Yet Another Predefined Class Strings

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Yet Another Predefined Class - Strings
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Topics to Cover

• Modeling Strings
• Objects of type String
• The Empty String
• A String Example
• String Methods
• Method Prototypes
• Variable Names and Objects
• More String Methods
• Conversions Numbers ? Strings
• Strings and Equality

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Modeling Strings

• A string is any sequence of characters (letters,
  digits, punctuation marks, spaces, and so on)
• Consider a string object
• What attributes would it have?
• Sequence of characters
• What behaviours would it have?
• Get length,

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Modeling Strings

• We have seen String constants as actual
  parameters to the methods print and println.
• For exampleSystem.out.print("Welcome to
  Java")System.out.println(" and to CS026!)
• Java uses the double quotation mark to delimit
  strings.
• Remember the quotation marks are not part of the
  String
• Java has a predefined class, String, which models
  a string of characters

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Modeling Strings

• Note the Java String class has subtle
  differences in usage from other predefined
  classes
• Why? Because Strings are used so often, they are
  designed to save space
• The differences will be pointed out as we go
  along watch for them!

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Objects of Type String

• Recall the steps we have seen so far to acquire
  an object
• Declare a reference variable of the class type
  from which we want an object
• Use the keyword new to create the object and to
  invoke the constructor
• The String class is different!
• We can have String constants (i.e. we can have a
  String object without having a reference
  variable)
• Example"Welcome to Java" is a constant
  (literal) String object

System.out.println("Welcome to Java")
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Objects of Type String

• The keyword new is not typically required (and,
  usually, should not be used) to create a String
  object
• The previous example demonstrated this with a
  String constant
• Moreover, the keyword new is not required when we
  create a reference to a string with a reference
  variable, e.g
• Remember because the variable welcomeMessage
  refers (points) to a String object, it is called
  a reference variable

String welcomeMessagewelcomeMessage "Welcome
to Java"System.out.println(welcomeMessage)
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Objects of Type String

• Discussion Points
• Considering the previous example,what does a
  trace look like after each statement is executed?

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Objects of Type String

• The keyword new can be used to create String
  objects and invoke the String constructor
  however, the keyword new may cause more memory
  than is really necessary to be required (more on
  this in a moment )
• The previous example could be written as
• How do the two differences discussed make using
  String objects easier?
• How do reference variables differ from simple
  variables?

String welcomeMessagewelcomeMessage
new String("Welcome to Java")System.out.printl
n(welcomeMessage)
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The Empty String

• The empty string
• Sometimes we want to initialize a String variable
  to nothing, that is, to contain no characters
  at all
• Examples
• String message new String()
• or
• String message ""
• When might one want to initialize a string to the
  empty string?

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Example
String firstName, lastName, usualNamefirstName
"Donald"lastName "Duck"usualName
firstName

• To what does the variable firstName refer?
• To what does the variable lastName refer?
• To what does the variable usualName refer?
• Recall What happens when there is a variable
  name on the right hand side of the assignment
  operator?
• Show what a trace would like after each statement

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String Methods

• What can we do with String objects? In other
  words, what behaviours do they have?
• Where do we find information about predefined
  classes?

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String Methods

• Example toUpperCase is a predefined method of
  the String class
• What kind of method is toUpperCase static or
  instance?
• Are there any parameters required by the method
  toUpperCase?
• What happens when the first println is executed?

String wordword "java"System.out.println(wor
d)System.out.println(word.toUpperCase())
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String Methods

• What happens when the method toUpperCase is
  executed?
• It does not change the variable word itself
• Instead, it creates a new String object with
  attributes "JAVA"
• It then returns the new String object
• What is printed on the screen?
• What happens if the following statements are
  executed after the previous code segment?
• All instance methods in the String class are
  accessor methods therefore, String objects are
  said to be immutable that is, unchangeable

word word.toUpperCase() String anotherWord
word.toUpperCase()
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Reference Variables and Objects

