
Classes are categories, and Objects are items within each category.

Eg:

Vehicle car;

Encapsulation

Encapsulation in Java is a powerful mechanism for storing the data members and data methods of a class together.
It is done in the form of a secure field accessible by only the members of the same class.

Encapsulation in Java is the process by which data (variables) and the code that acts upon them (methods) are integrated as a single unit.
By encapsulating a class's variables, other classes cannot access them, and only the methods of the class can access them.

Access Modifiers

public(visibility in entire env)
protected(visibility within package)
private(visibility within its class)

Uses:

Data hiding
Flexibility
Control and reusability
Security

Polymorphism

Polymorphism means "many forms", and it occurs when we have many classes that are related to each other by inheritance.

Overloading and Overriding

When the method signature (name and parameters) are the same in the superclass and the child class, it's called overriding.
When two or more methods in the same class have the same name but different parameters, it's called overloading.

Inheritance

Inheritance in Java is the method to create a hierarchy between classes by inheriting from other classes.
Java Inheritance is transitive -
so if Sedan extends Car and Car extends Vehicle, then Sedan is also inherited from the Vehicle class.
The Vehicle becomes the superclass of both Car and Sedan.

Single
Multilevel
Hierarchical
Multiple(Not possible with class to class - so Interface introduced)

Keywords

extends : class inherits class / interface inherits interface implements : class inherits interface / vice versa

Abstraction

Data abstraction is the process of hiding certain details and showing only essential information to the user.
Abstraction can be achieved with either abstract classes or interfaces.

The abstract keyword is a non-access modifier, used for classes and methods:

Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class).
An abstract class can have both abstract and regular methods
Abstract method: can only be used in an abstract class, and it does not have a body.
The body is provided by the subclass (inherited from).

Uses

Security

It can also acheives through interface.

Difference between Abstract class and Interface

An abstract class allows you to create functionality that subclasses can implement or override.
An interface only allows you to define functionality, not implement it but default method allows impl from java 8.

Non access Modifier Keywords

static

The static variable can be used to refer to the common property of all objects (which is not unique for each object),
for example, the company name of employees, college name of students, etc.
The static variable gets memory only once in the class area at the time of class loading.
Access directly by class no need to create instance/object.

final

The final keyword is a non-access modifier used for classes, attributes and methods, which makes them non-changeable (impossible to inherit or override)

PassByValue and PassByReference

PassByValue refers that the value of variable to be passed while method calling.
PassByReference refers that the reference(memory) of variable to be passed while method calling.
Java by default supports pass by value for primitive types whereas for objects it is pass by reference.