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Here is the list of Java Design Patterns. I am putting all together for now, will try to add explanation and code samples shortly -

Creational Patterns

• Abstract Factory
• Builder
• Factory
• Prototype
• Singleton

Structural Patterns

• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

Behavioral Patterns

• Chain of Responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• State
• Strategy
• Template Method
• Visitor

Java Design Patterns, Java

Threads and Processes helps in concurrent execution of any application.

Process:

• an architectural construct : can affect the architecture of an application.
• processes are independend execution units
• they have their own state, address space
• use interprocess communication to communicate between threads

Threads:

• coding construct : doesn’t affect the architecture.
• a single process might contain multiple threds
• all threds within a process share process’s address space and state
• threads can communicate with each other directly

Java process, threads

Q. What is System.out.println?

• System = final System class
• out          =  static PrintStream object
• println = method of a printstrem class

Q. What is difference between abstract class and interfaces.

• abstract class can provide implementation, interface has no implementation at all
• used using extends, used using implements
• both can not be instantiated

Read more…

Java core java, interview

Q: What are basic OOPS Concepts?

A: - Inheritance : When a child class acquires all the methods and properties of parent class it is called inheritance. Java supports inheritance either using classes or interfaces.

Encapsulation: In above class, we have declared id as private and defined two methods that will act as mutator and accessors for this id propert. Thus we are encapsulateing objects property and  methods with in an object, keeping it safe from outside world.

Polymorphism: In below scenario, we have added 2 declaration of a function add() to child class, one is for adding  integers and another is to add float values. This kind of behavior when same name is used to perform different tasks as per the parameters passed to it is called polymorphism.

class Parent
{
	private int id;
	public Parent()
	{
	   aMethod();
	}
 
	public void aMethod()
	{
	   System.out.println("I am in Parent");
	}
 
	public int getId()
	{
	  return this.id;
	}
 
	public void setId(int id)
	{
	  this.id = id;
	}
}
 
class Child extends Parent
{
	public Child()
	{
	  bMethod();
	}     	
 
	public void bMethod()
	{
	  System.out.println("I am in Child");
	}
 
	public void add(int a, int b)
	{
	}
 
	public void add(float a, float b)
	{
	}
}

In java polymorphism is achieved by function overloading. It is also called Compile time Polymorphism. Compile decides at compile time that which version of function is called depending up on the parameters passed.

function overriding is also type of polymorphism, some times it is called Run time polymorphism, because the methods version is decided depending on the reference type of object at run time.

Important Notes:

public void aMethod(int a, long b)
{
   System.out.println("int a, long b");
}
public void aMethod(long a, int b)
{
   System.out.println("long a, int b");
}

For above methods to be polymorphic you need to use them as -

someobject.aMethod(1,2L);
someobject.aMethod(1L,2);
//someobject.aMethod(1,2); //compile time error
//someobject.aMethod(2,1); //compile time error

Java encapsulation, inheritance, oops, polymorphism, runtime polymorphism

Lets see how we can sort our Employee objects using different criteria like id, name, salary.

SortTest remains the main class that will perform creating and filling objects for us. This time employee class is plain old java classs that holds some data about each employee we create.

Here I have created scenarios or you can say strategies OR classes for sorting as SortById,  SortByName, SortBySalary. Each of above class implements comparator interface and overrides compare method, which takes two objects to compare and return eithre -1, 1 or 0.

Note: I have added few helper functions that really helps while coding like  print - this takes array list of objects and prints in format I want instead of class id and all that. toString -  overridden this function so I can return the class string as required

Here goes the code -
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Java comparator, Java, sorting