Download String

CS 201 Lecture 6:
John Hurley
Cal State LA
Character Codes
 Computers store information in bits, not as characters
 Numeric values are easy to map to bits; just convert them to
base 2 and then map each digit to a bit
 There are several code systems to assign numeric values to
characters
Character Codes
 ASCII
 developed in the early 1960s, when memory was much more
expensive than it is now. Originally used to send teletype
data over phone lines, which are noisy
 128 (27) possible characters
 The first 33 are control characters, originally used to control
teletypes
Character Codes
 Unicode
 Used in Java
 Superset of ASCII
 65536 values
 The first 128 are the same as ASCII
 Adds other alphabets, mathematical symbols,etc.
Unicode and Hardware
Memory address
Memory content
.
.
.
.
.
.
2000
01001010
Encoding for character ‘J’
2001
01100001
Encoding for character ‘a’
2002
01110110
Encoding for character ‘v’
2003
01100001
Encoding for character ‘a’
2004
00000011
Encoding for number 3
Character Codes
public class ASCIIUnicodeDemo{
public static void main(String[] args){
for(int charNum = 0; charNum < 128; charNum++){
System.out.println("Character " + charNum + " is
" + (char) charNum);
}
}
}
 Why do we get a blank line after 10 and before 11?
Strings
 A String consists of zero or more characters
 String is a class that contains many methods
 Strings are more complicated than they
sound!
Constructing Strings
String newString = new String(stringLiteral);
String message = new String("Welcome to Java");
Since strings are used frequently, Java provides a shorthand initializer for
creating a string:
String message = "Welcome to Java";
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Variables vs Objects
Consider this code:
int myInt = 1;
 1 is a value
 myInt is a variable
String s1 = "The moon is made of green cheese."
 "The moon is made of green cheese" is a String, an object of class
String.
 s1 is a variable
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Null
A null value is one that does not exist. Variables
whose data type is a class, like String, may exist
without having any value:
String s = null;
A variable whose type is a primitive data type will be
null if no value is set, but can't be set to null
int x;
// ok
int x = null; //syntax error
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Strings Are Immutable
A String object is immutable; its contents cannot be changed.
The following code does not change the contents of the String, it
makes the variable s point to a different String in memory.
String s = "Java";
s = "HTML";
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animation
Trace Code
String s = "Java";
s = "HTML";
After executing s = "HTML";
After executing String s = "Java";
s
: String
String object for "Java"
Contents cannot be changed
s
: String
String object for "Java"
: String
String object for "HTML"
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This string object is
now unreferenced
animation
Trace Code
String s = "Java";
s = "HTML";
After executing s = "HTML";
After executing String s = "Java";
s
: String
String object for "Java"
Contents cannot be changed
s
: String
String object for "Java"
: String
String object for "HTML"
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This string object is
now unreferenced
Interned Strings
 Since strings are immutable, if two different variables need to point to identical Strings,
there is no need to save two separate Strings in memory.
 The JVM usually uses a unique instance for each set of string literals with the same
character sequence. In other words, the following code sets up one String in memory,
but two variables that point to it:
String s1 = "John";
String s2 = "John";
Such an instance is said to be interned.
 If you set up a new String using the full new String syntax, however, the JVM will not use
an interned String even if there is one with identical text. This code sets up two separate
Strings with identical characters:
String s1 = "John"'
String s2 = new String("John");
14
Examples
String s1 = "Welcome to Java";
String s2 = new String("Welcome to Java");
s1
s3
: String
Interned string object for
"Welcome to Java"
String s3 = "Welcome to Java";
System.out.println("s1 == s2 is " + (s1 == s2)); s2
System.out.println("s1 == s3 is " + (s1 == s3));
: String
A string object for
"Welcome to Java"
 s1 == s2 is false, because the == operation for Strings tests
whether two Strings are the same object in memory, and we did
not use the interned String
 s1 == s3 is true because without the new String() syntax, we
used the interned String. Thus, s1 and s3 point to the same
object
15 in memory.
animation
Trace Code
String s1 = "Welcome to Java";
String s2 = new String("Welcome to Java");
String s3 = "Welcome to Java";
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s1
: String
Interned string object for
"Welcome to Java"
Trace Code
String s1 = "Welcome to Java";
s1
String s2 = new String("Welcome to Java");
: String
Interned string object for
"Welcome to Java"
String s3 = "Welcome to Java";
s2
: String
A string object for
"Welcome to Java"
17
Trace Code
String s1 = "Welcome to Java";
s1
s3
String s2 = new String("Welcome to Java");
: String
Interned string object for
"Welcome to Java"
String s3 = "Welcome to Java";
s2
: String
A string object for
"Welcome to Java"
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String Comparisons
.equals(String otherString) test whether the text of this String is the same
as the text of the other String
java.lang.String
+equals(s1: String): boolean
Returns true if this string is equal to string s1.
