Lesson 3.4: Object-Oriented Programming

Question 1

Why are primitive types used over objects?

Answer 1

Primitive types، rather than objects، are used for these quantities for the sake of performance. Using objects for these basic types would add an unacceptable overhead to even the simplest of calculations. Thus، the primitive types are not part of the object hierarchy، and they do not inherit Object.

Question 2

In layman’s terms، what are type wrappers?

Answer 2

type wrappers، which are classes that encapsulate a primitive type within an object.

Question 3

What are the 8 type wrappers and what are they packaged in?

Answer 3

The type wrappers are Double، Float، Long، Integer، Short، Byte، Character، and Boolean، which are packaged in java.lang. These classes offer a wide array of methods that allow you to fully integrate the primitive types into Java’s object hierarchy.

Question 4

What are the most used type wrappers and what abstract class do they inherit?

Answer 4

437Probably the most commonly used type wrappers are those that represent numeric values. These are Byte، Short، Integer، Long، Float، and Double. All of the numeric type wrappers inherit the abstract class Number.

Question 5

What does the abstract class “Number” declare? What is the general form for its methods?

Answer 5

Number declares methods that return the value of an object in each of the different numeric types. These methods are shown here: byte byteValue( ) double doubleValue( ) float floatValue( ) int intValue( ) long longValue( ) short shortValue( )

Question 6

Though not used anymore، what are the general forms for the integer and double wrapper constructors? Are there any other wrapper types that get a constructor?

Answer 6

All of the numeric type wrappers define constructors that allow an object to be constructed from a given value، or a string representation of that value. For example، here are the constructors defined for Integer and Double: Integer(int num) Integer(String str) throws NumberFormatException Double(double num) Double(String str) throws NumberFormatException If str does not contain a valid numeric value، then a NumberFormatException is thrown. However، beginning with JDK 9، the type-wrapper constructors have been deprecated.

Question 7

Why is using a type constructor outdated for obtaining a wrapper object? What should you do to obtain a wrapper object? What is it’s general form?

Answer 7

Today، it is recommended that you use one of the valueOf( ) methods to obtain a wrapper object. The valueOf( ) method is a static member of all of the wrapper classes and all numeric classes support forms that convert a numeric value or a string into an object. For example، here are two forms supported by Integer: static Integer valueOf(int val) static Integer valueOf(String valStr) throws NumberFormatException Here، val specifies an integer value and valStr specifies a string that represents a properly formatted numeric value in string form.

Question 8

What is boxing and in what version did doing it manually become outdated?

Answer 8

The process of encapsulating a value within an object is called boxing. Prior to JDK 5، all boxing took place manually، with the programmer explicitly constructing an instance of a wrapper with the desired value

Question 9

What is unboxing?

Answer 9

The process of extracting a value from a type wrapper is called unboxing. Again، prior to JDK 5، all unboxing also took place manually، with the programmer explicitly calling a method on the wrapper to obtain its value.

Question 10

What is autoboxing and auto-unboxing?

Answer 10

Autoboxing is the process by which a primitive type is automatically encapsulated (boxed) into its equivalent type wrapper whenever an object of that type is needed. There is no need to explicitly obtain an object. Auto-unboxing is the process by which the value of a boxed object is automatically extracted (unboxed) from a type wrapper when its value is needed.

Question 11

How do you autobox a value?

Answer 11

You need only assign that value to a type-wrapper reference. Java automatically constructs the object for you. For example، here is the modern way to declare an Integer object that has the value 100: Integer iOb = 100; // autobox an int Notice that the object is not explicitly boxed. Java handles this for you، automatically.

Question 12

How do you auto-unbox a value?

Answer 12

To unbox an object، simply assign that object reference to a primitive-type variable. For example، to unbox iOb، you can use this line: int i = iOb; // auto-unbox Java handles the details for you.

Question 13

Give me an example of autoboxing/unboxing in a piece of code.

Answer 13

For example، consider the following program: Code to the right >

Question 14

Why are primitive types used instead of autoboxing & unboxing?

