You have to move the input.next() inside of the loop and I would recommand to use a while instead of a for loop:

public static void main(String[] args) {

    Scanner input = new Scanner(System.in);
    String secret = "Please", guess = "";

    System.out.print("Secret word?");

    while (!guess.equals(secret)) {
        guess = input.next();

        if (guess.equals(secret)) {
            System.out.println("enter");
        } else {
            System.out.println("try again");
        }
    }
}

Easy with do-while:

Scanner keyboard = new Scanner(System.in);
int startr, endr;
boolean good = false;
do
{
  System.out.println("Enter the Starting Number of the Range: ");
  startr = keyboard.nextInt();
  if(startr % 10 == 0 && startr >= 0)
    good = true;
  else
    System.out.println("Numbers is not divisible by 10");
}
while (!good);

good = false;
do
{
    System.out.println("Enter the Ending Number of the Range: ");
    endr = keyboard.nextInt();
    if(endr % 10 == 0 && endr <= 1000)
      good = true;
    else
      System.out.println("Numbers is not divisible by 10");
}
while (!good);

// do stuff

You are seeing outputs for each input because you call main recursively. If you immediately type a correct int, the flow is this:

main
  ask for input -> int
  print i (1)

But in your case the input is not an int. This is what happens: you type the first input, it fails. You do not print yet because you first call main again, asking for the next input. Only when you get a correct int you print, and then finish and allow the previous main-execution to finish by printing, which then allows the previous... and so on:

main(args)
    ask for input -> a !int
    main(args)
        ask for input -> d !int
        main(args)
            ask for input -> 2.0 !int
            main(args)
                ask for input -> 1 int
                print 1 (1)
            print 0 (2.0)
        print 0 (d)
    print 0 (a)

Look at Ravi's answer for a proper way to repeatedly ask for input without using try/catch (which is discouraged).

but here I want to loop until valid value is entered

Since you want to continuously receive a value until it is valid. It is an obvious hint you need to enclose your prompting of input within your loop:

System.out.println("enter a number");  
int a=sc.nextInt(); 

while(!validateValue(a)){                   //so long value is invalid, repeat.
    System.out.println("enter a number");  
    a=sc.nextInt();                         //prompt for input
}
System.out.println("valid value");

But if I were you, I will use a do-while loop, so we do not have to prompt for input twice.

do{                   
    System.out.println("enter a number");  
    a=sc.nextInt();                         
}while(!validateValue(a));
System.out.println("valid value");

Start by assuming the input is valid (and set valid to true on every iteration of the loop). Only set valid to false when you encounter an exception (hopefully the one you raised).

do {
    valid = true;
    try {
        System.out.println("Enter wall height (feet): ");
        wallHeight = scnr.nextDouble();
        if (wallHeight <= 0) {
            throw new Exception("Invalid Input");
        }
    } catch (Exception e) {
        valid = false;
        System.out.println("Invalid Input");
    }
} while (!valid);

Note that you do not appear to need an exception here, as

do {
    valid = true;
    System.out.println("Enter wall height (feet): ");
    wallHeight = scnr.nextDouble();
    if (wallHeight <= 0) {
        System.out.println("Invalid Input");
        valid = false;
    }
} while (!valid);

would also work. Of course, that assumes the user only inputs valid double(s). If you need handle arbitrary input, you should check that there is a double before you attempt to consume it (and you must consume anything that isn't a double or you have an infinite loop). Like,

do {
    valid = true;
    System.out.println("Enter wall height (feet): ");
    if (scnr.hasNextDouble()) {
        wallHeight = scnr.nextDouble();
        if (wallHeight <= 0) {
            System.out.println("Invalid Input");
            valid = false;
        }
    } else {
        System.out.println("Invalid Input " + scnr.nextLine());
        valid = false;
    }
} while (!valid);

Use a while loop above input line as:

 while(true)

And, use if condition to break.