+equalsIgnoreCase(s1: String):
boolean
Returns true if this string is equal to string s1 caseinsensitive.
+compareTo(s1: String): int
Returns an integer greater than 0, equal to 0, or less than 0
to indicate whether this string is greater than, equal to, or
less than s1.
+compareToIgnoreCase(s1: String):
int
Same as compareTo except that the comparison is caseinsensitive.
+regionMatches(toffset: int, s1: String,
offset: int, len: int): boolean
Returns true if the specified subregion of this string exactly
matches the specified subregion in string s1.
+regionMatches(ignoreCase: boolean,
toffset: int, s1: String, offset: int,
len: int): boolean
Same as the preceding method except that you can specify
whether the match is case-sensitive.
+startsWith(prefix: String): boolean
Returns true if this string starts with the specified prefix.
+endsWith(suffix: String): boolean
Returns true if this string ends with the specified suffix.
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String Comparisons
 String .equals() method
if (s1.equals(s2)){
// executed if s1 and s2 have the same contents
}
if (s1 == s2) {
// executed if s1 and s2 have the same memory
reference
}
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String Comparisons, cont.
 compareTo(Object object)
String s1 = new String("Welcome");
String s2 = "welcome";
if (s1.compareTo(s2) > 0) {
// s1 is greater than s2
}
else if (s1.compareTo(s2) == 0) {
// s1 and s2 have the same contents
}
else
// s1 is less than s2
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public class StringDemo{
public static void main(String[] args){
String s1 = new String("Welcome");
String s2 = "welcome";
String s3 = s1;
String s4 = new String("Welcome");
if (s1.equals(s2)){
System.out.println("s1 equals s2");
}
else System.out.println("s1 does not equal s2");
if (s1 == s2) {
System.out.println("s1 == s2");
}
else System.out.println("s1 != s2");
if (s1.equals(s3)){
System.out.println("s1 equals s3");
}
if (s1 == s3) {
System.out.println("s1 == s3");
}
else System.out.println("s1 != s3");
if (s1.equals(s4)){
System.out.println("s1 equals s4");
}
else System.out.println("s1 does not equal s4");
if (s1 == s4){
System.out.println("s1 == s4");
}
else System.out.println("s1 != s4");
if (s1.compareTo(s2) < 0) {
System.out.println("s1 < s2");
}
}
}
Escape Sequences
• Some characters will cause confusion in output
• What will happen if we try to execute this:
•System.out.println(""Hi,Mom"");
• Others, like backspace and tab, can’t be unambiguously
typed
•Use escape characters for these cases
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Escape Sequences
Description
Escape Sequence
Backspace
\b
Tab
\t
Linefeed
\n
Carriage return \r
(think of a typewriter)
Backslash
\\
Single Quote
\'
Double Quote
\"
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String Concatenation
Appending a String or character to the end of another String is
called concatenation.
Three strings concatenated:
String message = "Welcome " + "to " + "Java";
String s1 concatenated with character B:
String s1 = "Supplement" + 'B'; // s1 becomes SupplementB
To concatenate to existing String, create a new String and use
concat():
String myString = "Good";
String myOtherString = myString.concat(" Morning");
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String Concatenation
String s3 = s1.concat(s2);
String s3 = s1 + s2;
s1 + s2 + s3 + s4 + s5 same as
(((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);
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String Length, Characters, and
Combining Strings
java.lang.String
+length(): int
Returns the number of characters in this string.
+charAt(index: int): char
Returns the character at the specified index from this string.
+concat(s1: String): String
Returns a new string that concatenate this string with string s1.
string.
