Question

Please answer with a fully detailed solution with examples.

In Java, write a method to determine if a string contains only digit characters.
In Java, write a method on how to find each occurrence of a letter given a string.
In Java, given a list of numbers, write an algorithm to find all the triples and pairs of numbers in the list that equal exactly 13.
In Java, write a function to find the total number of each character in a given string.

Answer 1

Java code for above problems

import java.util.*;
class Main
{
   // testing main method
   public static void main(String args[])
   {
       System.out.println("Does \"123\" has all digit characters? "+allDigits("123"));
       System.out.println("Does \"12a3c\" has all digit characters? "+allDigits("12a3c"));
       System.out.println();
       countChars("alphabets");
       System.out.println();
       List<Integer> list=new ArrayList<Integer>();
       list.add(8);
       list.add(2);
       list.add(4);
       list.add(13);
       list.add(3);
       list.add(5);
       tripletsAndDoubles(list);
   }
   // method that returns true if given string has all characters as digits
   public static boolean allDigits(String str)
   {
       for(int i=0;i<str.length();i++)
       {
           char ch=str.charAt(i);
           if(!(ch>='0' && ch<='9'))
           {
               return false;
           }  
       }  
       return true;
   }
   // method that prints the occurences count of all the characters of given string
   public static void countChars(String str)
   {
       int [] count=new int[256];
       for(int i=0;i<str.length();i++)
       {
           count[str.charAt(i)]++;
       }
       for(int i=0;i<256;i++)
       {
           if(count[i]>0)
           {
               System.out.println((char)i+" ---> "+count[i]);
           }
       }
   }
   // method that prints the triplets and pairs of given list whose sum is 13
   public static void tripletsAndDoubles(List<Integer> list)
   {
       System.out.println("Triplets: ");
       printTriplets(list);
       System.out.println();
       System.out.println("Pairs: ");
       printPairs(list);
       System.out.println();
   }
   // method that prints the triplets whose sum is 13
   public static void printTriplets(List<Integer> list)
   {
       int len=list.size();
       for(int i=0;i<len;i++)
       {
           for(int j=i+1;j<len;j++)
           {
               for(int k=j+1;k<len;k++)
               {
                   if(list.get(i)+list.get(j)+list.get(k)==13)
                   {
                       System.out.println(list.get(i)+"+"+list.get(j)+"+"+list.get(k)+"=13");
                   }
               }
           }
       }
   }
   // method that prints the pairs whose sum is 13
   public static void printPairs(List<Integer> list)
   {
       int len=list.size();
       for(int i=0;i<len;i++)
       {
           for(int j=i+1;j<len;j++)
           {
               if(list.get(i)+list.get(j)==13)
               {
                   System.out.println(list.get(i)+"+"+list.get(j)+"=13");
               }
           }
       }
   }
}

Sample output

Note: Question 2 and 4 are same. So same will be repeated for both.

Mention in comments if any mistakes or errors are found. Thank you.