Logic tasks in the interview

You have to parse 5 logical problems plus 4 Java problems from me. The tasks that were met at the interviews and the solutions to them are on the Internet. Here is a top list of those that I encountered in interviews.

1. Four people in hats

2. Problem about the bridge and the flashlight

3. Out of 9 coins, one is fake: it is lighter. How to find it for two weighings on the scales?

4. Almost the same as the previous problem, only the scales can be any. Task "5 jars of pills" .

5. "Endless Train"

To fill in the questions that were asked at the interviews:

1. Write multiple ways to implement a singleton(singleton) in java.

2. What will happen in this code?

   class MyExc1 extends Exception{}
   class MyExc2 extends Exception{}
   class MyExc3 extends Exception{}
   
   public class Test {
       public static void main(String[] args) throws Exception {
           try {
               System.out.print(1);
               p();
           }catch (MyExc2 e){
           }
           finally {
               throw new MyExc3();
               System.out.print(2);
           }
       }
       public static void p() throws Exception {
           try {
               throw new MyExc1();
                } catch (MyExc1 myExc1) {
               throw new MyExc2();
           }finally {
               System.out.println(3);
           }
       }
   }

3. A task from me, because I don’t remember what example was there

   public class Test {
       public static void main(String[] args)  {
           String s = "Hello";
           String s1 = "Hello";
           String s2 = new String("Hello");
   
           System.out.println(s == s1);
           System.out.println(s1 == s2);
   
           Integer i = 111;
           Integer i1 = 111;
           Integer i2 = new Integer(111);
           Integer i3 = new Integer(111);
   
           Integer i4 = -129;
           Integer i5 = -129;
           Integer i6 = new Integer(-129);
   
           System.out.println(i == i1);
           System.out.println(i1 == i2);
           System.out.println(i2 == i3);
   
           System.out.println(i4 == i5);
           System.out.println(i5 == i6);
   
           System.out.println(i6 == -129);
       }
   }

4. What will be the result of running the program? And what result will we get if we uncomment the last two lines?

   class Test{
       private int id;
       private Integer id2;
   
       public Test(int id, Integer id2) {
           System.out.println("Создаем екзепляр Test");
           this.id = id;
           this.id2 = id2;
       }
   
       public Integer doIt(){
           return id + id2;
       }
   }
   class Test1{
       private Test test;
       private static int i;
   
       public Test1() {
           System.out.println("Создаем екзепляр Test1");
           this.test = new Test(i, 10);
       }
       public Test getTest() {
           return test;
       }
   }
   
   class Main {
       public static void main(String[] args)  {
           Test1 test1 = new Test1();
           System.out.println(test1.getTest().doIt());
          // Test test = new Test(null, 1);
          // test.doIt();
       }
   }

PS: Answers to logic problems can all be found on the Internet. For those who are interested, I am attaching a previous post about java developer interviews