Chapter 4:Object-Oriented Basic Concepts

Object-Oriented Basic Concepts

1

• Exam Objective 1.1 Describe, compare, and contrast
primitives (integer, floating point,boolean, and character),
enumeration types, and objects.
• Exam Objective 3.1 Develop code that uses primitives,
enumeration types, and object references, and recognize
literals of these types.

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Primitive Data Types
All numeric primitive types are signed.

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1. A signed data type has an equal number of non-zero
positive and negative values available.
A. True
B. False

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1. B. The range of negative numbers is greater by one
than the range of positive numbers.

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9. What is the range of values that can be assigned to a
variable of type short?
A. Depends on the underlying hardware
B. 0 through 216 − 1
C. 0 through 232 − 1
D. −215 through 215 − 1
E. −231 through 231 − 1

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9. D. The range for a 16-bit short is −215 through 215 −
1. This range is part of the Java
specification,regardless of the underlying hardware.

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10. What is the range of values that can be assigned to
a variable of type byte?
A. Depends on the underlying hardware
B. 0 through 28 − 1
C. 0 through 216 − 1
D. −27 through 27 − 1
E. −215 through 215 − 1

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10. D. The range for an 8-bit byte is −27 through 27 −1.
Table 1.3 lists the ranges for Java’s integral primitive
data types.

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Literals
• Literals can represent primitive or string variables and may appear on the right side of assignments
or in method calls.
• The only valid literals of boolean type are true and false. For example:
• 1. boolean isBig = true;
• 2. boolean isLittle = false;
• A chararacter literal (char) represents a single Unicode character. char c = ’w’;
• Of course, this technique works only if the desired character is available on the keyboard at
hand.Another way to express a char literal is as a Unicode value specified using four hexadecimal
digits, preceded by u, with the entire expression in single quotes. For example:
• Char c1 = ’u4567’;
• ’n’ for new line
• ’r’ for return
• ’t’ for tab
• ’b’ for backspace
• ’f’ for formfeed
• ’’’ for single quote
• ’” ’ for double quote
• ’’ for backslash

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• Integral literals may be assigned to any numeric primitive data type. They may be expressed
in decimal, octal, or hexadecimal. The default is decimal. To indicate octal, prefix the literal
with 0 (zero). To indicate hexadecimal, prefix the literal with 0x or 0X; the hex digits may be
upper- or lowercase. The value 28 may thus be expressed six ways:
• 28
•

034

•

0x1c

•

0x1C

•

0X1c

•

0X1C

• By default, an integral literal is a 32-bit value. To indicate a long (64-bit) literal, append the
suffix L to the literal expression. (The suffix can be lowercase, but then it looks so much like a
one that your readers are bound to be confused.)
• The letter E or e, indicating scientific notation, such as 4.23E+21
•

The suffix F or f, indicating a float literal, such as 1.828f

•

The suffix D or d, indicating a double literal, such as 1234d

A floating-point literal with no F or D suffix defaults to double type.

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String Literals
• A string literal is a sequence of characters enclosed in
double quotes. For example:
• String s = “Characters in strings are 16-bit Unicode.”;

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2. Choose the valid identifiers from those listed here.
(Choose all that apply.)
A.BigOlLongStringWithMeaninglessName
B. $int
C. bytes
D. $1
E. finalist

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2. A, B, C, D, E. All of the identifiers are valid. An
identifier begins with a letter, a dollar sign, or an
underscore; subsequent characters may be letters, dollar
signs, underscores, or digits. And ofcourse keywords and
their kin may not be identifiers.

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16. Which of the following are legal? (Choose all that
apply.)
A. int a = abcd;
B. int b = ABCD;
C. int c = 0xabcd;
D. int d = 0XABCD;
E. int e = 0abcd;
F. int f = 0ABCD;
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16. C, D. The characters a–f and A–F may be combined
with the digits 0–9 to create a hexadecimal literal, which
must begin with 0x.

