if statement is used when you want to execute a block of code under any condition check. There is 4 kind of if block-

1.  Simple if Statement :
       if(test expression){
       ??.
       ?..
      }
2.   if-else statement:
      if(test expression){
       ??.
       ?..
      }
else{
?.
?
}
3.  Nested If .. else: you can also put another if with in if block.
if(test expression){
if(test expression){
          ??.
       ?..
    }
else{ 
          ??.
       ?..
   }
}
else{
          ??.
       ?..
}

4.  else if ladder:
      if(condition1){
       ?.
       }
        else if(condition2){
        ?.
       }
        else if(condition3){
         ?.
       }
       else {
       ?..
       }

Example:

   import java.util.Scanner;

   public class IfElse{
    public static void main(String [] args){
        int a=50,b;
        Scanner scanner=new Scanner(System.in);
        System.out.println("Input any number: ");
        b=scanner.nextInt();                      //input 
        if(a<b){
            System.out.println("a is less than b");
        }
        else{
            System.out.println("a is greater than b");

        }
    }
}

Description : For user input you can use Scanner object. It parses user input entered on the console or from a file. The work of Scanner is to break its input into separate tokens and then returns them one at a time.