1. 1 Java basics Chapter 2 Spring 2005 CS 101 Aaron Bloomfield

2. 2 DisplayForecast.java // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } Three statements make up the action of method main() Method main() is part of class DisplayForecast // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } A method is a named piece of code that performs some action or implements a behavior // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } An application program is required to have a public static void method named main(). // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } public, static, and void are keywords. They cannot be used as names public means the method is shareable // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } We will discuss static and void later // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } Java allows a statement to be made up of multiple lines of text Semicolons delimit one statement from the next // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } A class defines an object form. An object can have methods and attributes Keyword class indicates a class definition follows // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } A class like a method must have a name // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } A class like a method must have a name // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } Programs are read by people – make sure they are readable. Use whitespace, comments, and indentation to aid understanding // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } // indicates rest of the line is a comment Comments are used to document authors, purpose, and program elements Three comments

3. 3 Indentation // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } Indentation indicates subcomponents Method main() is part of DisplayForecast Statements are part of method main()

4. 4 Good whitespacing // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } Whitespace separates program elements Whitespace between program elements is ignored by Java Whitespace

5. 5 Bad whitespacing  The same program without any whitespacing or comments: public class DisplayForecast2 { public static void main (String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } }

6. 6 A whitespacing aside: IOCCC The International Obfuscated C Code Contest The International Obfuscated C Code Contest –Online at http://www.ioccc.org C has very terse syntax C has very terse syntax –So the contest tries to make it terser! One common method is by modifying the whitespace One common method is by modifying the whitespace