Answer 14

. It is far less efficient than the equivalent code written using the primitive type double. The reason is that each autobox and auto-unbox adds overhead that is not present if the primitive type is used.

Question 15

Why are parameterized types important? What is its relationship with generics?

Answer 15

At its core، the term generics means parameterized types. Parameterized types are important because they enable you to create classes، interfaces، and methods in which the type of data Page 453upon which they operate is specified as a parameter. A class، interface، or method that operates on a type parameter is called generic، as in generic class or generic method.

Question 16

What is similar in Java to C++’s “templates”?

Answer 16

Java generics are similar to templates in C++. What Java calls a parameterized type، C++ calls a template. However، Java generics and C++ templates are not the same، and there are some fundamental differences between the two approaches to generic types. For the most part، Java’s approach is simpler to use. A word of warning: If you have a background in C++، it is important not to jump to conclusions about how generics work in Java. The two approaches to generic code differ in subtle but fundamental ways.

Question 17

What is similar in Java to C++’s “templates”?

Answer 17

Java generics are similar to templates in C++. What Java calls a parameterized type، C++ calls a template. However، Java generics and C++ templates are not the same، and there are some fundamental differences between the two approaches to generic types. For the most part، Java’s approach is simpler to use. A word of warning: If you have a background in C++، it is important not to jump to conclusions about how generics work in Java. The two approaches to generic code differ in subtle but fundamental ways.

Question 18

What was the problem that created the need for generics in Java

Answer 18

Because Object is the superclass of all other classes، an Object reference can refer to any type of object. Thus، in pre-generics code، generalized classes، interfaces، and methods used Object references to operate on various types of data. The problem was that they could not do so with type safety because casts were needed to explicitly convert from Object to the actual type of data being operated upon. Thus، it was possible to accidentally create type mismatches.

Question 19

Give me an example of a “generic class”.

Answer 19

The following program defines two classes. The first is the generic class Gen، and the second is GenDemo، which uses Gen. Code example to the right > Here، T is the name of a type parameter. This name is used as a placeholder for the actual type that will be passed to Gen when an object is created. Notice that T is contained within < >. This syntax can be generalized. Whenever a type parameter is being declared، it is specified within angle brackets. In the declaration of Gen، there is no special significance to the name T. Any valid identifier could have been used، but T is traditional. it is recommended that type parameter names be single-character، capital letters.

Question 20

Give me a code fragment of a type argument. What do type arguments do?

Answer 20

Gen iOb; the type Integer is specified within the angle brackets after Gen. In this case، Integer is a type argument that is passed to Gen’s type parameter، T. This effectively creates a version of Gen in which all references to T are translated into references to Integer.

Question 21

What is the process of removing generic type information called?

Answer 21

The process of removing generic type information is called erasure

Question 22

What will happen if the type argument doesn’t match match the constructor type?

Answer 22

iOb = new Gen(88); when the Gen constructor is called، the type argument Integer is also specified. This is because the type of the object (in this case iOb) to which the reference is being assigned is of type Gen. Thus، the reference returned by new must also be of type Gen. If it isn’t، a compile-time error will result.

Question 23

When declaring an instance of a generic type، what types can you not use?

Answer 23

When declaring an instance of a generic type، the type argument passed to the type parameter must be a reference type. You cannot use a primitive type، such as int or char.

Question 24

Why is not being able to use primitive types not a restriction when declaring an instance of a generic type?

Answer 24

Of course، not being able to specify a primitive type is not a serious restriction because you can use the type wrappers (as the preceding example did) to encapsulate a primitive type. Further، Java’s autoboxing and auto-unboxing mechanism makes the use of the type wrapper transparent.

Question 25

How can you make a version of a generic constructor that conceptually has a different type?

Answer 25

Gen strOb = new Gen(“Generics Test”); Because the type argument is String، String is substituted for T inside Gen. This creates (conceptually) a String version of Gen، as the remaining lines in the program demonstrate.

Question 26

What is the syntax for declaring a generic class?

