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When we declare a method static, it means that "this belongs to class as whole and not particular instance". The whole purpose of constructor is to initialize a object and hence there is no sense having static constructor.
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Ans. yes, we can.
exa: public class Java{
public static void main(String... args) {
JAva j= null;
j.greeting(); // call with null reference
}
public static void greeting() {
System.out.println("Hello World");
}
} // output: Hello World
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Q12. Which of following is / are valid static final declaration ?
public static final String MAX_NUM = "10";
public static final Object NULL = null;
public static final MathContext MATH_CONTEXT = new MathContext(2,RoundingMode.CEILING);
Ans. Objective of constant variable is to provide a context to the value and provide a single point of change. In terms of context, i don't see much need for having constant name for null as null is already self explanatory. In terms of providing single point of change, it doesn't make much sense as it would necessitate that all such placeholders either hold null or a common value.
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Q14. If we have to maintain a value "31" denoting "number of days in a month" as constant variable, Which of the following is the best constant name and Why ?
Constant should be such that it uniquely identifies the value it holds and context of the value. THIRTY_ONE uniquely identifies the value but doesn't express the context. NUMBER_OF_DAYS_IN_MONTH expresses the context but doesn't specify the value clearly as different month can have different number of days.
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Q15. If we have to maintain a value "7" denoting "number of days in a week" as constant variable, Which of the following is the best constant name and Why ?
Constant should be such that it uniquely identifies the value it holds and context of the value. SEVEN uniquely identifies the value but doesn't express the context.
Though NUMBER_OF_DAYS_IN_WEEK and NUMBER_OF_DAYS_IN_WEEK_7 makes sense in terms of unique value identification and context but 7 at the end of constant name appears useless as all weeks have 7 days and hence it's the only value it can hold. If it would have been "number of days in a month", it would have made sense to include number of days at the end like
Ans. Though it's useful but it's not as useful as overriding member or object methods. We cannot achieve polymorphic behavior with static methods by overriding their definition in derived class.
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Ans. It's weird that compiler doesn't complain if we declare transient with static variable because it makes no sense. At least a warning message saying "transient is useless in this situation" would have helped with code cleaning.
Static variables are never serialized and transient is an indication that the specified variable shouldn't be serialized so its kind of double enforcement not to serialize.
It could be that as it makes no different to the variable behavior and hence using both keywords with a variable are permitted.
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Ans. static is the keyword that makes it accessible even without creating any object and using class name only. Making it non static would like creation of object upfront before calling the method.
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Ans. static keyword is used to specify that the respective programming construct ( method , variable ) belongs to the class and not to its instance and is supposed to be shared by all instances of the class.
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Ans. Better Control - If the value is being used at multiple locations , that can be controlled better from single place. Any change would only require making single change.
Meaning , Aliasing and Better Readability - Sometimes its easy to read the value by its meaning or alias ( 0 as ZERO or 0 as NEUTRAL_VALUE ).
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Ans. static methods and static elements are shared by all objects of a class and hence could create problem.
Static methods are not synchronized by default and hence could create problem when accessed through multiple threads. Static elements are shareable by class and hence state of one object could be altered by another object.
Some limitations with Unit testing as not all mocking framework facilitate mocking them. Power mock allows but Mockito doesn't
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Ans. 1. Static class is a class which cannot be instantiated and all its members are static whereas Singleton is the class that only permit creation of single object and then the object is reused.
2. As there is no object in Static class, it cannot participate in runtime Polymorphism.
3. As Static class doesnt allow creating objects and hence it cannot be serialized.
4. Static class body is initialized eagerly at application load time whereas Singleton object can be initiated eagerly using static blocks or lazily on first need.
5. Its not recommended to use pure static class as it fails to use many OOPs concepts.
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Ans. In first case we are trying to initialize Inner class object using the instance of Outer Class whereas in second case we are trying to initialize the Inner class object directly using the Outer class name.
In second case , Inner class is "static inner class" as we cannot access "non static inner class" using Classname alone.
In first case, the inner class could be either "static inner class" or "non static inner class".
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Which of following are serialized ?