7. 7 A whitespacing aside: IOCCC #define _ -F<00||--F-OO--; int F=00,OO=00;main(){F_OO();printf("%1.3f\n",4.*-F/OO/OO);}F_OO() { _-_-_-_ _-_-_-_ _-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-__-_-_-_-_-_-_-_-_-_-_-_-_-_-_-__-_-_-_-_-_-_-_-_-_-_-_-_-_-_-__-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_ _-_-_-_-_-_-_-_ _-_-_-_ _-_-_-_} #define X #define XX #define XXX #define XXXX #define XXXXX #define XXXXXX #define XXXXXXX #define orfa for #define XXXXXXXXX #define archa char #define ainma main #define etcharga getchar #define utcharpa putchar #include #define Q r=R[*p++-'0'];while( #define B ;break;case char*s="Qjou!s\\311^-g\\311^-n\\311^-c\\::^-q-ma%mO1JBHm%BQ-aP1J[O1HB%[Q<nbj\ o)*|gps)<<*txjudi)m*|aQdbtf!::::;sfuvso<aQefgbvmu;aQ<m,,a%CQ<csfbla%bQ<aN2!Q\ \ndbtf!aP2Q;m>aP2Q aP4HC%T\ Qs\\q,,^>m,2 aP4HC%SD12N1\nJNQm>s\\..q^aHC%NHb%GN1!D32P3%RN1UP1D12JPQUaP1H\ R%PN4\nQ aP2Q,2 aP4Hb%OD12D12N2!N3\nJVP3Q,, n\ \\(aP3Q(^*m>g\\(aP3Q(^<fmtf!m,,aHC%QN1!N1\nJ#Qqsjoug)#&e]o#-aP1Q*aHb%#Qqvut)\ aP1Q*aHb%FN1\nQm>::::aHC%VP3Q>bupj)hfut)c**aHb%JD12JON1!Qjg)a%LN1UP1D12JIQUa\ P1HL%IQ*m>aN2!N2\nP2Q P2Q>aN2\nP2Hbdd!b/d";k;char R[4][99] ;main(c,v)char**v;{char*p,*r,*q;for(q=s;*q;q++)*q>' '&&(*q)--;{FILE*i=fopen(v [1],"r"),*o=fopen(q-3,"w");for(p=s;;p++)switch(*p++){B'M':Q(k=fgetc(i))!=EOF &&k!=*p)*r++=k;if(k==EOF){fputs("}}\n",o);fclose(o);return system(q-6);}*r=0 B'P':while(*p!='`')fputc(*p++,o)B'O':Q*r)fputc(*r++,o);p--B'C':k=0;Q k<*p-'0' )(*r++=fgetc(i),k++);*r=0 B'I':k= *p;if(**R==k)goto G B'G':k= *p;G:p=s;while( *p!='$'||p[1]!= k)p++;p++B'N':R[*p-'0'][0]++;}}} X X X X X X X X X X X X X X X X X X X X X X X X X XX X X XX X X XX X X XX X X XXX X XXXXXXXXX X XXX X X XXX X XXXX XXXX X XXX X X XXXX X XX ainma(){ archa XX X XXXX X X XXXX X oink[9],*igpa, X XXXX X X XXXXXX atinla=etcharga(),iocccwa XXXXXX X X XXXX,apca='A',owla='a',umna=26 XXXX X X XXX ; orfa(; (atinla+1)&&(!((( XXX X X XX atinla-apca)*(apca+umna-atinla) XX X X X >=0)+((atinla-owla)*(owla+umna- X X X X >=0)+((atinla-owla)*(owla+umna- X X X atinla)>=0))); utcharpa(atinla), X X atinla)>=0))); utcharpa(atinla), X X X atinla=etcharga()); orfa(; atinla+1; X X X X atinla=etcharga()); orfa(; atinla+1; X X X X ){ orfa( igpa=oink,iocccwa=( X X X X ){ orfa( igpa=oink,iocccwa=( X X X X (atinla- XXX apca)*( XXX apca+umna- X X X X (atinla- XXX apca)*( XXX apca+umna- X X X atinla)>=0) XXX XXX ; (((( X X atinla)>=0) XXX XXX ; (((( X X atinla-apca XXXXX XXXXXXX XXXXX )*(apca+ X X atinla-apca XXXXX XXXXXXX XXXXX )*(apca+ X X umna-atinla XXXXXX )>=0) XXXXXX +((atinla- X X umna-atinla XXXXXX )>=0) XXXXXX +((atinla- X X owla)*(owla+ XXXX umna- XXXX atinla)>=0)) X X owla)*(owla+ XXXX umna- XXXX atinla)>=0)) X X &&"-Pig-" XX "Lat-in" XX "COb-fus" X X &&"-Pig-" XX "Lat-in" XX "COb-fus" X X "ca-tion!!"[ X (((atinla- X apca)*(apca+ X X "ca-tion!!"[ X (((atinla- X apca)*(apca+ X X umna-atinla) X >=0)?atinla- X apca+owla: X X umna-atinla) X >=0)?atinla- X apca+owla: X X atinla)-owla X ]-'-')||((igpa== X oink)&&!