Java Banking Interview Questions

What is the difference between creating String as new() and literal?

When we create string with new() Operator, it’s created in heap and not added into string pool while String created using literal are created in String pool itself which exists in PermGen area of heap.

String str = new String("Test");
 
does not  put the object str in String pool , we need to call String.intern() method which is used to put  them into String pool explicitly. its only when you create String object as String literal e.g. String s = "Test" Java automatically put that into String pool. By the way there is a catch here, Since we are passing arguments as "Test", which is a String literal, it will also create another object as "Test" on string pool. This is the one point, which has gone unnoticed, until knowledgeable readers of Javarevisited blog suggested it

What is the difference when String is gets created using literal or new() operator ?
When we create string with new() operator,  its created in heap only and not added into string pool, while String created using literal are created in String pool itself which exists in PermGen area of heap. You can put such string object into pool by calling intern() method. If you happen to create same String object multiple times, intern() can save some memory for you.

What will be the problem if you don't override hashcode() method ?
If you don't override equals method, than contract between equals and hashcode will not work, according to which, two object which are equal by equals() must have same hashcode. In this case other object may return different hashcode and will be stored on that location, which breaks invariant of HashMap class, because they are not supposed to allow duplicate keys. When you add object using put() method, it iterate through all Map.Entry object present in that bucket location, and update value of previous mapping, if Map already contains that key. This will not work if hashcode is not overridden

When do you override hashcode and equals() ?
Whenever necessary especially if you want to do equality check  based upon business logic rather than object equality e.g. two employee object are equal if they have same emp_id, despite the fact that they are two different object, created by different part of code. Also overriding both these methods are must if you want to use them as key in HashMap. Now as part of equals-hashcode contract in Java, when you override equals, you must overide hashcode as well, otherwise your object will not break invariant of classes e.g. Set, Map which relies on equals() method for functioning properly.

How does substring () inside String works?

Another good Java interview question, I think answer is not sufficient but here it is “Substring creates new object out of source string by taking a portion of original string”.  This question was mainly asked to see if developer is familiar with risk of memory leak, which substring can create. Until Java 1.7, substring holds reference of original character array, which means even a substring of 5 character long, can prevent 1GB character array from garbage collection, by holding a strong reference. This issue is fixed in Java 1.7, where original character array is not referenced any more, but that change also made creation of substring bit costly in terms of time. Earlier it was on the range of O(1), which could be O(n) in worst case on Java 7.