Type Safety and Enumerations Type Checking and Type

Type Safety and Enumerations

Type Checking and Type Errors The type system defines data types and the operations on data types. q A type error occurs if an operation is performed on a value where the operation isn't defined or not welldefined. n char c; scanf("%f", &c); /* error: double => char */ q The translator (compiler, interpreter) seeks to avoid type errors by type checking. n Type checking can be done statically or dynamically. q Languages like Java also catch type errors at run-time and raise ("throw") exceptions. q

Run-time Type Checking Student s = new Student("joe"); Object o = s. clone(); JFrame t = (JFrame) o; throws a Class. Cast. Exception

Type Safety (1( A program is considered "type safe" if it can be verified that it is free of type errors. q In strongly typed languages like Java, this would seem like an easy task. . . but its not! q Is the java. util. Array. List class type safe? q

Type Safety (2( q Is the java. util. Array. List class type safe? java. util. Arrays from the Java 2 SDK API static void sort( Object [ ] array ); import java. awt. Color; . . . Object [ ] a = { new Double(0. 5), Color. RED, "hello" }; java. util. Arrays. sort( a ); // run-time Class. Cast. Exception !

Type Safety (2( Arrays. sort throws a Class. Cast. Exception. q Programmer isn't required to catch Runtime. Exceptions, so this will compile OK but fail at run-time. Another example: q Is the java. util. Date class type safe? q java. util. Date from the Java 2 SDK API public Date(int year, int month, int day); // Date(year-1900, month /* jan=0, feb=1 */, dom ) int year = 2000; int month = 0; // jan = 0, feb = 1, mar = 2, . . . int day = 1; // day of month Date newyear = new Date( year - 1900, month, day );

Type Safety (3( q But, its sooo easy to forget. . . n Date( year, month, day ) ? ? ? n Date( day, month, year ) ? ? ? n Date( month, day, year ) ? ? ? int year = 2000 - 1900, month = 0, day = 1; Date d 1 = new Date( year, month, day ); Date d 2 = new Date( day, month, year ); Date d 3 = new Date( month, day, year ); d 1. to. String() --> "Sat Jan 01 00: 00 ICT 2000" d 2. to. String() --> "Wed Apr 10 00: 00 ICT 1901" d 3. to. String() --> "Fri May 11 00: 00 ICT 1900" more coffee, please

Type Safety (4( q q Calendar class has static constants for the months, but the values are same as "int". n Calendar. JANUARY = 1 n Calendar. FEBRUARY = 2 They doesn't help type checking or type safety. int year = 2000 - 1900, month = 0, day = 1; Date d 1 = new Date( year, Calendar. JANUARY, day ); Date d 2 = new Date( day, Calendar. JANUARY, year ); Date d 3 = new Date( Calendar. JANUARY, day, year ); Its your fault!!!! You should have been more careful.

Enumerations: promoting type safety q Enumerated types were introduced in Pascal. type months = ( jan, feb, mar, apr, . . . , dec ); procedure date(Year: int; Month: months; Day: int) begin. . . end; (* main method *) var thismonth : months; (* variable of months type *) begin No brainer! thismonth : = jan; date(2000, thismonth, 1); compiler can verify argument is valid.

Java Enumerations: enum q "enum" is like a class when static instances of itself. public enum Month { /* the allowed values of this type: */ JANUARY, FEBRUARY, MARCH, The values of this enumeration APRIL, . . . DECEMBER; } public class My. Date { int year = 2005; Month month = Month. DECEMBER; int day = 9;

enum can have attributes, too q set the attributes in a constructor: public enum Month { /* the allowed values of this type: */ JANUARY("January", 0), FEBRUARY("February", 1), MARCH("March", 2), . . . DECEMBER; /* attributes */ public final String name; private int index; /* the constructor */ Month( String name, String index) { this. name = name; this. index = index; }

enum members are printable If an enum has a to. String( ) method, it is used. q If an enum doesn't have to. String( ), the enum member name is printed! q System. out. println( Month. JANUARY ); System. out. println( Month. JANUARY. name ); Output: JANUARY January

A Safer Date Class public class My. Date { /* attributes */ private int year; private Month month; private int day; /* the constructor */ public My. Date( int year, Month month, int day) { this. year = year; this. month = month; this. day = day; } public String to. String() { return ""+day+" "+month. name+" "+year; } }

My. Date q q My. Date requires that the 2 nd argument be a month: programmer can put in an invalid value. My. Date my = new Date(2005, Month. MAY, 1); Use the ". name" property to print month as string. My. Date d 1 = new My. Date( 2005, Month. DECEMBER, 9 ); My. Date d 2 = new My. Date( 2005, 12, 9 ); // Error System. out. println( "Today is " + d 1); Today is 9 December 2005

enum values() method q Java automatically supplies a method named values() that returns an array of all values in the enumeration. For the Month enum : Month [ ] mlist = Month. values( ); for( Month m : Month. values() ) // loop for all values System. out. println( m ); JANUARY FEBRUARY MARCH APRIL MAY JUNE. . .

enum for Token Types q Good OO design: object contains its own properties public enum Token. Type { /* the allowed values of this type: */ IDENT( "[A-Za-z_]\w*" ), NUMBER( "(\d+\. ? |\. \d)\d*([Ee][+-]? \d+)? " ), OPERATOR( "[-+*/^]" ), ASSIGN( "=" ), SEPARATOR( "[()]" ); /* attributes */ public final String regex; public final Pattern pattern; /* constructor */ Tokens( String regex ) { this. regex = regex; this. pattern = Pattern. compile(regex); }

Using Token Types q enum members are unique. You can use '==' to test if ( token. type() == Token. Type. IDENT ). . . if ( token. type() == Token. Type. NUMBER ). . . public class Token { /* token has a type and value */ private Token. Type type; private String value; Token(Token. Type type, Double value) { this. type = type; this. value = value; } /* return the token type */ Token. Type type() { return type; } }

Advantages of using Enumeration 1. Type safety. 2. Collect all properties of a token in a single place (token name, token regular expression, . . . ) 3. More efficient than using String for testing. 4. Avoid mistyping of String name in comparison. Compare: if ( token. type. equals("NUMBER") ). . . if ( token. type. equals("IDENT") ). . . If you use the wrong string value then the error is not detected, but the code won't work. if ( token. type == Token. NUMBER ). . . if ( token. type == Token. IDENT ). . .

How to Hide the enum q To minimize changes in your program, you can try. . . public class Token { public static Token. Type IDENT = Token. Type. IDENT; public static Token. Type NUMBER = Token. Type. NUMBER; . . . /* can make "type" attribute immutable public */ public final Token. Type type; . . . } if ( token. type == Token. IDENT ). . . if ( token type == Token. NUMBER ). . . same as Token. Type. NUMBER

Compare all token types q In your tokenizer class, you can use values: () public class Tokenizer { String s; Token. Type type = null; . . . /* can make "type" attribute immutable public */ for ( Token. Type t : Token. Type. values() ) if ( s. matches( t. regex ) ) type = t; token = new Token( s, type );