(*( X X igpa++)='w') X )||! X (*( X igpa X ++)=owla); * X X (igpa++)=(( X ( XXX XXX X atinla-apca X X )*(apca+ X umna XXX - XXX X atinla)>=0) X X ?atinla- X apca XXX + XXX owla X :atinla), X X atinla= X X X X etcharga()) X X atinla= X X X X etcharga()) X X ; orfa( X atinla=iocccwa?(( X (atinla- X X ; orfa( X atinla=iocccwa?(( X (atinla- X X owla)*(owla+ X umna-atinla)>=0 X )?atinla- X X owla)*(owla+ X umna-atinla)>=0 X )?atinla- X X owla+apca: X atinla): X atinla; ((( X X owla+apca: X atinla): X atinla; ((( X X atinla-apca)* X (apca+umna- X atinla)>=0)+( X X atinla-apca)* X (apca+umna- X atinla)>=0)+( X X (atinla-owla)* X (owla+ X umna-atinla)>= X X (atinla-owla)* X (owla+ X umna-atinla)>= X X 0)); utcharpa( XX XX atinla),atinla X X 0)); utcharpa( XX XX atinla),atinla X X =etcharga()); XXXXXXX orfa(*igpa=0, X X =etcharga()); XXXXXXX orfa(*igpa=0, X X igpa=oink; * igpa; utcharpa( X X igpa=oink; * igpa; utcharpa( X X *(igpa++))); orfa(; (atinla+1)&&(!((( X X *(igpa++))); orfa(; (atinla+1)&&(!((( X X atinla-apca )*(apca+ X X atinla-apca )*(apca+ X X umna- XXXXX XXXXX atinla)>=0 X X umna- XXXXX XXXXX atinla)>=0 X X )+(( XXXXX atinla- X X )+(( XXXXX atinla- X XX owla)*( owla+umna- XX XX owla)*( owla+umna- XX XX atinla)>=0))); utcharpa XX XX atinla)>=0))); utcharpa XX XX (atinla),atinla= XX XX (atinla),atinla= XX XX etcharga()); } XX XX etcharga()); } XX XXXX } XXXX XXXX } XXXX XXXXXXXXX XXXXXXXXX a(X){/*/X=-a(X){/*/X=- -1;F;X=--1;F;X=- -1;F;}/*/-1;F;}/*/ char*z[]={"char*z[]={","a(X){/*/X=-","-1;F;X=-","-1;F;}/*/","9999999999 :-| ", "int q,i,j,k,X,O=0,H;S(x)int*x;{X+=X;O+=O;*x+1?*x+2||X++:O++;*x=1;}L(n){for(*", "z[i=1]=n+97;i<4;i++)M(256),s(i),M(128),s(i),M(64),N;X*=8;O*=8;}s(R){char*r=z","[R];for(q&&Q;*r;)P(*r++);q&&(Q,P(44));}M(m){P(9);i-2||P(X&m?88:O&m?48:32);P(","9);}y(A){for(j=8;j;)~A&w[--j]||(q=0);}e(W,Z){for(i-=i*q;i<9&&q;)y(W|(1<<i++&","~Z));}R(){for(k=J[*J-48]-40;k;)e(w[k--],X|O);}main(u,v)char**v;{a(q=1);b(1);","c(1);*J=--u?O?*J:*v[1]:53;X|=u<<57-*v[u];y(X);K=40+q;q?e(O,X),q&&(K='|'),e(X",",O),R(),O|=1<<--i:J[*J-48+(X=O=0)]--;L(q=0);for(s(i=0);q=i<12;)s(i++),i>4&&N",";s(q=12);P(48);P('}');P(59);N;q=0;L(1);for(i=5;i<13;)s(i++),N;L(2);}",0}; b(X){/*/X=-b(X){/*/X=- -1;F;X=--1;F;X=- -1;F;}/*/-1;F;}/*/ int q,i,j,k,X,O=0,H;S(x)int*x;{X+=X;O+=O;*x+1?*x+2||X++:O++;*x=1;}L(n){for(* z[i=1]=n+97;i<4;i++)M(256),s(i),M(128),s(i),M(64),N;X*=8;O*=8;}s(R){char*r=z[R];for(q&&Q;*r;)P(*r++);q&&(Q,P(44));}M(m){P(9);i-2||P(X&m?88:O&m?48:32);P(9);}y(A){for(j=8;j;)~A&w[--j]||(q=0);}e(W,Z){for(i-=i*q;i<9&&q;)y(W|(1<<i++&~Z));}R(){for(k=J[*J-48]-40;k;)e(w[k--],X|O);}main(u,v)char**v;{a(q=1);b(1);c(1);*J=--u?O?*J:*v[1]:53;X|=u<<57-*v[u];y(X);K=40+q;q?e(O,X),q&&(K='|'),e(X,O),R(),O|=1<<--i:J[*J-48+(X=O=0)]--;L(q=0);for(s(i=0);q=i<12;)s(i++),i>4&&N;s(q=12);P(48);P('}');P(59);N;q=0;L(1);for(i=5;i<13;)s(i++),N;L(2);} c(X){/*/X=-c(X){/*/X=- -1;F;X=--1;F;X=- -1;F;}/*/-1;F;}/*/

