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Ans. When we create reference for primitive type, it's memory is allocated. So even if we don't assign any value to it, the default value is initialized.
int x; // default value 0 initialized
But for object references, it's different as references hold nothing till an object is assigned.
Object obj; // contains null
So all primitive types when declared , contains their respective default values on the basis of their type whereas all wrapper class References contains null irrespective of their types.
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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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Ans. Java 9 would provide an error at compile time if there are different modules with the same package mapped to the same class loader and hence would provide a compile time check for this issue.
NoClassDefFoundError exists because of ambiguity at realtime because of multiple versions of same package / class exist. As Java 9 would provide a compile time check for it through clearly defined dependencies and exports and a check on duplicate packages being loaded by single class loader, it will fix the problem.
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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. The problem with serialized singleton class is that whenever we deserialize it, it will create a new instance of the class. To overcome this scenario all we need to do is to provide the implementation of readResolve() method.
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Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains wil complain about it. And the JVM will internally load the class only once no matter how many times you import the same class.
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Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable.
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Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.
A static method is not associated with any instance of a class so the concept of overriding for runtime polymorphism using static methods is not applicable.
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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. member variable are loaded into heap, so they are initialized with default values when an instance of a class is created. In case of local variables, they are stored in stack until they are being used.
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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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Sample Code for enum
Which of the following is false for final ?
