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Java Strings
Presented By:
Rabia Saeed 301
Strings
Intr0duction
The String class
The Character class
The StringBuffer class
Introduction
• A string is sequence (series) of characters.
• A string is NOT an array of characters.
E.g. in C/C++:

char...
The String Class(1)
• String is in java.lang package.
• Since java.lang.* is always imported automatically, we don’t
need ...
String Class(2)
• Because String is a class, then s1 is an object.
• So there should be constructors, methods, or properti...
The String Class
• Constructing a String:
• String message = "Welcome to Java“;
• String message = new String("Welcome to ...
C0nstructing Strings
String newString = new String(stringLiteral);
String message = new String("Welcome to Java");
Since s...
The String Class
• How do I get those methods to use?
• A. You just declare a variable as String.
String s1=“”;
Then, you ...
Strings Are Immutable
A String object is immutable; its contents cannot be changed.
• has no setter method).
• The String ...
Trace C0de
String s=“Java”;
s = "HTML";

After executing s = "HTML";

After executing String s = "Java";
s

: String
Strin...
Trace C0de
String s = "Java“;

s= “HTML”;

After executing s = "HTML";

After executing String s = "Java";
s

: String
Str...
Interned Strings
• Since strings are immutable and are frequently used, to improve
efficiency and save memory, the JVM use...
Examples
String s = "Welcome to Java";

s
s2

String s1 = new String("Welcome to Java");

• String
d

s3

: String
Interne...
Trace C0de
String s = "Welcome to Java";

•t
String s1 = new String("Welcome to Java");
String s2 = s1.intern();
String s3...
Trace C0de
String s = "Welcome to Java";

s

•d

: String
Interned string object for
"Welcome to Java"

String s1 = new St...
Trace C0de
•String s = "Welcome to Java";
d

s
s2

: String
Interned string object for
"Welcome to Java"

String s1 = new ...
Trace C0de
String s = "Welcome to Java";

s

•f
String s1 = new String("Welcome to Java");

s2
s3

: String
Interned strin...
Finiding String Length
Finding string length using the length() method:
message = "Welcome";
message.length() (returns 7)
Retrieving Individual Characters in a String
• Do not use message[0]
• Use message.charAt(index)
• Index starts from 0
Ind...
String C0ncatinati0n
String s3 = s1.concat(s2);
String s3 = s1 + s2;
s1 + s2 + s3 + s4 + s5 same as
(((s1.concat(s2)).conc...
Extracting Substring
You can extract a single character from a string
using the charAt method. You can also extract a
subs...
String Comparisons
• equals
String s1 = new String("Welcome“);
String s2 = "welcome";
if (s1.equals(s2)){
// s1 and s2 hav...
String Comparisons, cont.
• compareTo(Object object)
String s1 = new String("Welcome“);
String s2 = "welcome";
if (s1.comp...
String C0nversi0ns
The contents of a string cannot be changed once the string is
created. But you can convert a string to ...
Finding a Character or a Substring in a String
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome

to
to
to
to...
The StringBuffer Class
• The StringBuffer class is an alternative to the String class. In
general, a string buffer can be ...
Example
• Count the number of words in a given String
Example:
Input: National Polytechnic Institute of Cambodia
Output: W...
java.lang.StringBuffer
+StringBuffer()

Constructs an empty string buffer with capacity 16

+StringBuffer(capacity: int)

...
StringBuffer Constructors
• public StringBuffer()
No characters, initial capacity 16 characters.
• public StringBuffer(int...
Appending New Contents
into a String Buffer
StringBuffer strBuf = new StringBuffer();
strBuf.append("Welcome");
strBuf.app...
The StringTokenizer Class
java.util.StringTokenizer
+StringTokenizer(s: String)

Constructs a string tokenizer for the str...
Example
String s = "Java is cool.";
StringTokenizer tokenizer = new StringTokenizer(s);
System.out.println("The total numb...
Java Strings
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Java Strings

