1 ans) 
Wait() : This function takes parameters as milliseconds and 
puts the thread in wait state for the desired time of the 
programmer after time passes the execution starts again.

Sleep() : This function is also used for same purpose using 
his function by java you can put a thread in sleep 
state .sleep does not contains any parameters so the thread 
will not be automatically start execution It needs a wake 
up signal again which can be Notify().or other function are 
also provided by java.

So the main difference in Wait() and sleep() is wait takes 
time parameter and wait for specific time only and sleep 
throws a thread in sleep mode for unspecified time.

2)Static Block  is executed when the Program starts.

Example :

public class StaticExample {

	static{
		System.out.println("Hello");
	}
	
	public static void main(String args[]){
		
	}
}

When we run this program it will print Hello.

Static Methods are executed when those methods are called 
from another static class or method

Example :

public class StaticExample {

	static void printString(){
		System.out.println("Hello");
	}
	
	static void testStaticMethod(){
		printString();
	}
	
	public static void main(String args[]){
		testStaticMethod();
	}
}
  
4) singleton class:

A singleton class is one which is having a single object.It 
is mainly used for the management purpose such as database 
connection.


public class SingletonPattern{
  private static SingletonPattern instance;
  private SingletonPattern(){} 
public static synchronized SingletonPattern getInstance(){
  if (instance == null)
    {
    instance = new SingletonPattern();
    }
    return instance;
  }
public static void main(String arg[]){
      System.out.println("The output of two instance:");
    SingletonPattern sp=new SingletonPattern();
    System.out.println("First Instance: "+sp.getInstance());
    sp=new SingletonPattern();
    System.out.println("Second Instance:"+sp.getInstance());
  }
}