Enum Types. Built-in support for types of discrete values Advantages over C++ s enum: Type-safety Body can include methods and fields

Transcription

1 Enum Types Built-in support for types of discrete values Advantages over C++ s enum: Enum declaration defines a class Type-safety Body can include methods and fields Values may be objects Support for iteration, naming 1

2 Representing a Set of Predefined Values The Old Way public class OldTrafficLight { public static final int RED = 1; public static final int YELLOW = 2; public static final int GREEN = 3; private int c = RED; public int getcolor() { return c; public void setcolor(int newcol) { if(newcol >= RED && newcol <= GREEN) c = newcol; 2

3 Representing a Set of Predefined Values With Enum Types public class TrafficLight { public enum Colors { RED, YELLOW, GREEN private Colors c = Colors.RED; public Colors getcolor() { return c; public void setcolor(colors newcol) { c = newcol; 3

4 The TrafficLight class - cont. public class TrafficLight {... public void setcolor(int n) { Colors[] temp = Colors.values(); setcolor(temp[n]); public boolean iscolor(int n) { return c.ordinal() == n; public void setcolor(string colorname) { setcolor(colors.valueof(colorname)); public boolean iscolor(string colorname) { return c.name().equals(colorname); 4

5 Using enum types import static TrafficLight.Colors.*; public void main() { TrafficLight tl = new TrafficLight(); System.out.println(tl.getColor()); tl.setcolor(yellow); // <-- Note the static import System.out.println(tl.getColor()); // YELLOW System.out.println(tl.isColor("YELLOW")); // true System.out.println(tl.isColor(2)); // false tl.setcolor(2); System.out.println(tl.getColor()); // GREEN tl.setcolor("red"); System.out.println(tl.getColor()); // RED { 5

6 Switching on Enums public class TrafficLight {... public enum Colors { RED, YELLOW, GREEN; public Colors next() { switch(this) { case RED: return YELLOW; case YELLOW: return GREEN; default: return null; 6

7 An enum type with methods public class TrafficLight { public enum Colors { RED { public Colors next() { return YELLOW;, YELLOW { public Colors next() { return GREEN;, GREEN; public Colors next() { return null; private Colors c = Colors.RED; public Colors getcolor() { return c; public void setcolor(colors nc) { c = nc; 7

8 Maps of Enums EnumMap is a high performance map implementation for enums. Implemented as an array. public class TrafficLight { EnumMap<Colors, String> colormsgs = new EnumMap<Colors, String>(Colors.class); colormsgs.put(red, stop ); colormsgs.put(yellow, get ready ); colormsgs.put(green, go ); 8

9 Enums - Summary Enums are classes Extend java.lang.enum Might implement interfaces Enums have no public constructor removes the ability to create additional instances of the enum not defined at compile-time Enum values are public, static, and final Values cannot be changed The enum can t be subclassed Enums override tostring( ) TrafficLight.Colors.RED.toString() returns the String RED. 9

10 Annotations The goal: Allow the programmer to provide additional information about the program This information can be used by software engineering tools An annotation is a type defined using an interface-like syntax An annotation can be specified whenever a modifier is allowed Convention: before the public/static modifiers Annotations do not affect semantics of the class But may affects semantics of things using the class (tools, code generation, runtime options, etc.) 10

11 Predefined Assert intention to override a method in a superclass Compiler fails if not actually overriding Checks spelling, override vs. Indicates that an element should not be Tells the compiler to suppress specific warnings 11

12 Definition of two annotation types // Author.java: Author { String value() default "Unknown"; // MethodKind.java: import java.lang.annotation.elementtype; import MethodKind { boolean composite() default false; boolean mutator() default false; 12

13 package my.prg; 13 public class Rational extends Number { private long a, b; public Rational(long a, long b) { this.a = a; this.b = b; private Double asdouble() { return new Double(a * 1.0 / public short shortvalue() { return asdouble().shortvalue();

14 Using public class Rational extends mutator=true) public void assign(long n) { a = n; b = @MethodKind(composite=true, mutator=false) public byte bytevalue() { return asdouble().bytevalue(); 14

15 Marker annotations Kinds of Annotations Have Single value annotations Provide a single piece of data. Attribute type can be primitive, class, enum, or Pazit unchecked, deprecation ) Multi valued annotations mutator=false)

16 What Can Be Annotated? Any program element Package Types Class, Interface, Enum definition, Annotation type Method, Constructor, Field, Enum constant, Method parameter Local varible declaration 16

17 Meta-Annotations Annotations that annotate annotations Specify how the annotation should be Javadoc should be generated when this annotation is applied to an Does the annotation get applied to Where the annotation can be used (source elements) Default is Where is the annotation retained 17

18 Meta-Annotations CONSTRUCTOR, class foo { public foo(){ int x; public void int y; //NO 18

19 Reflection and Annotations Annotations marked are available via reflection. Class, Constructor, Field, Method, Package have methods to handle annotations: isannotationpresent getannotations getdeclaredannotations 19

20 Inspecting Annotations using Reflection public class AuthorPrinter { static void printmethodsauthor(class c){ for(method m: c.getmethods()) if(m.isannotationpresent(author.class)) { Author a = m.getannotation(author.class); System.out.println(m.getName() + " by " + a.value()); Author annotation will be accessible only if its declaration was annotated 20

21 Usage Example class TestAnnotations public void f() { Output public void g(){ public static void main(string[] args){ AuthorPrinter.printMethodsAuthor (TestAnnotations.class); f by Unknown g by Pazit 21

22 VarArgs private static String format(object... values) { StringBuilder sb = new StringBuilder( ); for (Object o : values) sb.append(o).append(" "); return sb.tostring( ); public static void main(string[] args) { System.out.print(format(args)); System.out.print(format()); System.out.print(format(new Integer(7), new Date(), hello, 2)); 22