Use interfaces to define constants?
public interface ScienceConstants {
static final double PI = 3.14;
static final double BOLTZMANN_CONSTANT = 1.3806503e-23;
}
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Alternative 1
 Use Enums

Enums in Java are full fledged classes (Java 1.5)

More powerful than enums in C++
•
where they are just int values
 By nature, Enums are immutable


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All fields should be final
Make the fields private and provide accessors/mutators
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Do NOT use interfaces to define
constants
 The use of constants in a class should be
considered implementation detail
 If a non-final class implements an interface, all its
subclasses will have their namespaces polluted by
the constants in the interface
 Java uses constant interfaces in its libraries
(example: java.io.ObjectStreamConstants)

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treat these as anomalies
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Alternative 2
 Use Constant Utility class
public class PhsyicalConstants {
private PhysicalConstants() { } //prevent instantiation
static final double PI = 3.14;
static final double BOLTZMANN_CONSTANT = 1.3806503e-23;
}
Usage:
double circ = 2 * PhysicalConstants.PI * radius;
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Alternative 2: continued
 Use Constant Utility class
 If it is heavily used, then you don’t need to qualify it
each time

use static import facility, introduced in java 1.5
public class MyTest {
import static PhysicalConstants.*;
double myMethod () {
double circ = 2 * PI * radius;
…
}
}
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More on Enums
 Java Enums are classes that export one instance
for each enumeration constant via a public static
final field

Enum types are effectively final



no accessible constructor
clients can neither create instances or extend it
so, enum types are instance controlled
 Enums are generalization of Singletons
 Singleton is a single element enum
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More on Enums (2)
 Enums provide compile-time type safety
 Enums with identically named constants can coexist

as each type has its own namespace
 Can augment Enums with methods

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just like any Java class
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More on Enums (3)
 Has a static values(…) method that returns an
array of its values in the order it was declared

example in next slides
 Default toString() returns the declared name of
each enum value

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can be overridden
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Enum Example
public enum CourseAnalysis {
cs240 (4, 81.5, 55),
cs342 (4, 84.3, 35),
cs654 (3, 79.8, 41); // semi colon here
private final int numCredits;
private final double avgScore;
private final int numStudents;
private static final double NO_OF_EXAMS = 3;
// Constructor
CourseAnalysis(int numCreditsIn, double avgScoreIn, int numStudentsIn) {
numCredits = numCreditsIn;
avgScore = avgScoreIn;
numStudents = numStudentsIn;
}
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Enum Example (continued)
public int getNumCredits() { return numCredits; }
public double getAvgScore() { return avgScore; }
public int getNumStudents() { return numStudents; }
// dummy method, just an example
// note: calculation is not intended to make sense …
public double someWeightedAverage(double weight) {
return weight * numCredits * avgScore / numStudents;
}
}
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Enum: can’t instantiate like a Java object
// Try calling the constructor
CourseAnalysis cA = new CourseAnalysis(3, 3.14, 7);
Driver.java:91: enum types may not be instantiated
[javac]
CourseAnalysis cA = new CourseAnalysis(3, 3.14, 7);
[javac]
^
[javac] 1 error
BUILD FAILED
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Enum: cannot directly access
private data members
// try to access the private data member directly
System.out.printf("Enum member: average score is %f ",
CourseAnalysis.cs342.avgScore);
Error:
avgScore has private access in test.util.CourseAnalysis
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Enum: default toString()
System.out.printf("Enum member is %s %n", CourseAnalysis.cs342);
Output:
[java] Enum member is cs342
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Enum: accessing methods
// access methods like a standard Java object
System.out.printf("Enum member: average score is %f %n",
CourseAnalysis.cs342.getAvgScore());
Output:
[java] Enum member: average score is 84.300000
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Enum: foreach
double weightValue = 0.77;
for (CourseAnalysis c : CourseAnalysis.values()) {
System.out.printf("WeightedAvg of %s is %f %n", c,
c.someWeightedAverage(weightValue));
}
[java] WeightedAvg of cs240 is 4.564000
[java] WeightedAvg of cs342 is 7.418400
[java] WeightedAvg of cs654 is 4.496049
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Design Guideline
The constant interface pattern is a poor use of
interfaces
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