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Java Interview Questions and Answers
Can an application have multiple classes
having main method?
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.
Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method
is already defined in the class.
Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
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 abt it. And the JVM will internally load the
class only once no matter how many times you import the same class.
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. eg, 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. eg,
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.
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.
What are different types of inner classes?
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. Are the imports checked for validity at compile time? e.g. 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;
Does importing a package imports the subpackages as well? e.g. 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.
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.
e.g String s; is just a declaration while String s = new String ("abcd");
Or String s = "abcd"; are both definitions.
What is the default value of an object reference declared as an instance
variable?
Null unless we define it explicitly.
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