Classes and Objects 3

Question 1

What is the definition of package?

Answer 1

Allows to group classes and interfaces into bundles, that can then be handled together using an accepted naming convention

Question 2

Why would you group classes into packages?

Answer 2

• Allows reuse, rather than rewriting classes, you can use existing classes by importing them
• Prevents naming conflicts
  
  • classes with the same name can be used in a program, uniquely identifying them by specifying the package they belong to
• Allows access control
• Another level of encapsulation

Question 3

How to create packages?

Answer 3

package <directory_name_1>.<directory_name_2>;</directory_name_2></directory_name_1>

This implies that the class in directory_2, which is a sub-directory of directory_1

Question 4

How to use java packages?

Answer 4

To use classes in a package, the import statement, which can take one of the following forms must be used:

Question 5

What is the definition of default package?

Answer 5

All the classes in the current directory belong to an unnamed package called the default package - no package statement is needed

Question 6

What is the definition of Information Hiding?

Answer 6

Ability to “hide” the details of a class from the outside world

Referred to as Visibility Control

Question 7

What is the definition of access control?

Answer 7

Preventing an outside class from manipulating the properties of another class in undesired ways

Question 8

Why do we use information hiding?

Answer 8

• Allows to safely seal data within the capsule of the class
• Prevents programmers from relying on details of class implementation
• Helps in protecting against accidental or wrong usage
• Keeps code elegant and clean (easier to maintain)
• Enables to provide access to the object through a clean interface

Question 9

What is the definition of visibility modifiers?

Answer 9

a list of keywords that are used to hide information

Question 10

What is the definition of public?

Answer 10

Keyword when applied to a class, method or attribute makes it available/visible everywhere (within the class and outside the class)

Question 11

What is the definition of private?

Answer 11

Keyword when applied to a method or attribute of a class, makes them only visible within that class.
- Private methods and attributes are not visible within subclasses and are not inherited

Question 12

What is the definition of protected?

Answer 12

Keyword when applied to a method or attribute of a class, makes them only visible within that class, subclasses and also within all classes that are in the same package as that class. They are also visible to subclasses in other packages.

Question 13

What is the definition of mutable?

Answer 13

A class that contains public mutator methods or other public methods that can change the instance variables is called a mutable class, and its objects are called mutable objects

Question 14

What is the definition of immutable?

Answer 14

A class that contains no methods (other than constructors) that change any of the instance variables is called an immutable class, and its objects are called immutable objects

Question 15

What is the definition of delegation?

Answer 15

When a class delegates its responsibilities to other classes?

This is an Association relationship between the classes

Question 16

What is the definition of wrapper?

Answer 16

A class that gives extra functionality to primitives like int, and lets them behave like objects

Question 17

What is the definition of primitive?

Answer 17

A unit of information that contains only data, and has no attributes or methods

Question 18

What does wrapper classes do?

Answer 18

• Allows primitive data types to be “packaged” or “boxed” into object
• Allows primitives to “pretend” that they are classes
• Provides extra functionality

Question 19

What methods does the Integer Class provide?

Answer 19

• Reverse: Integer.reverse(10)
• Rotate Left: Integer.rotateLeft(10, 2)
• Signum: Integer.signum(-10)
• Parsing Integer.parseInt(”10”)

Question 20

What is the definition of parsing?

Answer 20

Processing one data type into another