1 .Implement a Java method highestFreq(String) String, that is given a string and returns a String with the most frequently occurring letter(s). If there are multiple letters that occur with the same maximum frequency, the result String should include them all. For example, if the given word is "mississippi", the String "is" is returned because i and s occur four times each, and no letter occurs more frequently.

2. Write a program that reads a word and prints all substrings, sorted by length. For example, if the user provides the input "rum", the program prints

r u m ru um rum

in JAVA