The While Statement. If boolean expression is true execute statement; continue to execute statement so long as boolean expression is true.

Transcription

1 Variables and Loops

2 The While Statement while ( boolean-expression ) statement If boolean expression is true execute statement; continue to execute statement so long as boolean expression is true.

3 While Statement Example public class GoToEnd public static void main( String[] args ) Vic.reset( args ); Vic tim = new Vic(); while ( tim.seesslot() ) tim.moveon();

4 The While Statement and Blocks To do more than one thing in a while loop use a compound statement: public class GoToEnd public static void main( String[] args ) Vic.reset( args ); Vic tim = new Vic(); while ( tim.seesslot() ) tim.moveon(); tim.say( "try again" );

5 1. Textbook, Chapter 3, page 3 3: Exercises Exercises

6 The String Type To declare a String object, use String followed by an identifier followed optionally by an initial value: public class Test public static void main( String[] args ) String str = "this is a very clever string"; System.out.println( str );

7 Testing Strings for Equality To test two strings for equality use the.equals( String ) method: public class Test public static void main (String[ ] args) String fred = "See spot run!"; String wilma = "Run spot, run!"; String betty = "See spot run!"; Question: what is the type of.equals? System.out.println( fred.equals( wilma ) ); System.out.println( fred.equals( betty ) );

8 Vic Method: Get Position This method returns a String which tells you which position an "arm" is. public static void main( String[] args ) Vic tim = new Vic(); String pos = tim.getposition(); tim.say( pos ); or public static void main( String[] args ) Vic tim = new Vic(); tim.say( tim.getposition() );

9 Has Some Filled Slot This method will return true if there is a CD at or after the current position. On completion, the arm will return to the position it started in. public class VicPlus extends Vic public boolean hassomefilledslot() String savepos = getposition(); while ( seesslot() &&!seescd() ) moveon(); boolean rval = seesslot(); while (!savepos.equals( getposition() ) ) backup(); return rval;

10 Go To Last CD public boolean gotolastcd() String pos = getposition(); boolean rval = false; while ( seesslot() ) if ( seescd() ) rval = true; pos = getposition(); moveon(); This method will return true if there is a CD anywhere in its sequence. On completion, the arm will be at the position of the last CD, or at its original position if no CD is found. while (!pos.equals( getposition() ) ) backup(); return rval;

11 1. Textbook, Chapter 3, page 3 7: Exercises Exercises

12 Practice: Move To Front public class MoveToFront public static void main( String[] args ) Vic.reset( args ); Vic tim = new Vic(); String pos = tim.getposition(); while ( tim.seesslot() ) tim.takecd(); tim.moveon(); Move all CDs in a row to the front of the row. while (!pos.equals( tim.getposition() ) ) tim.backup(); while ( Vic.stackHasCD() ) tim.putcd(); tim.moveon();

13 1. Textbook, Chapter 3, page 3 8: Exercise Exercises

14 The Looper Class: Back Up To Given a Vic position (from the Vic getposition() method) back up until the position is reached: public class Looper extends Vic private void backupto( String pos ) while (!pos.equals( getposition() ) ) backup(); This is a private method; it can only be used by other methods in the Looper class.

15 The Looper Class: Fill Slots Fill all slots in a sequence from the current position forward, then return to the original position. public void fillslots() String pos = getposition(); while ( seesslot() ) putcd(); moveon(); backupto( pos ); Question: what object do putcd and backupto refer to?

16 The Looper Class: Clear Slots To Stack Take all the CDs from the current position forward and put them on the stack, then return to the original position. public void clearslotstostack() String pos = getposition(); while ( seesslot() ) takecd(); moveon(); backupto( pos );

17 The Looper Class: Fill Odd Slots Beginning at the current position, fill all the odd numbered slots with CDs from the stack, then return to the original position. public void filloddslots() String pos = getposition(); while ( seesslot() && stackhascd() ) putcd(); moveon(); if ( seesslot() ); moveon(); backupto( pos );

18 The Looper Class: Sees All Filled Returns true if all slots from the current position forward are filled, false otherwise. At completion, the arm will be left at the original position. public boolean seesallfilled() String pos = getposition(); while ( seesslot() && seescd() ) moveon(); boolean rval =!seesslot(); backupto( pos ); return rval; Questions: 1. What is the type of seesallfilled? 2. Is this an instance method? Why?

19 Private Methods Declaring a method private means it is for internal use only; programmers of other classes cannot access. This is called data hiding, and it makes maintaining your code easier. private void backupto( String pos ) while (!pos.equals( getposition() ) ) backup();

20 Method Parameters Method Parameters make your methods more versatile; it enables the same logic can be applied to many different objects. private void backupto( String pos ) while (!pos.equals( getposition() ) ) backup(); Every parameter has a type. When you invoke a method, you must provide an argument of a compatible type.

21 Exercises 1. Enter and compile the code for the Looper class. 2. Textbook, Chapter 3, page 3 13: Exercise

22 Variables Local variables can be declared inside a method s braces: public void mymethod() Vic tim; // local variable declaration Turtle socrates; // local variable declaration...

23 Parameters Parameters can be declared inside a method s parentheses; when you invoke a method, you must pass arguments (values) of a compatible type. private void backupto( String pos ) while (!pos.equals( getposition() ) ) backup(); public void loopermethod() String spot = getposition();... backupto( spot ); parameter argument

24 Using Parameters (1) The method hasasmanyslotsas returns true if this Vic object has the same number of slots as another Vic object. public class TwoVicUser extends Vic public boolean hasasmanyslotsas( Vic parm ) String pos = getposition(); while ( seesslot() && parm.seesslot() ) moveon(); parm.moveon(); (continued on next slide)

25 Using Parameters (2) (continued from previous slide) boolean rval =!seesslot() &&!parm.seesslot(); while (!pos.equals( getposition() ) ) backup(); parm.backup(); return rval;

26 Arguments A value passed to a method is called an argument. public class Test public static void main( String[] args ) Vic.reset( args ); TwoVicUser tim = new TwoVicUser(); TwoVicUser sam = new TwoVicUser(); if ( tim.hasasmanyslotsas( sam ) ) Vic.say( "slots same" ); else Vic.say( "slots different" );

27 Garbage Collection Every time you new something memory gets used. If you new too many objects without releasing the ones that you don't need anymore you will eventually run out of memory. Java has a mechanism called garbage collection that automatically looks for unused objects and deletes them.

28 1. Textbook, Chapter 3, page 3 17: Exercise 3.28, 3.29 Exercises

29 The Int Type An int is used to store and manipulate integers. It is declared using the int keyword and an optional initial value: int alice = 5; int jane = 5;

30 The Int Type: Limits An int object can hold values between approximately 2.1 billion and 2.1 billion.

31 The Increment Operator The increment operator (++) increases the value of an int object by one. The increment operator comes in two flavors, pre increment (++inx) and postincrement (inx++): int alice = 5; int jane = ++alice; // pre-increment int rabbit = alice++; // post-increment

32 The Pre Increment Operator When the pre increment operator is used on a variable in an expression, first the variable is incremented then the rest of the expression is evaluated: public static void main( String[] args ) int inx = 5; int jnx = 10; int knx = ++inx * jnx; System.out.println( inx ); // 6 System.out.println( knx ); // 60

33 The Post Increment Operator When the post increment operator is used on a variable in an expression, first the expression is evaluated, then the variable is incremented: public static void main( String[] args ) int inx = 5; int jnx = 10; int knx = inx++ * jnx; System.out.println( inx ); // 6 System.out.println( knx ); // 50

34 The Decrement Operator The decrement operator ( ) decreases the value of an int variable by one. It comes in two flavors, pre decrement ( inx) and post decrement (inx ). int inx = 5; int jnx = 10; int knx = --inx * jnx; System.out.println( inx ); // 4 System.out.println( knx ); // 40 int inx = 5; int jnx = 10; int knx = inx-- * jnx; System.out.println( inx ); // 4 System.out.println( knx ); // 50

35 Unary Operators The increment and decrement operators are called unary operators because they take a single argument. Another unary operator is negation ( ) which changes the sign of its argument: public class Test public static void main( String[] args ) int inx = -5; int jnx = 20; System.out.println( -inx ); //5 System.out.println( -jnx ); //-20

36 Binary Operators A binary operator is an operator that takes two arguments. Some of the common binary operators in java are: Operator Example Addition (+) int inx = 5; int jnx = inx + 15; // jnx = 20 Subtraction ( ) int inx = 15; int jnx = inx - 3; // jnx = 12 Multiplication (*) int inx = 15; int jnx = inx * 4; // jnx = 60 Division (/) int inx = 15; int jnx = inx / 2; // jnx = 7 Modulo (%) int inx = 25; int jnx = inx % 4; // jnx == 1