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17. Which of the following are legal? (Choose all that
apply.)
A. double d = 1.2d;
B. double d = 1.2D;
C. double d = 1.2d5;
D. double d = 1.2D5;

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17. A, B. The d suffix in option A and the D suffix in
option B are optional. Options C and D are illegal
because the notation requires e or E, not d or D.

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18. Which of the following are legal?
A. char c = 0x1234;
B. char c = u1234;
C. char c = ‘u1234’;

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18. C. A Unicode literal character must be enclosed in
single quotes and must begin with u.

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Arrays
• A Java array is an ordered collection of primitives, object references, or other arrays.
• A method that takes an array of doubles could be declared as myMethod(double dubs[]) or as
myMethod(double[] dubs);
• a method that returns an array of doubles may be declared as either double[] anotherMethod()
or as double anotherMethod()[].
• When an array is constructed, its elements are automatically initialized to their default values.

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int[][] myInts = { {1, 2, 3}, {91, 92, 93, 94}, {2001, 2002} };
When you realize that the outermost array is a single-dimension array containing references,
you understand that you can replace any of the references with a reference to a different
subordinate array, provided the new subordinate array is of the right type. For example, you
can do the following:
int[][] myInts = { {1, 2, 3}, {91, 92, 93, 94}, {2001, 2002} };
int[] replacement = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};

myInts[1] = replacement;

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Variables and Initialization
Member variable A member variable of a class is created when an
instance is created, and it is destroyed when the object is destroyed.
Subject to accessibility rules and the need for a reference to the object,
member variables are accessible as long as the enclosing object exists.
Automatic variable An automatic variable of a method is created on entry
to the method and exists only during execution of the method, and
therefore it is accessible only during the execution of that method. (You’ll see
an exception to this rule when you look at inner classes, but don’t worry
about that for now.)
Class variable A class variable (also known as a static variable) is created
when the class is loaded and is destroyed when the class is unloaded.
There is only one copy of a class variable, and it exists regardless of the
number of instances of the class, even if the class is never instantiated.
Static variables are initialized at class load time
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All member variables that are not explicitly assigned a value upon declaration are
automatically assigned an initial value. The initialization value for member
variables depends on the member variable’s type.
A member value may be initialized in its own declaration line:
1. class HasVariables {
2. int x = 20;
3. static int y = 30;
When this technique is used, nonstatic instance variables are initialized just before
the class constructor is executed; here x would be set to 20 just before invocation
of any HasVariables constructor.

Static variables are initialized at class load time; here y would be set to 30 when the
HasVariables class is loaded.

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Automatic variables (also known as method local variables are not initialized by the system; every automatic variable must be explicitly
initialized before being used. For example, this method will not compile:

1. public int wrong() {

2. int i;

3. return i+5;

4. }
The compiler error at line 3 is, “Variable i may not have been initialized.” This error often appears when initialization of an automatic variable
occurs at a lower level of curly braces than the use of that variable. For example, the following method returns the fourth root of a
positive number:
1. public double fourthRoot(double d) { 2. double result;
3. if (d >= 0) {
4. result = Math.sqrt(Math.sqrt(d));
5. } 6. return result; 7. }
Here the result is initialized on line 4, but the initialization takes place within the curly braces of lines 3 and 5. The compiler will flag line 6,
complaining that “Variable result may not have been initialized.” A common solution is to initialize result to some reasonable default as
soon as it is declared:
1. public double fourthRoot(double d) {

2. double result = 0.0; // Initialize
3. if (d >= 0) {
4. result = Math.sqrt(Math.sqrt(d));
5. } 6. return result; 7. }

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5. Consider the following line of code:
int[] x = new int[25];
After execution, which statements are true? (Choose all that
apply.)
A. x[24] is 0
B. x[24] is undefined
C. x[25] is 0
D. x[0] is null
E. x.length is 25
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5. A, E. The array has 25 elements, indexed from 0
through 24. All elements are initialized to 0.

