3. Java Classes and Objects

Q: What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.

Q: What is the difference between String and StringBuffer?
String objects are immutable whereas StringBuffer objects are not. StringBuffer unlike Strings support growable and modifiable strings.

Q: Can a private method of a superclass be declared within a subclass?
Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses.There is no way for private stuff to have a runtime overloading or overriding (polymorphism) features.

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

Q: What is the difference between a constructor and a method? Or
Q: How can a subclass call a method or a constructor defined in a superclass?
A constructor is a member function of a class that is used to create objects of that class, invoked using the new operator. It has the same name as the class and has no return type. They are only called once, whereas member functions can be called many times. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator. Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
super.method(); is used to call a super class method from a sub class. To call a constructor of the super class, we use the super(); statement as the first line of the subclass’s constructor.

Q: 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/protected the compiler will complain that the “modifier private is not allowed here”.

Q: Why Java does not support multiple inheritance?
Java does support multiple inheritance via interface implementation.

Q: Where and how can you use a private constructor?
Private constructor can be used if you do not want any other class to instantiate the class. This concept is generally used in Singleton Design Pattern. The instantiation of such classes is done from a static public method.

Q: How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

Q: What is Method Overriding? What restrictions are placed on method overriding?
When a class defines a method using the same name, return type, and argument list as that of a method in its superclass, the method in the subclass is said to override the method present in the Superclass. When the method is invoked for an object of theclass, it is the new definition of the method that is called, and not the method definition from superclass.Restrictions placed on method overriding• Overridden methods must have the same name, argument list, and return type.• The overriding method may not limit the access of the method it overrides. Methods may be overridden to be more public, not more private.• The overriding method may not throw any exceptions that may not be thrown by the overridden method.

Q: What are the Object and Class classes used for? Which class should you use to obtain design information about an object? or
Q: Differentiate between a Class and an Object?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program. The Class class is used to obtain information about an object’s design. A Class is only a definition or prototype of real life object. Whereas an object is an instance or living representation of real life object. Every object belongs to a class and every class contains one or more related objects.

Q: What is a singleton class? Or
Q: What is singleton pattern?
This design pattern is used by an application to ensure that at any time there is only one instance of a class created. You can achieve this by having the private constructor in the class and having a getter method which returns an object of the class and creates one for the first time if its null.

Q: What is method overloading and method overriding? Or
Q: What is difference between overloading and overriding?
Method overloading: When 2 or more methods in a class have the same method names with different arguments, it is said to be method overloading. Overloading does not block inheritance from the superclass. Overloaded methods must have different method signatures
Method overriding : When a method in a class has the same method name with same arguments as that of the superclass,it is said to be method overriding. Overriding blocks inheritance from the superclass. Overridden methods must have same signature.
Basically overloading and overriding are different aspects of polymorphism.
static/early binding polymorphism: overloadingdynamic/late binding polymorphism: overriding
Q: If a class is declared without any access modifiers, where may the class be accessed?
A class that is declared without any access modifiers is said to have package or default access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

Q: Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its super classes.

Q: Can an object’s finalize() method be invoked while it is reachable?
An object’s finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object’s finalize() method may be invoked by other objects.

Q: What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
It returns the runtime information like memory availability.
* Runtime.freeMemory() –> Returns JVM Free Memory* Runtime.maxMemory() –> Returns the maximum amount of memory that the JVM will attempt to use. It also helps to run the garbage collector* Runtime.gc()

Q: What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.

Q: Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object’s finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable object.

Q: What is a bean? Where can it be used?
A Bean is a reusable and self-contained software component. Beans created using java take advantage of all the security and platform independent features of java. Bean can be plugged into any software application. Bean is a simple class which has set and get methods. It could be used within a JSP using JSP tags to use them.

Q: What is the functionality of instanceOf() ?
instanceOf opertaor is used to check whether an object can be cast to a specific type without throwing ClassCastException.

Q: What would happen if you say this = null?
It will come up with Error Message. “The left-hand side of an assignment must be a variable”.

Q: I want to create two instances of a class ,But when trying for creating third instance it should not allow me to create . What i have to do for making this?
One way of doing this would be:
public class test1
{
static int cntr=0;
test1()
{ cntr++;
if(cntr>2)
throw new NullPointerException();//u can define a new exception // for this
}
public static void main(String args[])
{
test1 t1= new test1();
System.out.println(”hello 1″);
test1 t2= new test1();
System.out.println(”hello 2″);
test1 t3= new test1();
}
}

Q: What is the difference between an object and an instance?
An Object May not have a class definition. eg int a[] where a is an array.
An Instance should have a class definition.
eg MyClass my=new MyClass();
my is an instance.

Q: What is heap in Java?
It is a memory area which stores all the objects created by an executing program.

Q: Why default constructor of base class will be called first in java?
A subclass inherits all the methods and fields (eligible one) from the base class, so base class is constructed in the process of creation of subclass object (subclass is also an object of superclass). Hence before initializing the default value of sub class the super class should be initialized using the default constructor.

Q: What are the other ways to create an object other than creating as new object?
We can create object in different ways;
1.new operator
2.class.forName: Classname obj = Class.forName(”Fully Qualified class Name”).newInstance();
3.newInstance
4.object.clone

Q: What is the difference between instance, object, reference and a class?
Class: A class is a user defined data type with set of data members & member functions
Object: An Object is an instance of a class
Reference: A reference is just like a pointer pointing to an object
Instance: This represents the values of data members of a class at a particular time