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Finding String Length
Finding string length using the length() method:
message = "Welcome";
message.length() (returns 7)
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Retrieving Individual Characters in
a String
 Do not use message[0]
 Use message.charAt(index)
 Index starts from 0
Indices
0
1
2
3
4
5
6
message
W
e
l
c
o
m
e
message.charAt(0)
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7
8
9
t
o
message.length() is 15
10 11 12 13 14
J
a
v
a
message.charAt(14)
Strings
public static void main(String[] args){
String s1 = new String("Welcome");
String s2 = " Shriners";
String s3 = s1.concat(s2);
System.out.println(s3);
String s4 = s1 + s2;
System.out.println(s4);
s1 = s1 + s2;
System.out.println(s1);
if(s1.startsWith("Wel")) System.out.println("\"" + s1 + "\"" + " starts with \"Wel!\"");
System.out.println("Length of \"" + s1 + "\" is " + s1.length());
System.out.println("In the string \"" + s1 + "\", the character at position 4 is " + s1.charAt(4)); }
Extracting Substrings
You can extract a single character from a string using the charAt
method.You can also extract a substring from a string using the
substring method in the String class.
String s1 = "Welcome to Java";
String s2 = s1.substring(0, 11) + "HTML";
Indices
0
1
2
3
4
5
6
message
W
e
l
c
o
m
e
7
8
9
t
o
message.substring(0, 11)
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10 11 12 13 14
J
a
v
a
message.substring(11)
Converting, Replacing, and Splitting
Strings
java.lang.String
+toLowerCase(): String
Returns a new string with all characters converted to lowercase.
+toUpperCase(): String
Returns a new string with all characters converted to uppercase.
+trim(): String
Returns a new string with blank characters trimmed on both sides.
+replace(oldChar: char,
newChar: char): String
Returns a new string that replaces all matching character in this
string with the new character.
+replaceFirst(oldString: String, Returns a new string that replaces the first matching substring in
newString: String): String
this string with the new substring.
+replaceAll(oldString: String, Returns a new string that replace all matching substrings in this
newString: String): String
string with the new substring.
+split(delimiter: String):
Returns an array of strings consisting of the substrings split by the
String[]
delimiter.
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Examples
"Welcome".toLowerCase() returns a new string, welcome.
"Welcome".toUpperCase() returns a new string, WELCOME.
" Welcome ".trim() returns a new string, Welcome.
"Welcome".replace('e', 'A') returns a new string, WAlcomA.
"Welcome".replaceFirst("e", "AB") returns a new string,
WABlcome.
"Welcome".replace("e", "AB") returns a new string,
WABlcomAB.
"Welcome".replace("el", "AB") returns a new string, WABlcome.
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Finding a Character or a Substring in
a String
java.lang.String
+indexOf(ch: char): int
Returns the index of the first occurrence of ch in the string.
Returns -1 if not matched.
+indexOf(ch: char, fromIndex:
int): int
Returns the index of the first occurrence of ch after fromIndex in
the string. Returns -1 if not matched.
+indexOf(s: String): int
Returns the index of the first occurrence of string s in this string.
Returns -1 if not matched.
+indexOf(s: String, fromIndex: Returns the index of the first occurrence of string s in this string
int): int
after fromIndex. Returns -1 if not matched.
+lastIndexOf(ch: int): int
Returns the index of the last occurrence of ch in the string.
Returns -1 if not matched.
+lastIndexOf(ch: int,
Returns the index of the last occurrence of ch before fromIndex
fromIndex: int): int
in this string. Returns -1 if not matched.
+lastIndexOf(s: String): int
Returns the index of the last occurrence of string s. Returns -1 if
not matched.
+lastIndexOf(s: String,
Returns the index of the last occurrence of string s before
fromIndex: int): int
fromIndex. Returns -1 if not matched.
34
Finding a Character or a Substring
in a String
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
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to
to
to
to
to
to
to
Java".indexOf('W') returns 0.
Java".indexOf('x') returns -1.
Java".indexOf('o', 5) returns 9.
Java".indexOf("come") returns 3.
Java".indexOf("Java", 5) returns 11.
Java".indexOf("java", 5) returns -1.
Java".lastIndexOf('a') returns 14.
StringBuilder
 The StringBuilder class is a supplement to the
String class.
 StringBuilder is more flexible than String.You can add, insert, or
append new contents into a string buffer, whereas the value of a
String object is fixed once the string is created.
 You will often have to parse the StringBuilder to a String when you
are done constructing it.
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StringBuilder Constructors
java.lang.StringBuilder
+StringBuilder()
Constructs an empty string builder with capacity 16.
+StringBuilder(capacity: int)
Constructs a string builder with the specified capacity.
+StringBuilder(s: String)
Constructs a string builder with the specified string.
37
Modifying Strings in the Builder
java.lang.StringBuilder
+append(data: char[]): StringBuilder
Appends a char array into this string builder.
+append(data: char[], offset: int, len: int):
StringBuilder
Appends a subarray in data into this string builder.
+append(v: aPrimitiveType): StringBuilder
Appends a primitive type value as a string to this
builder.
+append(s: String): StringBuilder
Appends a string to this string builder.
+delete(startIndex: int, endIndex: int):
StringBuilder
Deletes characters from startIndex to endIndex.
+deleteCharAt(index: int): StringBuilder
Deletes a character at the specified index.
+insert(index: int, data: char[], offset: int,
len: int): StringBuilder
Inserts a subarray of the data in the array to the builder
at the specified index.
+insert(offset: int, data: char[]):
StringBuilder
Inserts data into this builder at the position offset.
+insert(offset: int, b: aPrimitiveType):
StringBuilder
Inserts a value converted to a string into this builder.
+insert(offset: int, s: String): StringBuilder
Inserts a string into this builder at the position offset.
+replace(startIndex: int, endIndex: int, s:
String): StringBuilder
Replaces the characters in this builder from startIndex
to endIndex with the specified string.
+reverse(): StringBuilder
Reverses the characters in the builder.
+setCharAt(index: int, ch: char): void
Sets a new character at the specified index in this
builder.
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The toString, capacity, length,
setLength, and charAt Methods
java.lang.StringBuilder
+toString(): String
Returns a string object from the string builder.
+capacity(): int
Returns the capacity of this string builder.
+charAt(index: int): char
Returns the character at the specified index.
+length(): int
Returns the number of characters in this builder.
+setLength(newLength: int): void
Sets a new length in this builder.
+substring(startIndex: int): String
Returns a substring starting at startIndex.
+substring(startIndex: int, endIndex: int):
String
Returns a substring from startIndex to endIndex-1.
+trimToSize(): void
Reduces the storage size used for the string builder.
39
Examples
public static void main(String[] args) {
StringBuilder stringBuilder = new StringBuilder("Welcome to ");
System.out.println(stringBuilder);
stringBuilder.append("Java");
System.out.println(stringBuilder);
stringBuilder.insert(11, "HTML and ");
System.out.println(stringBuilder);
stringBuilder.delete(8, 11); // changes the builder to Welcome HTML and Java.
System.out.println(stringBuilder);
stringBuilder.deleteCharAt(8); // changes the builder to Welcome TML and Java.
System.out.println(stringBuilder);
stringBuilder.reverse(); // changes the builder to avaJ dna LMT emocleW
System.out.println(stringBuilder);
stringBuilder.reverse(); // undoes the reverse()
System.out.println(stringBuilder);
stringBuilder.replace(11, 15, " to"); // changes the builder to Welcome TML to Java.
System.out.println(stringBuilder);
stringBuilder.delete(7,11); // back to Welcome to Java
System.out.println(stringBuilder);
stringBuilder.setCharAt(0, 'w'); // sets the builder to welcome to Java.
System.out.println(stringBuilder);
}
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StringBuilder
public class StringBuilderDemo{
public static void main(String[] Args){
StringBuilder sb = new StringBuilder("Bakey");
System.out.println(sb);
sb.insert(4, "ry");
System.out.println(sb);
sb.deleteCharAt(5);
System.out.println(sb);
// this will not do what you expect!
sb.append(" " + sb.reverse());
System.out.println(sb);
sb.delete(sb.indexOf(" "), sb.length());
System.out.println(sb);
StringBuilder sb2 = new StringBuilder(sb);
sb.deleteCharAt(5);
sb2.reverse();
sb.append(sb2);
System.out.println(sb);
sb.insert(5," ");
System.out.println(sb);
sb.replace(0,0,"Y");
System.out.println(sb);
sb.deleteCharAt(1);
System.out.println(sb);
sb.reverse();
System.out.println(sb);
}
}