Important questions can be asked in competitive exams...!!

                                 Important questions

Q:	What if the main method is declared as private?
A:	The program compiles properly but at runtime it will give "Main method not public." message.
Q:	What if the static modifier is removed from the signature of the main method?
A:	Program compiles. But at runtime throws an error "NoSuchMethodError".
Q:	What if I write static public void instead of public static void?
A:	Program compiles and runs properly.
Q:	What if I do not provide the String array as the argument to the method?
A:	Program compiles but throws a runtime error "NoSuchMethodError".
Q:	How can one prove that the array is not null but empty using one line of code?
A:	Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.
Q:	Can an application have multiple classes having main method?
A:	Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
Q:	Do I need to import java.lang package any time? Why ?
A:	No. It is by default loaded internally by the JVM.
Q:	What are different types of inner classes?
A:	Nested top-level classes, Member classes, Local classes, Anonymous classes Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety. Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class. Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable. Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Q:	Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
A:	No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
Q:	What is the difference between declaring a variable and defining a variable?
A:	In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.

Q:	What is the default value of an object reference declared as an instance variable?
A:	null unless we define it explicitly.

Q:	What type of parameter passing does Java support?
A:	In Java the arguments are always passed by value .

Q:	Primitive data types are passed by reference or pass by value?
A:	Primitive data types are passed by value.

:

Objects are passed by value or by reference?

A:

Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object . Q Why do we need wrapper classes? It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object. Q: What are checked exceptions? A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions. Q: What are runtime exceptions? A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time. Q: What is the difference between error and an exception? A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.). Q: Is it necessary that each try block must be followed by a catch block? A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method. Q: If I write System.exit (0); at the end of the try block, will the finally block still execute? A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.