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Ans. Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes | ||||
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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations. | ||||
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Ans. StringBuffer is synchronized whereas StringBuilder is not. | ||||
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Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance. | ||||
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Ans. 1.this is a reference to the current object in which this keyword is used whereas super is a reference used to access members specific to the parent Class. 2.this is primarily used for accessing member variables if local variables have same name, for constructor chaining and for passing itself to some method whereas super is primarily used to initialize base class members within derived class constructor. | ||||
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Ans. The jsp scriptlet tag can only declare variables not methods whereas jsp declaration tag can declare variables as well as methods. The declaration of scriptlet tag is placed inside the _jspService() method whereas The declaration of jsp declaration tag is placed outside the _jspService() method. | ||||
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Ans. Collections can only hold objects, It can't hold primitive data types. Collections have performance overheads as they deal with objects and offer dynamic memory expansion. This dynamic expansion could be a bigger overhead if the collection class needs consecutive memory location like Vectors. Collections doesn't allow modification while traversal as it may lead to concurrentModificationException. | ||||
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Ans. It could be worthy to move a method to util class if the method needs to be shared, doesn't require polymorphic behavior and need not be overridden in special cases. Don't belong to one group through is-a relationship ( You can share through parent class method ) Don't implement a specific interface ( java 8 default methods ) Doesn't involve complex computing as you will be loosing the benefit of object state with just static method. Doesn't require polymorphic behavior as static methods don't participate in runtime polymorphism. | ||||
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Like Discuss Correct / Improve  utility classes   util classes   static methods   application design   rare | ||||
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Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements. | ||||
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Ans. No. Because java internally treats var args as arrays and hence both method declarations will generate the same byte code and hence would result in ambiguity while determining call binding. | ||||
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Ans. 1. Default definition for some of the methods , like equals, hashcode etc that gets carried to all objects even if you don't define anything. But that default definition is kind of like assigning 0 to integer, it just provide a safe state , nothing much for comparison. 2. Places where you have no idea about what object you may receive and just want to perform out of 8 basic methods of object class, something like printing their string representation ( defined by their toString method ), or equality ( using their equals method ) | ||||
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Like Discuss Correct / Improve  object class | ||||
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Ans. ClassNotFoundException is an exception that occurs when we try to load a class at run time using Class.forName() or loadClass() methods and mentioned classes are not found in the classpath. NoClassDefFoundError is an error that occurs when a particular class is present at compile time, but was missing at run time. | ||||
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Ans. For top level class we can only use "public" and "default". We can use private with inner class. | ||||
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Ans. To display the current CLASSPATH variable, use these commands in UNIX (Bourne shell): % echo $CLASSPATH To delete the current contents of the CLASSPATH variable, In UNIX: % unset CLASSPATH; export CLASSPATH To set the CLASSPATH variable, In UNIX: % CLASSPATH=/home/george/java/classes; export CLASSPATH | ||||
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Very Frequently asked.Favorite question in Walk in drive for many Indian service companies. | ||||
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Ans. A Class that cannot be sub classed. | ||||
Sample Code for final class | ||||
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Ans. boolean is a primitive type whereas Boolean is a class. | ||||
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Ans. A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component. A Wrapper Class that wraps or encapsulates the primitive data type is called Primitive Wrapper Class. | ||||
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Ans. Adapter. | ||||
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Ans. Part of JVM which is used to load classes and interfaces. Bootstrap , Extension and System are the class loaders used by JVM. | ||||
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Ans. Inner class is a type of nested class. Inner classes are non static nested classes i.e the one that are associated with the instance of outer class. | ||||
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Ans. Any interface declared inside a class or an interface. It is static by default. | ||||
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Ans. Keep it as a Abstract Class if its a "Is a" Relationsship and should do subset/all of the functionality. Keep it as Interface if its a "Should Do" relationship. | ||||
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Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection. | ||||
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Ans. Class loaders are hierarchical. The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running. | ||||
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Ans. static loading - Classes are statically loaded with Java new operator. dynamic class loading - Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time. Class.forName (Test className); | ||||
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Ans. Collections are re-sizable in nature. We can increase or decrease the size as per recruitment. Collections can hold both homogeneous and heterogeneous data's. Every collection follows some standard data structures. Collection provides many useful built in methods for traversing,sorting and search. | ||||
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Ans. private static boolean isPalindrome(String str) { if (str == null) return false; StringBuilder strBuilder = new StringBuilder(str); strBuilder.reverse(); return strBuilder.toString().equals(str); } | ||||
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Ans. private static String removeChar(String str, char c) { | ||||
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Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesnt need to be calculated again. This makes it a great candidate for key in a Map and its processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys. | ||||
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Ans. intern() method keeps the string in an internal cache that is usually not garbage collected. Moreover provide reference for scp object for corresponding string object present in heap memory. | ||||
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Ans. substring method would build a new String object keeping a reference to the whole char array, to avoid copying it. Hence you can inadvertently keep a reference to a very big character array with just a one character string. | ||||
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