i just made a problem which should return if a number "isHappy" or not. A number is happy if it meets some criteria:

A happy number is a number defined by the following process:

-Starting with any positive integer, replace the number by the sum of the squares of its digits. -Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. -Those numbers for which this process ends in 1 are happy.

`

import java.util.HashSet;
import java.util.Set;

class Solution{
    public int getNext(int n){
        int totalSum = 0;
        while(n > 0){
            int last = n % 10;
            n = n / 10;
            totalSum  = last * last;
        }
        return totalSum;
    }

    public boolean isHappy(int n){
        Set<Integer> seen = new HashSet<>();
        while( n!=1 && !seen.contains(n)){
            seen.add(n);
            n = getNext(n);
        }
        return n==1;
    }
}

 class Main {
    public static void main(String[] args){
        isHappy(23);  //this doesnt work.
    }
}

`

I don't know how to call this function, tried different methods like int number = 23 for ex and then isHappy(number) sout(isHappy(number)); , number.isHappy() although this makes no sense, intellij is telling me to make a method inside main but i dont think i must do this, think there s another way.

CodePudding user response：

There are a couple of problems here. When you run a Java application it looks for the main method in the class you run. Your class has no main method, instead it has an inner class that has a main method.

class Solution {
    // existing methods

    // no 'Main' class wrapping method
    public static void main(String[] argv) {

    }
}

The second problem is that main is a 'static' method but isHappy is an 'instance' method. To call it you need an instance of the class

// inside the `main` function
var sol = new Solution();
if (sol.isHappy(23)) {
    System.out.println("23 is happy");
} else {
    System.out.println("23 is not happy");
}

CodePudding user response：

You have completely solved the problem. You only have to decide what to do with the result of your calculations. You can simply print a string depending on the result.

Please note, that the methods in your Solution class are not static. So, you need to create an instance of the class and call the methods on that instance.

import java.util.HashSet;
import java.util.Set;

class Solution {

    public int sumOfDigitsSquares(int n) {
        int totalSum = 0;
        while (n > 0) {
            int last = n % 10;
            n = n / 10;
            totalSum  = last * last;
        }
        return totalSum;
    }

    public boolean isHappy(int n) {
        Set<Integer> seen = new HashSet<>();
        while (n != 1 && !seen.contains(n)) {
            seen.add(n);
            n = sumOfDigitsSquares(n);
        }
        return n == 1;
    }
}

class Main {
    public static void main(String[] args) {
        var solution = new Solution();

        String answer = solution.isHappy(23) ? "Is happy" : "Not happy at all";
        System.out.println(answer);
    }
}

Here is one possible way to check all numbers from 1 to 100 for happiness and print the result for each number.

IntStream.rangeClosed(1, 100)
                .mapToObj(i -> ""   i   " "   (solution.isHappy(i) ? "is happy" : "not happy"))
                .forEach(System.out::println);

CodePudding user response：

To call non-static methods of a class, you need an instance:

 public static void main (String [] args) {
     foo = new Solution ();
     boolean bar = foo.isHappy (49);
     // do something with bar
 }

To use a static method, you don't use an instance:

   class Solution{
       public static int getNext(int n){  ... etc ... 
       }                   
       public static boolean isHappy(int n){ ... etc ...  
       }        
     
       public static void main (String [] args) { 
          boolean foo = isHappy (1024);
          // do something with foo 
       }
   

Another problem is you are throwing away the result of isHappy (23).

      System.out.println (isHappy(23));
      System.out.println ("is 23 happy?"   (isHappy(23) ? "Yes" : "No");

are two possibilities.