8. 8 Identifiers  Identifiers are names for variables, classes, etc.  Good ones are compact, but inidicate what they stand for radius, width, height, length  Bad ones are either too long theRadiusOfTheCircle theWidthOfTheBoxThatIsBeingUsed the_width_of_the_box_that_is_being_used  Or too short a, b, c, d, e  Good identifiers will help the graders understand your program!

9. 9 // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); } Keywords // Authors: J. P. Cohoon and J. W. Davidson // Purpose: display a quotation in a console window public class DisplayForecast { // method main(): application entry point public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); }  Some words are reserved, and can’t be used as identifiers

10. 10 Capitalization  Case matters!  public ≠ Public ≠ PUBLIC This is different that FORTRAN and BASIC This is the same as C/C++  You can use Public as a identifier Not recommended, though!

11. 11 End of lecture on 24 January 2004

12. 12 Defining a method  All methods have the following syntax: modifers type name ( parameters ) { statements } Properties of the method Type that it returns A name for the method Any number (including zero) of parameters The body of the method (can be empty) public staticvoidmain(String[] args){... }

13. 13 Escape sequences  Java provides escape sequences for printing special characters \bbackspace \nnewline \ttab \rcarriage return \\backslash \"double quote \'single quote

14. 14 Escape sequences  What do these statements output? System.out.println("Person\tHeight\tShoe size"); System.out.println("========================="); System.out.println("Hannah\t5‘1\"\t7"); System.out.println("Jenna\t5'10\"\t9"); System.out.println("JJ\t6'1\"\t14");  Output Person Height Shoe size ========================= Hannah 5‘1" 7 Jenna 5'10" 9 JJ 6'1" 14

15. 15  Assignment operator = Allows the memory location for a variable to be updated  Consider int j = 11; j = 1985;  Assignment operator = Allows the memory location for a variable to be updated  Consider int j = 11; j = 1985; Primitive variable assignment

16. 16  Consider int a = 1; int aSquared = a * a; a = 5; aSquared = a * a;  Consider int i = 0; i = i + 1;  Consider int asaRating; asaRating = 400; Primitive variable assignment int a = 1; int aSquared = a * a; a = 5; aSquared = a * a; int i = 0; i = i + 1; int asaRating; asaRating = 400;

17. 17 Primitive variable assignment  Consider double x = 5.12; double y = 19.28; double rememberX = x; x = y; y = rememberX;  Consider double x = 5.12; double y = 19.28; double rememberX = x; x = y; y = rememberX;  Consider double x = 5.12; double y = 19.28; double rememberX = x; x = y; y = rememberX;  Consider double x = 5.12; double y = 19.28; double rememberX = x; x = y; y = rememberX;  Consider double x = 5.12; double y = 19.28; double rememberX = x; x = y; y = rememberX;  Consider double x = 5.12; double y = 19.28; double rememberX = x; x = y; y = rememberX;

18. 18 Primitive variable types  Java has 8 (or so) primitive types: float double boolean char byte short int long real numbers integer numbers two values: true and falsea a single character  Also the void “type”

19. 19 Primitive real (floating-point) types  A float takes up 4 bytes of space Has 6 decimal places of accuracy: 3.14159  A double takes up 8 bytes of space Has 15 decimal places of accuracy: 3.14159265358979  Always use doubles It will save you quite a headache!

20. 20 Primitive integer types  Consider a byte: 01000101  1 byte = 8 bits  Each bit has two possibilities: 0 or 1  2 8 = 256  Thus, a byte can have any one of 256 values  A Java byte can have values from -128 to 127 From -2 7 to 2 7 -1  C/C++ has unsigned versions; Java does not

21. 21 Primitive integer types TypeBytesMinimum valueMaximum value byte1-2 7 =-1282 7 -1=127 short2-2 15 = -32,768 2 15 -1= 32,767 int4-2 31 =-2,147,483,6482 31 -1=2,147,483,647 long8-2 63 =-9,223,372,036, 854,775,808 2 63 -1=9,223,372,036, 854,775,807