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Argument Passing: By Reference or by
Value
1. public void bumper(int bumpMe) {
2. bumpMe += 15;
3. }
Line 2 modifies a copy of the parameter passed by the
caller. For example
1. int xx = 12345;
2. bumper(xx);
3. System.out.println(“Now xx is “ + xx);
line 3 will report that xx is still 12345.
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When Java code appears to store objects in variables or pass
objects into method calls, the object references are stored or
1. Button btn;
passed.

2. btn = new Button(“Pink“);
3. replacer(btn);
4. System.out.println(btn.getLabel());
5.
6. public void replacer(Button replaceMe) {
7. replaceMe = new Button(“Blue“);
8. }
Line 2 constructs a button and stores a reference to that button in
btn. In line 3, a copy of the reference is passed into the replacer()
method.
the string printed out is “Pink”.

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1. Button btn;
2. btn = new Button(“Pink“);
3. changer(btn);
4. System.out.println(btn.getLabel());
5.
6. public void changer(Button changeMe) {
7. changeMe.setLabel(“Blue“);
8. }
the value printed out by line 4 is “Blue”.
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Arrays are objects, meaning that programs deal with
references to arrays, not with arrays themselves. What
gets passed into a method is a copy of a reference to an
array. It is therefore possible for a called method to
modify the contents of a caller’s array.

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6.Consider the following application:
class Q6 {
public static void main(String args[]) {
Holder h = new Holder();
h.held = 100;
h.bump(h);
System.out.println(h.held);
}
}
class Holder {
public int held;
public void bump(Holder theHolder) {
theHolder.held++; }
}
}
What value is printed out at line 6?
A. 0
B. 1
C. 100
D. 101

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6. D. A holder is constructed on line 3. A reference to
that holder is passed into method bump() on line 5.
Within the method call, the holder’s held variable is
bumped from 100 to 101.

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7. Consider the following application:
1. class Q7 {
2. public static void main(String args[]) {
3. double d = 12.3;
4. Decrementer dec = new Decrementer();
5. dec.decrement(d);
6. System.out.println(d);
7. }
8. }
9.
10. class Decrementer {
11. public void decrement(double decMe) {
12. decMe = decMe - 1.0;

13. }
14. }
Review Questions 31
What value is printed out at line 6?
A. 0.0
B. 1.0

C. 12.3
D. 11.3

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7. C. The decrement() method is passed a copy of the
argument d; the copy gets decremented, but the
original is untouched.

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20. Which of the following are true? (Choose all that
apply.)
A. Primitives are passed by reference.
B. Primitives are passed by value.
C. References are passed by reference.
D. References are passed by value.

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20. B, D. In Java, all arguments are passed by value.

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The Object Class
The Object class is the ultimate ancestor of all Java
classes. If a class does not contain the extends keyword
in its declaration, the compiler builds a class that extends
directly from Object
if (d1 == d2)
The comparison will be true if the reference in d1is equal to
the reference in d2 . Of course, this is the case only when
both variables refer to the same object.
public boolean equals(Object object)

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The Object class provides a clone() method, which returns a copy of the
current object. In other words, the clone has the same class as the
original, and all its data values are identical.
Thus all references in the clone point to the same objects as those pointed
to in the original.Object’s version of clone() is protected, so a class’
clone() may not be called by any code anywhere. If you want a class’
clone() to be public, you need to insert something like the following:
public Object clone() throws CloneNotSupportedException {
return super.clone();
}
Notice the CloneNotSupportedException. It is not a runtime exception, so it
must be dealt with. Classes that override clone() generally declare that
they implement java.lang.Cloneable, which defines the single clone()
method.
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13. Which of the following are legal clone() methods in a
class called Q13 that extends Object?
• A. public Object clone() { return super.clone(); }
• B. public Object clone() throws
CloneNotSupportedException
{ return super.clone(); }
• C. public Q13 clone() { return (Q13)super.clone(); }
• D. public Q13 clone()
throws CloneNotSupportedException { return
(Q13)super.clone(); }

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13. B, D. The CloneNotSupportedException must be dealt
with, so A and C are wrong. The version being overridden (in
Object) has return type Object, so prior to release 5.0 the return
type in D would be illegal; however, now that covariant returns
are legal, D is allowed.


