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Java - Interview Questions and Answers on Classes and Objects

Q1.  What are different types of inner classes ?

Ans. Simple Inner Class, Local Inner Class, Anonymous Inner Class , Static Nested Inner Class.

Q3.  Does Constructor creates the object ?

Ans. New operator in Java creates objects. Constructor is the later step in object creation. Constructor's job is to initialize the members after the object has reserved memory for itself.

Q4.  Difference between == and .equals() ?

Ans. "equals" is the member of object class which returns true if the content of objects are same whereas "==" evaluate to see if the object handlers on the left and right are pointing to the same object in memory.

Q5.  There are two objects a and b with same hashcode. I am inserting these two objects inside a hashmap.

hMap.put(a,a);
hMap.put(b,b);

where a.hashCode()==b.hashCode()

Now tell me how many objects will be there inside the hashmap?

Ans. There can be two different elements with the same hashcode. When two elements have the same hashcode then Java uses the equals to further differentation. So there can be one or two objects depending on the content of the objects.

Q6.  What are the common uses of "this" keyword in java ?

Ans. "this" keyword is a reference to the current object and can be used for following -

1. Passing itself to another method.
2. Referring to the instance variable when local variable has the same name.
3. Calling another constructor in constructor chaining.

Q7.  What are wrapper classes ?

Ans. They are wrappers to primitive data types. They allow us to access primitives as objects.

Q8.  What one should take care of, while serializing the object?

Ans. One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializable Exception.

Q9.  Describe what happens when an object is created in Java ?

Ans. 1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data.
2. The instance variables of the objects are initialized to their default values.
3. The constructor for the most derived class is invoked. The first thing a constructor does is call the constructor for its superclasses. This process continues until the constructor for java.lang.Object is called,
as java.lang.Object is the base class for all objects in java.
4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.

Q10.  What are the methods of Object Class ?

Ans. clone() - Creates and returns a copy of this object.
equals() - Indicates whether some other object is "equal to" this one.
finalize()  - Called by the garbage collector on an object when garbage collection determines that there are no more references to the object
getClass() - Returns the runtime class of an object.
hashCode() - Returns a hash code value for the object.
toString() - Returns a string representation of the object.
notify(), notifyAll(), and wait() - Play a part in synchronizing the activities of independently running threads in a program.

Q11.  Which access specifier can be used with Class ?

Ans. For top level class we can only use "public" and "default". We can use private with inner class.

Q12.  Difference between Abstract and Concrete Class ?

Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

Q13.  What is a Final Class ?

Ans. A Class that cannot be sub classed.

Q14.  What is an immutable class ?

Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

Q15.  Difference between object instantiation and construction ?

Ans. Though It's often confused with each other, Object Creation ( Instantiation ) and Initialization ( Construction ) are different things in Java. Construction follows object creation.

Object Creation is the process to create the object in memory and returning its handler. Java provides New keyword for object creation. 

Initialization is the process of setting the initial / default values to the members. Constructor is used for this purpose. If we don't provide any constructor, Java provides one default implementation to set the default values according to the member data types. 

Q16.  Difference between nested and inner classes ?

Ans. Inner classes are non static nested classes.

Q17.  When do you get ClassCastException?

Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance.

Q18.  Difference Between this() and super() ?

Ans. 1.this is a reference to the current object in which this keyword is used whereas super is a reference used to access members specific to the parent Class. 

2.this is primarily used for accessing member variables if local variables have same name, for constructor chaining and for passing itself to some method whereas super is primarily used to initialize base class members within derived class constructor. 

Q19.  What is the benefit of inner / nested classes ?

Ans. You can put related classes together as a single logical group.

Nested classes can access all class members of the enclosing class, which might be useful in certain cases.

Nested classes are sometimes useful for specific purposes. For example, anonymous inner classes are useful for writing simpler event-handling code with AWT/Swing.

Q20.  Explain Static nested Classes ?

Ans. The accessibility (public, protected, etc.) of the static nested class is defined by the outer class.

A static nested class is not an inner class, it's a top-level nested class.

The name of the static nested class is expressed with OuterClassName.NestedClassName syntax.

When you define an inner nested class (or interface) inside an interface, the nested class is declared implicitly public and static. 

Static nested classes can be declared abstract or final.

Static nested classes can extend another class or it can be used as a base class.

Static nested classes can have static members. 

Static nested classes can access the members of the outer class (only static members, obviously).

The outer class can also access the members (even private members) of the nested class through an object of nested class. If you don’t declare an instance of the nested class, the outer class cannot access nested class elements directly.

Q21.  Explain Inner Classes ?

Ans. The accessibility (public, protected, etc.) of the inner class is defined by the outer class. 

Just like top-level classes, an inner class can extend a class or can implement interfaces. Similarly, an inner class can be extended by other classes, and an inner interface can be implemented or extended by other classes or interfaces.

An inner class can be declared final or abstract.

Inner classes can have inner classes, but you’ll have a hard time reading or understanding such complex nesting of classes.

Q22.  Explain Method Local Inner Classes ?

Ans. You can create a non-static local class inside a body of code. Interfaces cannot have local  classes, and you cannot create local interfaces.

Local classes are accessible only from the body of the code in which the class is defined. The local classes are completely inaccessible outside the body of the code in which the class is defined.

You can extend a class or implement interfaces while defining a local class.

A local class can access all the variables available in the body of the code in which it is defined. You can pass only final variables to a local inner class.

Q23.  Explain about anonymous inner classes ?

Ans. Anonymous classes are defined in the new expression itself, so you cannot create multiple objects of an anonymous class.

You cannot explicitly extend a class or explicitly implement interfaces when defining an anonymous class.

An anonymous inner class is always created as part of a statement; don't forget to close the statement after the class definition with a curly brace. This is a rare case in Java, a curly brace followed by a semicolon.

Anonymous inner classes have no name, and their type must be either a subclass of the named type or an implementer of the named interface

Q24.  Difference between Class#getInstance() and new operator ?

Ans. Class.getInstance doesn't call the constructor whereas if we create an object using new operator , we need to have a matching constructor or copiler should provide a default constructor.

Q25.  Can we create an object if a Class doesn't have any constructor ( not even the default provided by constructor ) ?

Ans. Yes , using Class.getInstance.

Q26.  What are different ways of object creation in Java ?

Ans. Using new operator - new xyzClass()
Using factory methods - xyzFactory.getInstance( )
Using newInstance( ) method - (Class.forName(“xyzClass”))emp.newInstance( )
By cloning an already available object - (xyzClass)obj1.clone( )



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