Q: What is immutable object? Can you write immutable object?
Immutable classes are Java classes whose objects can not be modified once created. Any modification in Immutable object result in new object. For example is String is immutable in Java. Mostly Immutable are also final in Java, in order to prevent sub class from overriding methods in Java which can compromise Immutability. You can achieve same functionality by making member as non final but private and not modifying them except in constructor.
Q: Does all property of immutable object needs to be final?
Not necessary as stated above you can achieve same functionality by making member as non final but private and not modifying them except in constructor.
Q: 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 s = new String("Test");
does not put the object 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.
Q: 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”. see my post How SubString works in Java for detailed answer of this Java question.
Q: Which two method you need to implement for key Object in HashMap ?
In order to use any object as Key in HashMap, it must implements equals and hashcode method in Java. Read How HashMap works in Java for detailed explanation on how equals and hashcode method is used to put and get object from HashMap. You can also see my post 5 tips to correctly override equals in Java to learn more about equals.
Q: Where does these two method comes in picture during get operation?
This core Java interview question is follow-up of previous Java question and candidate should know that once you mention hashCode, people are most likely ask How its used in HashMap. See How HashMap works in Java for detailed explanation.
Q: How do you handle error condition while writing stored procedure or accessing stored procedure from java?
This is one of the tough Java interview question and its open for all, my friend didn't know the answer so he didn't mind telling me. my take is that stored procedure should return error code if some operation fails but if stored procedure itself fail than catching SQLException is only choice.
Q: What is Singleton? is it better to make whole method synchronized or only critical section synchronized ?
Singleton in Java is a class with just one instance in whole Java application, for example java.lang.Runtime is a Singleton class. Creating Singleton was tricky prior Java 4 but once Java 5 introduced Enum its very easy.
Q: What is the difference when String is gets created using literal or new() operator ?
When we create string with new() its created in heap and not added into string pool while String created using literal are created in String pool itself which exists in Perm area of heap.
Q: Does not overriding hashcode() method has any performance implication ?
Hashcode function will result in frequent collision in HashMap which eventually increase time for adding an object into Hash Map.