37 Integer Division When two integers are divided, the result is always an integer; any fractional remainder is discarded. public class Test public static void main (String[ ] args) System.out.println( 15 / 7 ); // 2 System.out.println( 20 / 7 ); // 2 System.out.println( 8 / 7 ); // 1

38 Modulo The expression: inx % jnx is read inx modulo jnx The value of A % B, where A and B are integers, is equal to the remainder of A / B. public class Test public static void main (String[ ] args) System.out.println( 15 % 7 ); // 1 System.out.println( 20 % 7 ); // 6 System.out.println( 8 % 7 ); // 1 System.out.println( 42 % 7 ); // 0

39 Comparison Operators The binary operators that perform comparisons are:: Operator Example Greater than (>) if ( inx > jnx ) do this Less than (<) if ( inx < jnx ) do this Equal to (==) if ( inx == jnx ) do this Not equal to (!=) if ( inx!= jnx ) do this Greater than or equal to (>=) Less than or equal to (<=) if ( inx >= jnx ) do this if ( inx <= jnx ) do this

40 Comparison Operator Example if ( inx == 0 ) System.out.println( the value is 0 ); boolean result = seesslot() && seescd();

41 The Conditional Expression Operator Choose between two outcomes depending on some condition: condition? outcome1 : outcome2 The above expression consists of one operator and three operands... If condition is true the value of the expression is outcome1, otherwise the value of the expression is outcome2. // Produce up to count lines of output, but no more than LIMIT. private static void printlines( int count ) int last = count < LIMIT? count : LIMIT; while ( last-- > 0 ) System.out.println( "Every good boy deserves favor" );

42 Exercises 1. Chapter 4, page Int 6: Exercise 1, 2, 3 (note: getnumslots is on page Int 1) 2. Download and complete ArithmeticActivity from the class web site.

43 String Concatenation Use the plus sign (+) to concatenate two Strings or a String and a number. public static void main( String[] args ) String prefix = "String demonstration: "; String demo1 = prefix + "concatenate two strings"; String demo2 = prefix + 25; int answer = 42; System.out.println( demo1 ); System.out.println( demo2 ); System.out.println( "The answer is: " + answer );

44 The For Statement (1) The for statement executes one statement (possibly a block statement) 0 or more times. for ( initialize ; test ; increment ) statement 1. Evaluates the initialization expression. 2. Evaluates the test expression. a. If the test is true goes to step 3; b. If the test is false goes to step Executes statement. 4. Evaluates the increment expression. 5. Goes back to step Terminates execution of the loop. // print 1 through 10 for ( int inx = 0 ; inx < 11 ; ++inx ) System.out.println( inx );

45 The For Statement (2) For statement example: public class Test public static void main( String[] args ) int fac = computefactorial( 10 ); System.out.println( "10! = " + fac ); static private int computefactorial( int num ) int rval = 1; int inx = 0; for ( inx = num ; inx > 0 ; --inx ) rval = rval * inx; return rval;

46 The For Statement (3) For statement example: public class Test public static void main( String[] args ) int fac = computefactorial( 10 ); System.out.println( "10! = " + fac ); static private int computefactorial( int num ) int rval = 1; for ( int inx = num ; inx > 0 ; --inx ) rval = rval * inx; return rval;

47 Exercises 1. Write a program that uses a for loop to print out the squares of the first 20 integers. 2. Textbook, Chapter 4, page Int 6: Exercise 4, 5

48 The For Statement (4) You can eliminate the initialization expression in a for loop: public static void main( String[] args ) Vic tim = new Vic(); int num = countfilledslots( tim ); System.out.println( "Tim has " + num + " filled slots" ); static private int countfilledslots( Vic parm ) int rval = 0; for ( ; parm.seesslot() ; parm.moveon() ) if ( parm.seescd() ) ++rval; return rval;

49 The For Statement (5) You can eliminate the increment expression in a for statement: public class Test public static void main( String[] args ) Vic tim = new Vic(); gotoend( tim ); static private void gotoend( Vic parm ) for ( ; parm.seesslot() ; ) parm.moveon();

50 The For Statement (6) To execute more than one statement in the body of a for loop use a block statement: for ( init ; test ; incr ) statement 1; statement 2;

51 The Do Statement The do statement is like the while statement, except: statement is always a block statement; statement always executes at least once. do statements while ( boolean expression ); 1. Executes the block statement. 2. Evaluates the Boolean expression. a) If boolean expression is true goes to step one. b) if boolean expression is false terminates the loop.

