21. Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
22. Can I import same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains about it. And the JVM will internally load the class only once no matter how many times you import the same class.
23. What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown.
Example: IOException thrown by java.io.FileInputStream's read() method·
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown.
Example: StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
24. What is Overriding?
When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.
25. Are the imports checked for validity at compile time? Example: will the code containing an import such as java.lang.ABCD compile?
Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying, can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
26. Does importing a package imports the subpackages as well? Example: Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
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.
27. What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
Example: String s; is just a declaration while String s . new String ("abcd"); Or String s . "abcd"; are both definitions.
28. What is the default value of an object reference declared as an instance variable?
The default value will be null unless we define it explicitly.
29. Can a top level class be private or protected?
No. A top level class cannot be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.
If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.
30. What type of parameter passing does Java support?
In Java the arguments are always passed by value.