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Enums
• 5.0 introduces the enum, which is a class with certain
added functionality and also certain restrictions.
• Enums are subclasses of java.lang.Enum.

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public class TrafficLight {
private int state; // 1 means red
// 2 means yellow
// 3 means green
public int getState() {
return state;
}
}
Assume that the class also contains timing code to change
the state at appropriate intervals.
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This class is extremely prone to error. Anyone who modifies the class or calls
getState() has to know that 1 means red, 2 means yellow, and 3 means green. If
anyone’s code gets confused even for a moment about which value means which
color, the resulting bug could be very difficult to track down.
The common solution is to introduce constants:
public class TrafficLight {
public final static int RED = 1;
public final static int YELLOW = 2;
public final static int GREEN = 3;
private int state;
public int getState() {
return state;
}
}
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This code compares the light’s state only to constants
defined in the TrafficLight class, never to literal integer
values. You can hope people who call getState() use this
approach;
you can insist they do so in daily e-mail memos; but you
can’t guarantee they will do so.
Our code so far has numerous drawbacks, especially these:
It is possible to assign an out-of-range value to a light’s
state. Printing a light’s state is not very informative.

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A more robust approach is shown here:

1. public class LightState {
2. protected final String name;
3.
4. public final static LightState RED = new LightState("red");
5. public final static LightState YELLOW = new LightState("yellow");
6. public final static LightState GREEN = new LightState("green");
7.
8. private LightState(String s) {
9. name = s;
10. }
11.
12. public String name() {
13. return name;
14. }
15. }

The new approach represents states by using instances of the LightState class, rather than
ints. Note that the constructor is private, so you can never construct any instances from
outside the class or its subclasses; the only instances you will ever need are constructed
statically.
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You can’t use the new class as a switch statement argument as you
could with the ints of the previous example, but that’s a small price
to pay. Our original switching code becomes
LightState state = nextTrafficLight.getState();
if (state == LightState.RED)
stop();
else if (state == LightState.YELLOW)
floorIt();
else if (state == LightState.GREEN)
proceed();
else
assert false : “null light state.”;
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• This design ensures that the only possibly unexpected
state value for a traffic light is null.
• Also, debugging code can print out any LightState
instance and produce an informative result. In fact, this
approach is so useful that it has a name: the typesafe
enumeration design pattern. It is considered typesafe
because there is a dedicated type whose sole purpose is
to encode a few enumerated states, thus avoiding the
dangers of using arbitrarily assigned int values.

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•
•
•
•

public class A extends Enum { } // erreur
public class A extends Option { } // erreur
public enum Option extends A { } // erreur
public enum Option extends Option { } // erreur

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public enum Age {
jeune(20), mure(40), agé(60), vieux(80), cadavérique(999);

Age(int année) {
this.année=année;
}
private final int année;
private static Age getAge(int année) {
for(Age age:Age.values())
if (age.année>=année)
return age;
return cadavérique;
}
public static void main(String[] args) {

System.out.println(getAge(
new Scanner(System.in).nextInt()));
} // 35 mure

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public enum Option {
l{
public void performs() {
System.out.println("long");
}
}, a {
System.out.println("all");
}
}, v {
System.out.println("verbose");
}
};
public abstract void performs();
public static void performs(Set<Option> set) {
for(Option option:set)
option.performs();
}
}

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public interface Performer {
public void performs();
}
public enum Option implements Performer {
l{
System.out.println("long");
}
},
a{
System.out.println("all");
}
};
}