Answer 26

Here is the syntax for declaring a generic class: class class-name { // …

Question 27

What do bound types do?

Answer 27

you need some way to tell the compiler that you intend to pass only numeric types to T. Furthermore، you need some way to ensure that only numeric types are actually passed. To handle such situations، Java provides bounded types. When specifying a type parameter، you can create an upper bound that declares the superclass from which all type arguments must be derived.

Question 28

How do you declare a bounded type?

Answer 28

through the use of an extends clause when specifying the type parameter، as shown here: This specifies that T can be replaced only by superclass، or subclasses of superclass. Thus، superclass defines an inclusive، upper limit.

Question 29

Give me a code that accepts multiple type parameters in a generic.

Answer 29

You can declare more than one type parameter in a generic type. To specify two or more type parameters، simply use a comma-separated list. For example، the following TwoGen class is a variation of the Gen class that has two type parameters: Code example to the right >

Question 30

What is a wildcard argument?

Answer 30

The wildcard argument is specified by the ?، and it represents an unknown type.

Question 31

What is the full syntax for declaring a reference to a generic class and creating a generic instance?

Answer 31

Here is the full syntax for declaring a reference to a generic class and creating a generic instance: class-name var-name = new class-name(cons-arg-list);

Question 32

Give me an example of a code that uses bounded types.

Answer 32

You can use an upper bound to fix the NumericFns class shown earlier by specifying Number as an upper bound، as shown here: Code example to the right >

Question 33

Give me a code fragment that uses a wildcard argument to determine if two generic objects are the same.

Answer 33

Using a wildcard، here is one way to write the absEqual( ) method: Code example to the right>

Question 34

How do you declare an upper bound for a wildcard?

Answer 34

In general، to establish an upper bound for a wildcard، use the following type of wildcard expression: extends superclass>

Question 35

How do you declare a lower bound for a wildcard?

Answer 35

You can also specify a lower bound for a wildcard by adding a super clause to a wildcard declaration. Here is its general form: super subclass> In this case، only classes that are superclasses of subclass are acceptable arguments. This is an inclusive clause.

Question 36

In relation to the return type، where is the type parameter declared?

Answer 36

The type parameters are declared before the return type of the method.

Question 37

What does the Comparable interface do?

Answer 37

Comparable is an interface declared in java.lang. A class that implements Comparable defines objects that can be ordered. Comparable is generic، and its type parameter specifies the type of objects that it compares.

Question 38

How do wildcards affect what type of objects can be created?

Answer 38

One last point: It is important to understand that the wildcard does not affect what type of NumericFns objects can be created. This is governed by the extends clause in the NumericFns declaration. The wildcard simply matches any valid NumericFns object.

Question 39

Give me an example of a very basic generic class.

Answer 39

the following very simple generic class: Code example to the right > Gen takes one type parameter، which specifies the type of object stored in ob. Because T is unbounded، the type of T is unrestricted. That is، T can be of any class type.

Question 40

What is the syntax for a generic method?

Answer 40

Here is the syntax for a generic method: ret-type meth-name(param-list) { // … In all cases، type-param-list is a comma-separated list of type parameters.

Question 41

If a class is not generic، can constructor in the class be generic?

Answer 41

A constructor can be generic، even if its class is not.

Question 42

Give me a basic code example that uses a generic constructor but the class in not generic.

Answer 42

For example، in the following program، the class Summation is not generic، but its constructor is. Code example to the right >

Question 43

If generic interfaces are specified like generic classes، give me a code example that uses a generic interface.

Answer 43

Generic interfaces are specified just like generic classes. Here is an example. It creates an interface called Containment، which can be implemented by classes that store one or more values. It declares a method called contains( ) that determines if a specified value is contained by the invoking object. Code example to the right >

Question 44

What is the syntax for a generic interface?

Answer 44

Once the type parameter has been established، it is simply passed to the interface without further modification. Here is the generalized syntax for a generic interface: interface interface-name { // … Here، type-param-list is a comma-separated list of type parameters. When a generic interface is implemented، you must specify the type arguments، as shown here: