1. Characters and Strings

2. Characters and Strings
The data type char represents a single character in Java.
Character values are written as a symbol: 'a', ')', '%', 'A', etc.
A char value in Java is really represented as an integer.
Each character has an associated 16-bit integer value*.
16 bits
*Digression: …
0* …………. + 0* * * * * *22 + 0*21 + 1*20
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Characters and Strings

3. Characters and Strings
So: a char value in Java is really represented as an integer. Thus:
The integer value associated with the character is based upon a code.
The ASCII code represents 256 characters including all upper and lower case English letters, the numbers between 0 and 9, punctuation, and some unprintable characters.
ASCII is a subset of the UNICODE character set.
The UNICODE character set contains 34,168 distinct characters.
The major languages from the Americas, Europe, Middle East, Africa, India, Asia, and Pacifica are represented.
Unicode (16 bits) support for every character in the world: ‘\u0000’ to ‘\uFFFF’
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Characters and Strings

4. Characters and Strings
The printable ASCII characters include tabs, new lines, carriage return, single and double quote.
New line = '\n'
Tab = '\t'
Carriage return = '\r'
Single quote = '\''
Double quote = '\ "‘
System.out.println(“Line one \nLine two”);
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Characters and Strings

5. Non-printable Characters
There are also characters contained in ASCII that are not printable.
Bell tone = bel (ASCII 7)
Characters 0 to 32 are non-printable characters.
Character 127 (delete) is also non-printable character
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Characters and Strings

6. Characters and Strings
To define a character use the char data type.
char firstChar = 'a', secondChar = 'A';
Notice that two integers are declared and initialized on the same lane.
To convert an integer into a character you can type cast the integer.
char thirdCharacter = (char) 120;
char thirdCharacter = ‘x’;
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Characters and Strings

7. Characters and Strings
You can print a char as an integer using type casting.
System.out.println( (int) 'C');
Output: 67
Comparing characters is done based upon their integer representation.
True or false? 'c' < 'C'
True or false? '1' < '4'
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Characters and Strings

8. Characters and Strings
import java.io.* ;
public class CharString {
public static void main( String args[] ) {
System.out.println('A');
System.out.println('\u0041');
System.out.println((char)65); }
Output:
A A A
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Characters and Strings

9. Characters and Strings
import java.io.* ;
public class CharString {
public static void main( String args[] ) {
int a = 98;
System.out.println(a);
System.out.println((char)a);
System.out.println('a'); }
Output:
98 b a
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Characters and Strings

10. Characters and Strings
A string is composed of individual characters that are treated as a single unit.
The individual characters 'h', 'e', 'l', 'l', and 'o' are combined into the string "hello".
A string may contain letters, digits, and special characters such as +, -, etc.
String Examples:
"My name is Matilda."
"1 + 2 = 3“
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Characters and Strings

11. Characters and Strings
The data type of a string is String.
The capital S of String indicates that this data type is not a primitive data type.
In fact, String is a complex data type.
When an individual string is created, it is an object of type String.
Here comes string content.
stringName
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Characters and Strings

12. Characters and Strings
String Constructors
Java provides various string constructors.
Assume String s1; (What’s the value of s1 so far?)
s1 = new String( );
This creates a string of length zero with no characters.
s1 = new String( s );
This creates a string that is a copy of the characters stored in String s that is passed to the constructor.
s1 = "This is a string";
This is a special shortcut constructor, ONLY available to Strings.
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Characters and Strings

13. Characters and Strings
Each character of a String has an associated index.
The first letter of a string has an index of zero (0), the second letter has an index of one (1), … the last letter has an index of (string length – 1).
What is the string length of "hello"?
What is the index of the second 'l' in the word "hello"?
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Characters and Strings

14. Characters and Strings
String Methods
The length of a string can be found by:
stringName.length();
The first element of a string is always zero.
A character at a specific position can be found by:
stringName.charAt( 3 );
Where 3 is an index into the string. 1 2 3 4 5 6 S u m a t r
stringName
stringName.charAt(3)
The variable refers to the whole string
The method returns the character at position #3
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Characters and Strings

15. Characters and Strings
Strings are immutable !!!
Once you create a string and initialize it you can not change the string.
You can assign a new string to the string variable.
The original string is lost (will be handled by the java garbage collection process.
You can not add new characters or remove existing characters.
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Characters and Strings

16. Characters and Strings
Changing Case
To change the case of characters in a string:
stringName.replace( 'l', 'L' );
This returns a String with all characters 'l' in the String replaced by 'L'. If there are no 'l's in the String, the original string is returned.
stringName.toUpperCase( );
This will return a String with all lower case letters to capital letters.
stringName.toLowerCase( );
This will return a String with all capital letters to lower case letters.
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Characters and Strings

17. Characters and Strings
String Comparison
Are two strings equal to one another?
stringName1.equals( stringName2 );
The result is true if the two strings are the same and false if the two strings are different.
Capital and Lower case letters are considered to be different.
stringName1 == stringName2;
The result is only true if stringName1 and stringName2 both refer to the same object in memory.
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Characters and Strings

18. Characters and Strings
String Comparison
You can ignore the case of letters during comparison using:
stringName1.equalsIgnoreCase( StringName2 );
That means that "hello" is equal to "HELLO"
You can also compare strings using
stringName1.compareTo( StringName2 );
This comparison returns 0 if the strings are equal, a negative number if stringName less than stringName2, and a positive number if stringName greater than stringName2.
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Characters and Strings

19. Characters and Strings
String Comparison
To compare portions of two strings:
stringName1.regionMatches( 0, StringName2, 0, 5 );
The first parameter 0 is the starting index in stringName1, the third parameter is the starting index in stringName2, and the last argument is the number of characters to compare.
This method returns true only if the members compared are equal.
"ello" == "ello" but "ello" != "Ello“
stringNam1e.regionMatches( true, 0, StringName2, 0, 5 );
Here, the true says we want to ignore case
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Characters and Strings

