How can I fix my IDE, which puts lots of squiggle lines over my Java code?

Question

I just started to learn Java using Eclipse. My current homework asks me to write codes where users can choose two games: game 1 is converting seconds value to the "hour:minute:second" format; game 2 is to convert input into the sum of individual digits.

My coding is below, but I can't run it, as it contains lots of problem, such as 'selection does not contain main type', 'cannot convert from int to boolean', among many others. I tried to correct them on Eclipse and also searched the solutions on Stack Overflow, but cannot resolve it.

import java.util.Scanner;

public class SimpleGame {

    /**
     * Write a method to convert the given seconds to hours:minutes:seconds.
     * @param seconds to convert
     * @return string for the converted seconds in the format: 23:59:59
     * 
     * Example(s): 
     * - If input seconds is 1432, print and return output in the format: 0:23:52
     * - If input seconds is 0, print and return output in the format: 0:0:0
     * - If input seconds is not valid (negative), print and return: -1:-1:-1.  
     *   So if input seconds is -2, print and return: -1:-1:-1 
     *   If input seconds is -3214, likewise print and return: -1:-1:-1
     */
    public String convertTime(int seconds){
        // TODO: Your code goes here
        if (seconds == 0) {
            return System.out.println("0:0:0");
        if (seconds < 0) {
            return System.out.println("-1:-1:-1");  
        if (seconds > 0) {
            int hour = seconds/3600;
            int min = (seconds%3600))/60;
            int sec = seconds % 60;
            String strhour = Integer.toString(hour);
            String strmin = Integer.toString(min);
            String strsec = Integer.toString(sec);
            return System.out.println(strhour + ":" + strmin + ":" + strsec);
        }
        }
        }
    }

    /**
     * Write a method that adds all the digits in the given non-negative integer.
     * 
     * @param integer to add digits
     * @return integer in which all the digits in the given non-negative integer are
     *         added.
     * 
     *         Example(s): - If input is 565, print and return 16. - If input is 7,
     *         print and return 7. - If input is 0, print and return 0.
     */
    public int digitsSum(int input) {
        // TODO: Your code goes here
        return 0;
    }

    public static void main(String[] args) {
        // Create an instance of the SimpleGame class.
        // TODO: Your code goes here

        Scanner sc = new Scanner(System.in);

        // Ask the user which game to play.
        System.out.println("which game to play?");
        // Then ask the user for input and pass the value to the corresponding method.
        String game = sc.next();
        // If the user enters 1, ask for an integer to convert and call the convertTime
        // method.
        if (game == "1") {
            System.out.println("what's your integer?");
            int num1 = sc.next();
            sc.convertTime(num1);
        }
        // If the user enters 2, ask for an integer and call the digitsSum method.
        if (game == "2") {
            System.out.println("what's your integer?");
            int num2 = sc.next();
            sc.digitsSum(num2);
        }
        // TODO: Your code goes here

        sc.close();
    }
}