22. 22 Increment and decrement operators  ++ Increments a number variable by 1  -- Decrements a numeric variable by 1  Consider int i = 4; // define ++i; System.out.println(i); System.out.print(++i); System.out.println(i++); System.out.println(i);  ++ Increments a number variable by 1  -- Decrements a numeric variable by 1  Consider int i = 4; ++i; // increment System.out.println(i); System.out.print(++i); System.out.println(i++); System.out.println(i);  ++ Increments a number variable by 1  -- Decrements a numeric variable by 1  Consider int i = 4; ++i; System.out.println(i); // display System.out.print(++i); System.out.println(i++); System.out.println(i);  ++ Increments a number variable by 1  -- Decrements a numeric variable by 1  Consider int i = 4; ++i; System.out.println(i); System.out.print(++i); // update then display System.out.println(i++); System.out.println(i);  ++ Increments a number variable by 1  -- Decrements a numeric variable by 1  Consider int i = 4; ++i; System.out.println(i); System.out.print(++i); System.out.println(i++); // display then update System.out.println(i);  ++ Increments a number variable by 1  -- Decrements a numeric variable by 1  Consider int i = 4; ++i; System.out.println(i); System.out.print(++i); System.out.println(i++); System.out.println(i); // display  ++ Increments a number variable by 1  -- Decrements a numeric variable by 1  Consider int i = 4; ++i; System.out.println(i); System.out.print(++i); System.out.println(i++); System.out.println(i);

23. 23 Why you should get the extended warranty

24. 24 Primitive character type  All characters have a integer equivalent ‘0’ = 48 ‘1’ = 49 ‘A’ = 65 ‘a’ = 97  Thus, you can refer to ‘B’ as ‘A’+1

25. 25 Primitive character type public class LowerToUpper { // main(): application entry point public static void main(String[] args) { // set lower case character of interest char lowerCaseLetter = 'c'; // convert to uppercase equivalent char upperCaseLetter = 'A' + (lowerCaseLetter - 'a'); // display result System.out.println("Uppercase equivalent of"); System.out.println(" " + lowerCaseLetter); System.out.println("is"); System.out.println(" " + upperCaseLetter); }

26. 26 Primitive boolean type  When is the following program valid in Java? Assume a and b have been properly declared... if ( a && b ) { // do something interesting }...  Answer: ONLY when a and b are boolean variables  In C/C++, a and b would be ints (or int variants) If you try making a and b ints in Java, you get the following: operator && cannot be applied to int,int

27. 27 Primitive void “type”  In Java, you can ONLY use void to specify that a method does not return a value  You cannot use it to specify that there are no parameters to a method:... int foo (void) {... This is different from C/C++  You cannot use it to declare a void “variable”, as in C/C++: void *foo;

28. 28 Variable initialization  Consider the following code: int x; System.out.println(x);  What happens?  Error message: variable x might not have been initialized

29. 29 Constants  Consider the following: final int x = 5;  The value of x can NEVER be changed! The value assigned to it is “final”  This is how Java defines constants

30. 30 Expressions  What is the value used to initialize expression int expression = 4 + 2 * 5;  What value is displayed System.out.println(5 / 2.0);  Java rules in a nutshell Each operator has a precedence level and an associativity  Operators with higher precedence are done first * and / have higher precedence than + and -  Associativity indicates how to handle ties When floating-point is used the result is floating point

31. 31 Question on expressions  Does the following statement compute the average of double variables a, b, and c? Why or why not? double average = a + b + c / 3.0;

32. 32 System.out.println() public static void main(String[] args) { System.out.print("I think there is a world market for"); System.out.println(" maybe five computers."); System.out.println(" Thomas Watson, IBM, 1943."); }  Class System supplies objects that can print and read values  System variable out references the standard printing object Known as the standard output stream  Variable out provides access to printing methods print(): displays a value println(): displays a value and moves cursor to the next line

33. 33 System.out + println(String s) : void + print(String s) : void +... System.out : PrintStream - destination = -... Variable System.out gives access to an output stream of type PrintStream The printing destination attribute for this PrintStream object is the console window The behaviors of a PrintStream object support a high-level view of printing

34. 34 Selection System. out. print ( " string " ) Literal character string that is the parameter to print(). Member out of System is an output stream object automatically associated with the console window running the application Class System is defined in the standard package java.lang The period indicates that we want to select an individual class member of System The period indicates that we want to select an individual class member of out Method member of out. The execution of member print() causes its parameter to be displayed to the output stream The method we are calling

35. 35 I/O streams  System.out Prints to standard output Equivalent to cout in C++, and print() in C  System.err Prints to standard error Equivalent to cerr in C++, and fprintf(stderr) in C  System.in Reads from standard input Equivalent to cin in C++, and scanf() in C

36. 36 Beware!!!

