How should I use Main and static for this block of code?

Question

I have a question regarding these two blocks of code: The first one:

class Solution {
public  int[] twoSum(int[] nums, int target) {
 int[] result= new int[2];
 for (int i=0; i<nums.length; i++){
    for (int j=i+1; j<nums.length; j++){
         if ( nums[i]+nums[j]==target){
            result[0]=i;
            result[1]=j;
         }
     }
 }
 return(result);
 }
}

and the second one:

import java. util.*;

public class Solution {

public static int firstUniqChar(String s) {
    HashMap<String, Integer> map = new HashMap<String, Integer>();
    
    String[] s_arr = s.split("");

    for (int i = 0; i < s_arr.length; i++) {
        if (map.containsKey(s_arr[i])) {
        
            map.put(s_arr[i], map.get(s_arr[i]) + 1);
        }
        else
            map.put(s_arr[i], 1);
    }
    
    for (int i = 0; i < s_arr.length; i++) {
        if (map.get(s_arr[i]) == 1) {
            return i;
        
        }
    }
    return -1;
}

public static void main(String[] args) {
    
    Scanner sc = new Scanner(System.in);
    System.out.println(firstUniqChar(sc.nextLine()));

}   

}

My question is why and how the first code is working without "main"!! And if I want to put "main" for the first code how should I do that? I follow the pattern of the second code regarding static and main and.. but it does not work. Thanks

It's not? It doesn't. Your assertions of the first one "working" are specious. — Elliott Frisch, Mar 09 '23 at 23:19
I am checking both in leetcode, and both are working but my questions are 1- How and Why the first one is working without main()?! According to my knowledge, we cannot execute without a main method because when we are running the java program. 2- How I can use main() for the first one? — Azi, Mar 09 '23 at 23:26
1- you can't. they must have a "tester" program (with a `main()`) that loads the class and executes it (with unseen code). 2- By writing one and calling `new Solution().twoSum()` with an array and a target value. Best of luck! — Elliott Frisch, Mar 09 '23 at 23:35

John Williams · Accepted Answer · 2023-03-10T08:26:16.480

0

Add this to the first definition of Solution and run it:

public static void main(String[] args) {
    
    Solution sol = new Solution();
    System.out.println(sol.twoSum(new int[]{1,2,3}, 4));

}

For any Java program to run it requires a main method with signature as follows. It’s the entry-point or start point. If it’s not in your code it is provided by the surrounding code - online or IDE.

public static void main(String[] args)

Any static method (firstUniqChar) can be called without referring to an object. A non-static method (twoSum) is linked to an object so has to be invoked from an object, in this case sol

edited Mar 10 '23 at 08:26

answered Mar 10 '23 at 00:12

John Williams

4,252
2
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I wrote: public static void main(String[] args) { Solutiontest1 sol = new Solutiontest1(); System.out.println(sol.twoSum({1,2,3}, 4)); } and recived : Exception in thread "main" java.lang.Error: Unresolved compilation problems: Syntax error on token ".", @ expected after this token Syntax error, insert ")" to complete SingleMemberAnnotation Syntax error, insert "SimpleName" to complete ArgumentList Syntax error on token ")", delete this token at java_test1/java_test1.Solutiontest1.main(Solutiontest1.java:20) – Azi Mar 10 '23 at 00:58
I left out new on new int[]. Fixes the answer. – John Williams Mar 10 '23 at 08:27

How should I use Main and static for this block of code?

1 Answers1