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• You can declare an enum anywhere you can declare a class.
• Compiling an enum generates a .class file whose name is derived from
the enum’s name.
• Enums inherit data and methods from Object.
• Enums may be converted and cast according to the same rules that
govern any class that extends Object.
• Enums may have main() methods and can be invoked as applications.
• Enums that have no explicit constructors get default no-args
constructors.
• Enums have restricted functionality. They are unlike traditional classes
in the following ways:
• Enums are declared with the enum keyword rather than the class
keyword.
• Enums may not be instantiated.
• Enums may not extend anything and may not be extended.
• Enums may be arguments in switch statements.
• Enums have built-in name() and toString() methods, both of which
return the name of the current instance.
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switch (nextTrafficLight.getState()) {
case LightState.RED:
stop();
break;
case LightState.YELLOW:
floorIt();
break;
case LightState.GREEN:
proceed();
break;
default:
assert false: “null light state”;
public enum LightState {
}
RED, YELLOW, GREEN;
}

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enum Quark {
UP('u', 2.0/3.0),
DOWN('d', -1.0/3.0),
CHARM('c', 2.0/3.0),
STRANGE('s', -1.0/3.0),
TRUTH('t', 2.0/3.0),
BEAUTY('b', -1.0/3.0);
private final char symbol;
private final double charge;

Quark(char symbol, double charge){
this.symbol = symbol;
this.charge = charge;
}
public char getSymbol(){
return symbol;
}
public double getCharge(){
return charge;
}
}

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10. Which of the following are legal enums?
A. enum Animals { LION, TIGER, BEAR }
B. enum Animals {
int age;
LION, TIGER, BEAR;
}
C. enum Animals {

LION, TIGER, BEAR;
int weight;
}
D. enum Animals {
LION(450), TIGER(450), BEAR;
int weight;
Animals(int w) {
weight = w;
}
}
E. enum Animals {
LION(450), TIGER(450), BEAR;
int weight;

Animals() { }
Animals(int w) {
weight = w;
}
}

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10. C, E. A is illegal because the list of names must be
terminated by a semicolon. B is illegal because the list
of names must be the first element in the enum body. C is
a legal enum that contains, in addition to its name list, a
variable. D is illegal because the declaration of Bear
requires the existence of a no-args constructor. E fixes
the bug in D by adding a no-args constructor.

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apply.)
• A. An enum definition should declare that it extends
java.lang.Enum.
• B. An enum may be subclassed.
• C. An enum may contain public method definitions.
• D. An enum may contain private data.

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12. C, D. Enums may not extend or be extended. They
may contain methods and data just like ordinary
classes.

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apply.)
A. An enum definition may contain the main() method of
an application.
B. You can call an enum’s toString() method.
C. You can call an enum’s wait() method.
D. You can call an enum’s notify() method.

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13. A, B, C, D. Enums may contain public static void
main() methods and may serve as application main
classes. Enums inherit from Object, so they have
toString(), wait(), and notify() methods.

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14. Suppose x and y are of type TrafficLightState, which
is an enum. What is the best way to
test whether x and y refer to the same constant?
A. if (x == y)
B. if (x.equals(y))
C. if (x.toString().equals(y.toString()))
D. if (x.hashCode() == y.hashCode())

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14. A. It is never possible to have two instances of an enum
that represent the same value. So the == operator is
reliable, and it’s faster than any method call.

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16. Given the following code, which of the following will
compile?
enum Spice { NUTMEG, CINNAMON, CORIANDER,
ROSEMARY; }
A. Spice sp = Spice.NUTMEG; Object ob = sp;
B. Spice sp = Spice.NUTMEG; Object ob = (Object)sp;
C. Object ob = new Object(); Spice sp = object;
D. Object ob = new Object(); Spice sp = (Spice)object;

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16. A, B, D. Enums may be converted to Object, just like
other objects. So A and B are legal, though the cast in
B is not necessary. Assigning an Object reference to an
enum requires a cast, so C is illegal, but D is legal

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18. Which methods return an enum constant’s name?
A. getName()
B. name()
C. toString()
D. nameString()
E. getNameString()

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18. B, C. Both name() and toString() return a constant’s
name. name() is final, but toString() can be
overridden.

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Chapter 4:Object-Oriented Basic Concepts

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Chapter 4:Object-Oriented Basic Concepts