37. 37 End of lecture on 26 January 2005

38. 38 Java operators  The following are the common operators for ints: + - / * % Division is integer division  6 / 2 yields 3  7 / 2 yields 3, not 3.5  Because everything is an int, the answer is an int Modulus is %  Returns the remainder  7 % 2 yields 1  6 % 2 yields 0  Floats and doubles use the same first four + - / * 7.0 / 2.0 yields 3.5 7.0 / 2 yields 3.5 7 / 2.0 yields 3.5 7 / 2 yields 3

39. 39 Java operators  Booleans have their own operators && is AND  Only true when both operands are true  true && true yields true  false && true yields false || is OR  True when either of the operands (or both) are true  true || false yields true  false || false yields false ! is NOT  Changes the value  !true yields false  !false yields true

40. 40 Quick survey I understand Java operators I understand Java operators a) Pretty much b) With a little review, I’ll have it down c) Not really d) 17

41. 41 System.out.println  Can print multiple things by using the + operator  Let int i = 7;  Example: System.out.println (“i = “ + i); Prints i = 7  Can also have the statement on multiple lines System.out.println ( “hello world!” ) ;  Can’t have the String on multiple lines System.out.println ( “hello world!” );

42. 42 System.out.println  System.out.println (“result: “ + 3/5); What does it print? result: 0  System.out.println (“result: “ + 5 % 3); What does it print? result: 2  System.out.println (“result: “ + 3/5.0); What does it print? result: 0.6  System.out.println (“result: “ + 3+4.0); What does it print? result: 34.0  System.out.println (“result: “ + (3+4.0)); What does it print? result: 7.0

43. 43 Casting  Consider the following code double d = 3.6; int x = Math.round(d);  Java complains (about loss of precision). Why?  Math.round() returns a long, not an int So this is forcing a long value into an int variable  How to fix this double d = 3.6; int x = (int) Math.round(d);  You are telling Java that it is okay to do this This is called “casting” The type name is in parenthesis

44. 44 More casting examples  Consider double d = 3.6; int x = (int) d;  At this point, x holds 3 (not 4!) This truncates the value!  Consider int x = 300; byte b = (byte) x; System.out.println (b);  What gets printed? Recall that a byte can hold values -128 to 127 44! This is the “loss of precision”

45. 45 Quick survey I understand casting I understand casting a) Totally! b) More or less c) Not really d) Huh?

46. 46 Example program: temperature conversion // Purpose: Convert a Celsius temperature to Fahrenheit public class CelsiusToFahrenheit { // main(): application entry point public static void main(String[] args) { // set Celsius temperature of interest int celsius = 28; // convert to Fahrenheit equivalent int fahrenheit = 32 + ((9 * celsius) / 5); // display result System.out.println("Celsius temperature"); System.out.println(" " + celsius); System.out.println("equals Fahrenheit temperature"); System.out.println(" " + fahrenheit); }

47. 47 Computation  Programmers frequently write small programs for computing useful things  Example – body mass index (BMI) Measure of fitness  Ratio of person’s weight to the square of the person’s height Weight in is kilograms, height is in meters  Person of interest is 4.5 feet and weighs 75.5 pounds  Metric conversions Kilograms per pound 0.454 Meters per foot 0.3046

48. 48 Program outline for BMI.java // Purpose: Compute BMI for given weight and height public class BMI { // main(): application entry point public static void main(String[] args) { // define constants // set up person's characteristics // convert to metric equivalents // perform bmi calculation // display result }

