Interview Questions on JAVA Classes and Objects- Part 1
Q: What is Class and Object?
Answer:
Java is an Object Oriented Language. As a language that has the Object Oriented feature Java supports the following fundamental concepts:
- Polymorphism
- Inheritance
- Encapsulation
- Abstraction
- Classes
- Objects
- Instance
- Method
In this chapter we will look into the concepts Classes and Objects.
· Object - Objects have states and behaviors. Example: A dog has states-color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
· Class - A class can be defined as a template/ blue print that describe the behaviors/states that object of its type support.
à A class is nothing but new data type
à Keyword class is used to create new classes (data types)
à Keyword extends is used to inherit the features of another class
à Any class declared, automatically extends java.lang.Object classes,
"The Mother of all classes"
public class Bicycle {
// the Bicycle class has
// three fields
public int cadence;
public int gear;
public int speed;
// the Bicycle class has
// one constructor
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
// the Bicycle class has
// four methods
public void setCadence(int newValue) {
cadence = newValue;
}
public void setGear(int newValue) {
gear = newValue;
}
public void applyBrake(int decrement) {
speed -= decrement;
}
public void speedUp(int increment) {
speed += increment;
}
}
Q: What are the Object and Class classes used for? Which class should you use to obtain design information about an object?
OR
Differentiate between a Class and an Object?
Answer:
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: Explain about Access Modifiers in Java?
Answer:
Access modifiers specifies who can access them. There are four access modifiers used in java. They are public, private, protected, no modifier/default (declaring without an access modifer). Usage of these access modifiers is restricted to two levels. The two levels are class level access modifiers and member level access modifiers.
I) Class level access modifiers (java classes only)
Only two access modifiers is allowed, public and no modifier
- If a class is ‘public’, then it CAN be accessed from ANYWHERE.
- If a class has ‘no modifer’, then it CAN ONLY be accessed from ‘same package’.
II) Member level access modifiers (java variables and java methods)
All the four public, private, protected and no modifer is allowed.
- public and no modifier – the same way as used in class level.
- private – members CAN ONLY access.
- protected – CAN be accessed from ‘same package’ and a subclass existing in any package can access.
For better understanding, member level access is formulated as a table:
Access Modifiers | Same Class | Same Package | Subclass | Other packages |
public | Y | Y | Y | Y |
protected | Y | Y | Y | N |
No modifier/default | Y | Y | N | N |
private | Y | N | N | N |
First row {public Y Y Y Y} should be interpreted as:
- Y – A member declared with ‘public’ access modifier CAN be accessed by the members of the ‘same class’.
- Y – A member declared with ‘public’ access modifier CAN be accessed by the members of the ‘same package’.
- Y – A member declared with ‘public’ access modifier CAN be accessed by the members of the ‘subclass’.
- Y – A member declared as ‘public’ CAN be accessed from ‘Other packages’.
Second row {protected Y Y Y N} should be interpreted as:
- Y – A member declared with ‘protected’ access modifier CAN be accessed by the members of the ‘same class’.
- Y – A member declared with ‘protected’ access modifier CAN be accessed by the members of the ‘same package’.
- Y – A member declared with ‘protected’ access modifier CAN be accessed by the members of the ‘subclass’.
- N – A member declared with ‘protected’ access modifier CANNOT be accessed by the members of the ‘Other package’.
similarly interpret the access modifiers table for the third (no access modifier) and fourth (private access modifier) records.
Q: Explain about static, instance and local variables?
OR
What are the different scopes for Java variables?
Answer:
There are various variable types available in Java Language. There are three kinds of variables in Java:
1. Local variables
2. Instance variables
3. Class/static variables
Local variables:
· Local variables are declared in methods, constructors, or blocks.
· Local variables are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor or block.
· Access modifiers cannot be used for local variables.
· Local variables are visible only within the declared method, constructor or block.
· Local variables are implemented at stack level internally.
· There is no default value for local variables so local variables should be declared and an initial value should be assigned before the first use.
Instance variables:
· Instance variables are declared in a class, but outside a method, constructor or any block.
· When a space is allocated for an object in the heap a slot for each instance variable value is created.
· Instance variables are created when an object is created with the use of the key word 'new' and destroyed when the object is destroyed.
· Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object.s state that must be present through out the class.
· Instance variables can be declared in class level before or after use.
· Access modifiers can be given for instance variables.
· The instance variables are visible for all methods, constructors and block in the class. Normally it is recommended to make these variables private (access level).However visibility for subclasses can be given for these variables with the use of access modifiers.
· Instance variables have default values. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor.
· Instance variables can be accessed directly by calling the variable name inside the class. However within static methods and different class ( when instance variables are given accessibility) the should be called using the fully qualified name .ObjectReference.VariableName.
Class/static variables:
· Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.
· There would only be one copy of each class variable per class, regardless of how many objects are created from it.
· Static variables are rarely used other than being declared as constants. Constants are variables that are declared as public/private, final and static. Constant variables never change from their initial value.
· Static variables are stored in static memory. It is rare to use static variables other than declared final and used as either public or private constants.
· Static variables are created when the program starts and destroyed when the program stops.
· Visibility is similar to instance variables. However, most static variables are declared public since they must be available for users of the class.
· Default values are same as instance variables. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor. Additionally values can be assigned in special static initializer blocks.
· Static variables can be accessed by calling with the class name .ClassName.VariableName.
· When declaring class variables as public static final, then variables names (constants) are all in upper case. If the static variables are not public and final the naming syntax is the same as instance and local variables.
Q: If a class is declared without any access modifiers, where may the class be accessed?
Answer
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: Can a top-level class be private or protected?
Answer: 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”.
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