if(year == 0)
    break;

Also, condition for leap year is wrong in your code. It should be:

if((year % 100 == 0 && year % 400 == 0) || (year % 4 == 0 && year % 100 != 0))
    //its a leap year
else
    //its not

PS: As in comments, I'll give a complete code:

import java.util.*;

public class Task10 {

public static void main(String[] args) {
    System.out.println("Enter a year to check if it is a leap year");
    while(true){
    Scanner input = new Scanner(System.in);
        int year = input.nextInt();
        if(year == 0)
            break;
        if((year % 100 == 0 && year % 400 == 0) || (year % 4 == 0 && year % 100 != 0))
            System.out.println(year + " is a leap year");
        else
            System.out.println(year + " is not a leap year");
    }
}

}

That's not how you compare strings in Java.

There is also a logical error in your code, as the string can't be both Y and N at the same time, you have to use && instead of ||. As long as the choice is neither Y or N, you want to continue the loop. If it is any of them, you want to stop. So && is the correct choice here.

To check if two strings are equal, you have to use .equals(obj)

while (!loopChoice.equals("Y") && !loopChoice.equals("N")) {

The reason for this is that == compares object references, and two Strings are most often not the same object reference. (Technically, they can be the same object reference, but that's not something you need to worry about now) To be safe, use .equals to compare Strings.

To avoid a possible NullPointerException in other situations, you can also use:

while (!"Y".equals(loopChoice) && !"N".equals(loopChoice)) {

Let's come up with an sample for you so you can follow as your blueprint

first, I chose do while loop because you need to ask this question at least once.

he syntax of a do...while loop is:

do
{
   //Statements
}while(Boolean_expression);

Notice that the Boolean expression appears at the end of the loop, so the statements in the loop execute once before the Boolean is tested.

If the Boolean expression is true, the flow of control jumps back up to do, and the statements in the loop execute again. This process repeats until the Boolean expression is false.

Next, you need to see how you can staisfy the boolean_experssion when the input is right, so you can stop looping or if it is wrong, you keep asking the question.

The way that I really like is to use sentinel value because using break keyword really scares me.

In programming, a special value that is used to terminate a loop. The sentinel value typically is chosen so as to not be a legitimate data value that the loop will encounter and attempt to perform with. For example, in a loop algorithm that computes non-negative integers, the value "-1" can be set as the sentinel value as the computation will never encounter that value as a legitimate processing output.

so when the input is right you change the value of i, so you can stop the looping or otherwise, showing the message and asking the question again and again till the use hits the right answer.

Code:

    int i = 0;
    Scanner input = new Scanner(System.in);
    while (i == 0) {
        System.out.println("Enter number zero plz");
        int result = input.nextInt();
        if(result == 0 ){
            System.out.println("I entered right number");
            i = 1;
        } else
            System.out.println("you entered the wrong number \nplz try again");
    }

output:

You should have an else case:

if(playerValue > 10 && playerValue < 21){

            System.out.println("Players Value so far " + playerValue + ", Do you want to draw another card? Y/N");
            //  input a y or n answer
            decision = sob.next();

            if(decision.equalsIgnoreCase("Y")){
                continue;

            }else if(decision.equalsIgnoreCase("N")){
                break;
            }
            else
            {
                 //whatever you want to happen if they don't enter either y or n
            }

        }
    }

Maybe in the else have another loop that says please enter a valid input and keep looping until they give a valid input...

In general, use break inside a while loop is considered a bad practice. You should have something like this

public static void main(String[] args) {

    Scanner input = new Scanner(System.in);

    int number;
    int minimum = 5;
    int maximum = 15;

    do{
        System.out.print("Enter a number between" + " " + minimum + " " + "and" + " " + maximum + ":" );
        number = input.nextInt();
        if (number < minimum || number > maximum) 
            System.out.print("Sorry, invalid");
    } while (number < minimum || number > maximum); 

}