52 Do Statement Example do String prompt = "guess a number between 0 and 10"; System.out.println( prompt ); num = getusersguess(); System.out.println( "Your guess: " + num ); while ( num < 0 num > 10 );

53 Reference Types All objects are reference types. A properly initialized object value is a pointer to an area of memory where the object data is stored. public class Test At this point tim is not an public static void main( String[] args ) object. Turtle tim; tim = new Turtle(); current color current position current direction etc.

54 The Null Keyword The null keyword can be used to distinguish between a proper reference to a block of memory, and an object value that is not a proper reference. public static void main( String[] args ) String data; data = promptuser( null ); data = promptuser( "Enter a number: " ); private static String promptuser( String prompt ) String actprompt = "enter> "; if ( prompt!= null ) actprompt = prompt; System.out.println( "testing: " + actprompt ); return null;

55 Testing Strings For Equality (1) The output of the following code is not equal, even though both strings contain the same value. Can you explain why? public static void main( String[] args ) String test1 = "My string"; String test2 = "My "; test2 = test2 + "string"; System.out.println( "Test 1> " + test1 ); System.out.println( "Test 2> " + test2 ); if ( test1 == test2 ) System.out.println( "equal" ); else System.out.println( "not equal" );

56 Testing Strings For Equality (2) The String method equals determines if two String objects contain the same value (even though they may be different objects). public static void main( String[] args ) String test1 = "My string"; String test2 = "My "; test2 = test2 + "string"; System.out.println( "Test 1> " + test1 ); System.out.println( "Test 2> " + test2 ); if ( test1.equalsignorecase( test2 ) ) System.out.println( "equal" ); else System.out.println( "not equal" ); The output is equal.

57 Testing Strings For Equality (3) The String method equalsignorecase determines if two String objects contain the same value independent of capitalization. public static void main( String[] args ) String test1 = "My string"; String test2 = "MY STRing"; System.out.println( "Test 1> " + test1 ); System.out.println( "Test 2> " + test2 ); if ( test1.equalsignorecase( test2 ) ) System.out.println( "equal" ); else System.out.println( "not equal" ); The output is equal.

58 Strings: Compare To Method The compareto method tells you in which order two strings would appear in a dictionary. private static void testguess( String str ) int result = str.compareto( "alligator" ); if ( result == 0 ) System.out.println( "equal to to the hidden word" ); else if ( result < 0 ) System.out.println( "less than the hidden word" ); else if ( result > 0 ) System.out.println( "greater than the hidden word" ); else System.out.println( shouldn t ever get here" );

59 Strings: Determining Order Comparing strings for magnitude is determined by the Unicode values of the individual characters: All lower case letters are greater than all upper case letters. All letters are greater than all numbers.

60 Strings: Compare To Ignore Case Method The method comparetoignorecase works like compareto except that it treats upper case and lower case letters as equal.

61 Strings: Contains Method The contains method tells whether one string contains another.. private static void testcontains( String str ) if ( str.contains( "coyote" ) ) System.out.println( "contains secret word" ); else System.out.println( "doesn t contain secret word" );

62 Strings: Ends With Method The endswith method tells whether one string ends with another.. static private void testdomain( String address ) if ( address.endswith( ".com" ) ) System.out.println( "company domain" ); else if ( address.endswith( ".mil" ) ) System.out.println( "military domain" ); else if ( address.endswith( ".edu" ) ) System.out.println( "education domain" ); else if ( address.endswith( ".org" ) ) System.out.println( "organization domain" ); else if ( address.endswith( ".gov" ) ) System.out.println( "government domain" ); else System.out.println( "unknown domain" );

63 More String Methods See the java API for complete documentation of the String class.

64 Exercises 1. Write a program that uses a for loop to print out the squares of the first twenty integers. Each printed line should look something like this: The square of 4 is Write a program that determines which of each of the following pairs of strings is greater than the other. For each pair print a line that looks something like this: string bravo is greater than string alpha Zulu Market Albatross market "200"