• A reference variable refers to exactly one
  object
• An object can be referred to by many reference
  variables
• Example
• The variable names firstName and usualName both
  refer to the same String object (what is it?)
• Show a trace after each statement

String firstName, usualNamefirstName
"Donald"usualName firstName
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Reference Variables and Objects

• What happens if we now make firstName refer to a
  different String object?
• What happens to usualName?Nothing, it still
  refers to the same String object "Donald"
• Perform a trace to clearly show the contents of
  the variables

firstName "Ronald"
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More String Methods

• We can manipulate String objects in many ways,
  using other predefined String methods e.g.
  method concat
• Concatenation means joining together
• What object is concatenated with lastName?
• What is the return type of the method concat ?
• What will be printed in the above example? (Be
  careful!)

String firstName, lastName, fullName firstName
"Donald"lastName "Duck"fullName
firstName.concat(lastName) System.out.println(ful
lName)
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More String Methods

• We have seen the term concatenation in class
  before.
• Java has the concatenation operator which
  can be used instead (in fact, its easier to
  use!)
• Rewrite the previous example using the
  concatenation operator
• Draw diagrams of main memory (i.e. perform a
  trace)

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More String Methods

• Method substring
• The character positions in a string are numbered
  from left to right, starting at 0
• In the string "Donald", what character is at
  position 0?
• At what position is the last character in the
  string?
• Example 1

String shortNameshortName firstName.substring(
0,3)System.out.println(shortName)
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More String Methods

• substring is an overloaded method it has two
  versions, depending on the number and type of
  parameters!
• The two-parameter version specifies position1 and
  position2
• The substring will consist of all characters
  starting at position1, up to but not including
  the character at position2
• What is the return type of the method substring?
• What will be printed in Example 1?
• Rewrite Example 1 as a single statement

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More String Methods

• Example 2
• The one-parameter version specifies a starting
  position
• The substring will consist of all characters
  starting at the given position, up to the end of
  the string
• What is the operator in this context?
• What will be printed in Example 2?
• Rewrite example 2 as a single statement

String letter, newNameletter "R"newName
letter firstName.substring(1)System.out.printl
n(newName)
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More String Methods

• Recall String objects are immutable once they
  are created, they do not change
• Therefore the String methods toUpperCase,
  concat, and substring return a new String object
• They do not change the existing String object

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More String Methods

• Method length
• length returns the number of characters in the
  string
• What is the return type of length?
• What would be output by this statement?

firstName "Bob" System.out.println(firstName.l
ength())
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Conversions Numbers ?? Strings

• int to String We have seen this before
• How are the numbers 1983, 6, and 20 represented
  in main memory?
• What parameter type is required by println?
• Each integer value is converted to a String
  object with the use of the concatenation operator
  ()

int birthYear 1983int birthMonth 6int
birthDay 20 System.out.println(birthYear "-"
birthMonth "-" birthDay)
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Conversions Numbers ?? Strings

• Observe using the concatenation operator causes
  the conversion, for example
• What is ""?
• Recall it is the empty string
• What are the differences between birthYear and
  birthYearString?

String birthYearString "" birthYear
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Conversions Numbers ?? Strings

• Now suppose we want to create a string called
  birthDate of the form 20-6-1983
• Each number is converted to a String object
• Then what?
• Now suppose we want birthDate to be of the form
  20-6-83 (fill in the blank)String birthDate
  

String birthDate birthDay "-"
birthMonth "-" birthYear
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Conversions Numbers ?? Strings

• Conversion String to int
• We have seen this before too!
• Use the static method parseInt from the Integer
  class
• Example (fill in the blank)int birthYear
  Integer.parseInt (birthDate.substring(
  ))

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Conversions Numbers ?? Strings

• Conversion String to double
• Similar to String to int
• Use the static method parseDouble from the Double
  class
• Example (fill in the blank)String priceString
  "49.99"double price Double.parseDouble
  (priceString.substring( ))

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Conversions Numbers ?? Strings

• When might it be useful to convert from a String
  to a number?
• Hint In what form is all input from the
  keyboard?