20. Locating Characters and Substrings
indexOf can be used to find characters in strings.
stringName.indexOf( (int) 'a' );
This returns the index of the first ‘a’ in the string if it is found. If it is not found the result is -1.
stringName.indexOf( (int) 'a', 2 );
This is similar to the first except the second parameter specifies which index of the string the search should begin.
stringName.indexOf( "a" );
This is the same as the first except the parameter is a String rather than an int.
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Characters and Strings

21. Characters and Strings
import java.io.* ;
public class CharString {
public static void main( String args[] ) {
String s = "Vladimir";
System.out.println( s.indexOf((int) 'i') );
System.out.println( s.indexOf((int) 'i',5) );
System.out.println( s.indexOf("i") );
System.out.println( s.indexOf('i') ); } }
Output: 4 6
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Characters and Strings

22. Extracting Substrings
Methods to get substrings out of strings are:
stringName.substring( 10 );
This returns the string that begins at index 10 and ends at the end of the original string.
stringName.substring( 10, 15 );
This returns the string that begins at index 10 and ends at one index before 15.
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Characters and Strings

23. Concatenating Strings
We have already used string concatenation with:
"this is a string" + stringName
To concatenate two string variables:
stringName3 = stringName1.concat( stringName2 );
This returns the second string added to the end of the first string.
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Characters and Strings

24. Characters and Strings
Name.java
public class Name {
public static void main( String args[] ) {
String name;
int midLoc;
name = "Nan";
name = name.concat( " Schaller" );
midLoc = name.indexOf( " " );
name = name.substring( 0, midLoc ) + " Carol" +
name.substring( midLoc );
System.out.println( name );
// Print out first name, a character per line
for (int i=0; i<name.length() && name.charAt(i) != ' '; i++ ) {
System.out.println( name.charAt(i) ); }
There is a simpler way to write this:
for(int i=0; i<midLoc; i++) {
System.out.println(name.charAt(i)); }
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Characters and Strings

25. Characters and Strings
Other String Methods
Using the Javadoc documentation you can learn about the many other String methods.
Methods for comparing regions of strings.
Converting variables of other data types to strings.
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Characters and Strings

26. Primitive vs. Complex Data Types
When you define a primitive data type (int, char, double, bool) the memory location is allocated.
The number of bytes is always the same to store a value.
char let = 'A';
let A
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Characters and Strings

27. Primitive vs. Complex Data Types
A complex data type is a data type defined by a class.
String is an example of a complex data type.
Complex data types usually begin with a capital letter.
The amount of storage required for a complex data type varies depending upon how large the actual values are.
Complex data types are also called reference data types.
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Characters and Strings

28. Primitive vs. Complex Data Types
When we define a String a memory location is allocated to hold a reference to the actual location of the information.
The reference is the location of the first item in memory.
The information is stored sequentially beginning at the reference location.
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Characters and Strings

29. Primitive vs. Complex Data Types
String nameA, nameB;
nameA = "Rochester";
nameB = nameA;
1008
nameB
2044
nameA
2044
1012 …
2044
nameA
Rochester
R o
c h
2048
e s
2052
t e
2056 r
2060
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Characters and Strings

30. Primitive vs. Complex Data Types
If we define another string and assign it equal to name then they will both point to the same location in memory.
string nameB = nameA;
Now nameA and nameB both point to memory location 2044.
nameA
Rochester
nameB
2044
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Characters and Strings

31. Passing Primitive Data to Methods
If a program passes a variable that has a primitive data type to a method, the actual value is passed using call-by-value.
The advantage is that the original value can not be modified by the method.
The disadvantage is that a copy of the original value is made, this requires more memory.
In fact, Java always passes method arguments by value!
Even if variables are reference types.
I’ll try to explain this – hope you’ll get it!
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Characters and Strings

32. Passing Objects to methods
When we pass a String to a method we are passing it using call-by-reference mechanism.
This means that we do not pass the actual string, we are passing the contents of the memory location that holds the reference (address) to the actual string.
A problem associated with call-by-reference is that the original object may be modified.
All objects (both Java defined and user defined) are passed using call-by-reference.
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Characters and Strings

33. Passing Objects to methods
Some of the String methods require a String as a parameter to the method.
For example, stringName1.equals(stringName2);
The method definition requires a String object to be passed to the method equals.
Sometimes == results in different value than stringName1.equals(stringName2);
When? We want some example !!!
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Characters and Strings

34. Passing Objects to methods
Sometimes == results in different value than *.equals !!!
word2
word1 :
String
Java
word1 == word2 is true
word1.equals(word2) is true
word2
word1 :
String
Java
word1 == word2 is false
word1.equals(word2) is true
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Characters and Strings

35. Returning Things from Methods
When a method returns an object, a memory reference is really returned.
Not the actual data.
When a method returns a primitive data type, then the actual value is returned.
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Characters and Strings

36. Characters and Strings
StringBuffer
The String class provides string objects that cannot be changed (are immutable).
The StringBuffer class provides mutable objects.
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Characters and Strings

37. Characters and Strings
Palindrome
// This program checks a given string to see if it is a palindrome
public class Palin {
public static void main( String args[] ) {
String original = "mom", reverse = "";
// Reverse it
for (int i=0; i<original.length(); i++) {
reverse = original.charAt( i ) + reverse; }
// Now check it ( note that orig == reverse does not work )
if (original.equalsIgnoreCase(reverse)) {
System.out.println( "Palindrome" );
} else {
System.out.println( "Not a palindrome !!!" );
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Characters and Strings