49. 49 // define constants final double KILOGRAMS_PER_POUND = 0.454; final double METERS_PER_FOOT = 0.3046; // define constants final double KILOGRAMS_PER_POUND = 0.454; final double METERS_PER_FOOT = 0.3046; // define constants final double KILOGRAMS_PER_POUND = 0.454; final double METERS_PER_FOOT = 0.3046; BMI.java: define constants

50. 50 BMI.java: personal characteristics // set up person's characteristics double weightInPounds = 75.5; // our person’s weight double heightInFeet = 4.5; // our person’s height // set up person's characteristics double weightInPounds = 75.5; // our person’s weight double heightInFeet = 4.5; // our person’s height // set up person's characteristics double weightInPounds = 75.5; // our person’s weight double heightInFeet = 4.5; // our person’s height

51. 51 BMI.java: convert to metric equivalents // convert to metric equivalents double metricWeight = weightInPounds * KILOGRAMS_PER_POUND; double metricHeight = heightInFeet * METERS_PER_FOOT; // convert to metric equivalents double metricWeight = weightInPounds * KILOGRAMS_PER_POUND; double metricHeight = heightInFeet * METERS_PER_FOOT; // convert to metric equivalents double metricWeight = weightInPounds * KILOGRAMS_PER_POUND; double metricHeight = heightInFeet * METERS_PER_FOOT;

52. 52 A bit of humor…

53. 53 BMI.java: perform BMI calculation // perform bmi calculation double bmi = metricWeight / (metricHeight * metricHeight);

54. 54 // display result System.out.println("A person with"); System.out.println(" weight " + weightInPounds + " lbs"); System.out.println(" height " + heightInFeet + " feet"); System.out.println("has a BMI of " + Math.round(bmi)); BMI.java: display result // display result System.out.println("A person with"); System.out.println(" weight " + weightInPounds + " lbs"); System.out.println(" height " + heightInFeet + " feet"); System.out.println("has a BMI of " + Math.round(bmi)); Operator evaluation depend upon its operands Math.round(bmi) is 18

55. public static void main(String[] args) { // define constants final double KILOGRAMS_PER_POUND = 0.454; final double METERS_PER_FOOT = 0.3046; // set up person's characteristics double weightInPounds = 75.5; // our person’s weight double heightInFeet = 4.5; // our person’s height // convert to metric equivalents double metricWeight = weightInPounds * KILOGRAMS_PER_POUND; double metricHeight = heightInFeet * METERS_PER_FOOT; // perform bmi calculation double bmi = metricWeight / (metricHeight * metricHeight); // display result System.out.println("A person with"); System.out.println(" weight " + weightInPounds + " lbs"); System.out.println(" height " + heightInFeet + " feet"); System.out.println("has a BMI of " + Math.round(bmi)); }

56. 56 Quick survey The BMI program makes sense to me The BMI program makes sense to me a) Pretty much b) With a little review, I’ll have it down c) Not really d) I’m so lost

57. 57 Common program elements  Type Set of values along with operators that can manipulate and create values from the set  Primitive types support numeric, character, logical values double and float  Values with decimals byte, short, int, long  Integers char  Characters (considered numeric) boolean  Logical values  Basic operators + addition- subtraction * multiplication/ division

58. 58 Common program elements  Constant Symbolic name for memory location whose value does not change  KILOGRAMS_PER_POUND  Variable Symbolic name for memory location whose value can change  weightInPounds

59. 59 Interactive programs  Programs that interact with their users through statements performing input and output  Temperature conversion Not interactive – Celsius temperature is fixed  BMI.java Not interactive – weight and height are fixed

60. 60 Un-reliable computers…

61. 61 Interactive programs  Programs that interact with their users through statements performing input and output  BMI.java Not interactive – weight and height are fixed

62. 62 Support for interactive console programs  Variable System.in Associated with the standard input stream – the keyboard  Class Scanner Makes obtaining input from the keyboard easy Scanner stdin = new Scanner (System.in); + nextDouble() : double +... stdin : Scanner - source = -... Variable stdin gives Scanner access to an input stream Input source attribute for this Scanner is the keyboard Behaviors of a Scanner support high-level view of inputting text

