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13.	How to compare two Strings in java program?
	Java String implements Comparable interface and it has two variants of compareTo() methods. compareTo(String another String) method compares the String object with the String argument passed lexicographically. If String object precedes the argument passed, it returns negative integer and if String object follows the argument String passed, it returns positive integer. It returns zero when both the String have same value, in this case equals (String str) method will also return true. compare ToIgnoreCase (String str): This method is similar to the first one, except that it ignores the case. It uses String CASE_INSENSITIVE_ORDER Comparator for case insensitive comparison. If the value is zero thenequalsIgnoreCase(String str) will also return true.
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14.	Why StringBuffer is called mutable?
	The string class is considered as immutable; so that once it is created a String object cannot be changed. If there is a necessity to make a lot of modifications to Strings of characters then StringBuffer should be used.
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15.	How do you check if two Strings are equal in Java?
	There are two ways to check if two Strings are equal or not – using "==" operator or using equals method. When we use "==" operator, it checks for value of Strings as well as reference but in our programming, most of the time we are checking equality of String for value only. So we should use equal method to check if two Strings are equal or not.
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16.	What is Hash Code collision?
	The idea of hashing is to store an element into array at a position index computed as below. obtain element_hash_code of the element by processing the element's data and generating an integer value. use a simple mod operation to map into the array's range: index = hash(element) = abs(element_hash_code % array_capacity) The hash code collision occurs when the index calculated by hash function results same for two or more elements.
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