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Strings and Equality

• Recall identity vs. state equality
• Recall strings are different! (e.g. they are
  immutable)
• Consider StringEquality.java

/ StringEquality.java CS026 Lecture Notes
Example Task demonstrates that String
equality is different than equality of
other objects/ import
java.io.import java.awt.Rectangle
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Strings and Equality
public class StringEquality public static
void main(String args) throws Exception
BufferedReader keyboard new BufferedReader
(new InputStreamReader(System.in),1)
/ dimensions for rectangle objects
/ final int WIDTH 5 final int
HEIGHT 10 / first, compare objects to
themselves / System.out.println("COMPARE
OBJECTS TO THEMSELVES ") String name1
"UWO" compare("Compare a string object to
itself", name1, name1) Rectangle shape1
new Rectangle(0, 0, WIDTH, HEIGHT)
compare("Compare a rectangle object to itself ",
shape1, shape1)
keyboard.readLine()
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Strings and Equality
/ second, compare objects to other objects with
different states / System.out.println("COM
PARE DIFFERENT OBJECTS "
"WITH DIFFERENT STATES ") String
name2 "London" compare("Compare a
string to another string composed of different
characters ", name1, name2) Rectangle
shape2 new Rectangle(1, 1, WIDTH2, HEIGHT2)
compare("Compare a rectangle object with
another rectangle in a different position of a
different size ", shape1, shape2)
keyboard.readLine()
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Strings and Equality
/ third, compare objects to other objects with
the same states / System.out.println("COMP
ARE DIFFERENT OBJECTS WITH "
"THE SAME STATES ") String name3
"UWO" compare("Compare a string to
another string " "composed of
the same characters ", name1,
name3) Rectangle shape3 new
Rectangle(0, 0, WIDTH, HEIGHT)
compare("Compare a rectangle to another rectangle
" "with the same
characteristics ", shape1,
shape3)
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Strings and Equality
public static void compare( String message,
Rectangle r1, Rectangle r2)
System.out.println(message)   if (r1
r2) System.out.println(" identity
equality") else
System.out.println(" identity inequality")  
if (r1.equals(r2))
System.out.println(" state equality")
else System.out.println(" state
inequality")   System.out.println()
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Strings and Equality
public static void compare( String message,
String s1, String s2) System.out.println(m
essage) if (s1 s2)
System.out.println(" identity equality")
else System.out.println(" identity
inequality")   if (s1.equals(s2))
System.out.println(" state equality")
else System.out.println(" state
inequality")   System.out.println()

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Strings and Equality
Screen shot of BlueJ running the StringEquality
program
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Why Is It So???

• Because Java strings are said to be interned
• Recall strings are immutable
• Therefore, when multiple strings have the same
  state, it makes sense to have them refer to
  exactly the same object. For example
• The two objects are equivalent with respect to
  both state and identity. That is

String name1 "UWO"String name2 "UWO"
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Why Is It So???
if (name1 name2) System.out.println("identity
equivalence")if (name1.equals(name2)) System.ou
t.println("state equivalence")

• will result in the output
• This referred to as interning of the strings,
  which is done to avoid duplicates (therefore,
  saving memory and making equality checks faster)

identity equivalencestate equivalence
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Why Is It So???

• Observe if one has the following
• then the objects are only equivalent with
  respect to state, that is,will result in the
  output

String name1 new String("UWO")String name2
new String("UWO")
if (name1 name2) System.out.println("identity
equivalence")if (name1.equals(name2)) System.ou
t.println("state equivalence")
state equivalence
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Why Is It So???

• In other words, to avoid the interning of
  strings, one can create strings with the keyword
  new
• It might not be as efficient to do this however
• Think of a situation where one would want to
  avoid the interning

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Further Reading

• The following sections from the Java 5.0 Program
  Design text supplement these notes nicely
• Section 7.6 (omit Listings 7.7, 7.8, 7.9,  7.10
  and discussions thereof)