63. 63 How to make Java work with the Scanner class  In Java 1.5, do a: import java.util.*;  To create a new Scanner: Scanner stdin = new Scanner (System.in);  Do NOT use the following (it won’t work): Scanner stdin = Scanner.create (System.in);

64. 64  Program outline import java.util.*; // Purpose: Compute BMI for user-specified // weight and height public class BMICalculator { // main(): application entry point public static void main(String[] args) { // defining constants // displaying legend // set up input stream // get person's characteristics // convert to metric equivalents // perform bmi calculation // display result } Interactive program for BMI  Program outline import java.util.*; // Purpose: Compute BMI for user-specified // weight and height public class BMICalculator { // main(): application entry point public static void main(String[] args) { // defining constants // displaying legend // set up input stream // get person's characteristics // convert to metric equivalents // perform bmi calculation // display result }

65. public static void main(String[] args) { // define constants //... // displaying legend System.out.println ("BMI Calculator\n"); // set up input stream Scanner stdin = new Scanner (System.in); // get person's characteristics System.out.print("Enter weight (lbs): "); double weight = stdin.nextDouble(); System.out.print("Enter height (feet): "); double height = stdin.nextDouble(); // convert to metric equivalents double metricWeight = weight * KILOGRAMS_PER_POUND; double metricHeight = height * METERS_PER_FOOT; // perform bmi calculation double bmi = metricWeight / (metricHeight * metricHeight); // display result //... }

66. import java.util.*; class BMICalculator { public static void main(String[] args) { // define constants final double KILOGRAMS_PER_POUND = 0.454; final double METERS_PER_FOOT = 0.3046; // displaying legend System.out.println ("BMI Calculator\n"); // set up input stream Scanner stdin = new Scanner (System.in); // get person's characteristics System.out.print("Enter weight (lbs): "); double weight = stdin.nextDouble(); System.out.print("Enter height (feet): "); double height = stdin.nextDouble(); // convert to metric equivalents double metricWeight = weight * KILOGRAMS_PER_POUND; double metricHeight = height * METERS_PER_FOOT; // perform bmi calculation double bmi = metricWeight / (metricHeight * metricHeight); // display result System.out.println("A person with"); System.out.println(" weight " + weight + " lbs"); System.out.println(" height " + height + " feet"); System.out.println("has a BMI of " + Math.round(bmi)); }

67. 67 Scanner API public Scanner(InputStream in)// Scanner(): convenience constructor for an // InputStream public Scanner(File s)// Scanner(): convenience constructor for a filename public int nextInt() // nextInt(): next input value as an int public short nextShort() // nextShort(): next input value as a short public long nextLong() // nextLong(): next input value as a long public double nextDouble() // nextDouble(): next next input value as a double public float nextFloat() // nextFloat(): next next input value as a float public String next() // next(): get next whitespace-free string public String nextLine() // nextLine(): return contents of input line buffer public boolean hasNext() // hasNext(): is there a value to next

68. 68 Class fields class BMICalculator { // define constants final static double KILOGRAMS_PER_POUND = 0.454; final static double METERS_PER_FOOT = 0.3046; public static void main(String[] args) { // displaying legend System.out.println ("BMI Calculator\n"); //... } class BMICalculator { // define constants final static double KILOGRAMS_PER_POUND = 0.454; final static double METERS_PER_FOOT = 0.3046; public static void main(String[] args) { // displaying legend System.out.println ("BMI Calculator\n"); //... } class BMICalculator { public static void main(String[] args) { // define constants final double KILOGRAMS_PER_POUND = 0.454; final double METERS_PER_FOOT = 0.3046; // displaying legend System.out.println ("BMI Calculator\n"); //... }

69. 69 Quick survey I felt I understood the material in this slide set… I felt I understood the material in this slide set… a) Very well b) With some review, I’ll be good c) Not really d) Not at all

70. 70 Quick survey The pace of the lecture for this slide set was… The pace of the lecture for this slide set was… a) Fast b) About right c) A little slow d) Too slow

71. 71 End of lecture on 31 January 2005