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Java Strings

  1. 1. Java Strings Presented By: Rabia Saeed 301
  2. 2. Strings Intr0duction The String class The Character class The StringBuffer class
  3. 3. Introduction • A string is sequence (series) of characters. • A string is NOT an array of characters. E.g. in C/C++: char s[20]; • In Java, A String is an object. • Java has 3 String classes: • String • StringBuffer • StringTokenizer
  4. 4. The String Class(1) • String is in java.lang package. • Since java.lang.* is always imported automatically, we don’t need to import the String class. • Declaration: String s1; • Initialization: s1=“Information Technology”; • Or, short-cut: String s1=“Information Technology”;
  5. 5. String Class(2) • Because String is a class, then s1 is an object. • So there should be constructors, methods, or properties. • String constructors: • String() • String(String value) • String(char[] value) Ex: String s1 = new String(“IT”); • You just say: String s1 = “IT”;
  6. 6. The String Class • Constructing a String: • String message = "Welcome to Java“; • String message = new String("Welcome to Java“); • String s = new String(); • Obtaining String length and Retrieving Individual Characters in a string
  7. 7. C0nstructing 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";
  8. 8. The String Class • How do I get those methods to use? • A. You just declare a variable as String. String s1=“”; Then, you call a method, say length(). System.out.println(s1.length()); • Let’s try this out: Public class TestString { public static void main(String[] args){ String s1=“npic”;
  9. 9. Strings Are Immutable A String object is immutable; its contents cannot be changed. • has no setter method). • The String class is final so we cannot inherit from it. Does the following code change the contents of the string? String s = "Java"; s = "HTML";
  10. 10. Trace C0de 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" This string object is now unreferenced
  11. 11. Trace C0de 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" This string object is now unreferenced
  12. 12. Interned Strings • Since strings are immutable and are frequently used, to improve efficiency and save memory, the JVM uses a unique instance for string literals with the same character sequence. Such an instance is called interned. You can also use a String object’s intern method to return an interned string. For example, the following statements:
  13. 13. Examples String s = "Welcome to Java"; s s2 String s1 = new String("Welcome to Java"); • String d s3 : String Interned string object for "Welcome to Java" s2 = s1.intern(); String s3 = "Welcome to Java"; System.out.println("s1 == s is " + (s1 == s)); System.out.println("s2 == s is " + (s2 == s)); System.out.println("s == s3 is " + (s == s3)); display s1 == s is false s2 == s is true s == s3 is true s1 : String A string object for "Welcome to Java" A new object is created if you use the new operator. If you use the string initializer, no new object is created if the interned object is already created.
  14. 14. Trace C0de String s = "Welcome to Java"; •t String s1 = new String("Welcome to Java"); String s2 = s1.intern(); String s3 = "Welcome to Java"; s : String Interned string object for "Welcome to Java"
  15. 15. Trace C0de String s = "Welcome to Java"; s •d : String Interned string object for "Welcome to Java" String s1 = new String("Welcome to Java"); String s2 = s1.intern(); String s3 = "Welcome to Java"; s1 : String A string object for "Welcome to Java"
  16. 16. Trace C0de •String s = "Welcome to Java"; d s s2 : String Interned string object for "Welcome to Java" String s1 = new String("Welcome to Java"); String s2 = s1.intern(); String s3 = "Welcome to Java"; s1 : String A string object for "Welcome to Java"
  17. 17. Trace C0de String s = "Welcome to Java"; s •f String s1 = new String("Welcome to Java"); s2 s3 : String Interned string object for "Welcome to Java" String s2 = s1.intern(); String s3 = "Welcome to Java"; s1 : String A string object for "Welcome to Java"
  18. 18. Finiding String Length Finding string length using the length() method: message = "Welcome"; message.length() (returns 7)
  19. 19. 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) 7 8 9 t o message.length() is 15 10 11 12 13 14 J a v a message.charAt(14)
  20. 20. String C0ncatinati0n 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);
  21. 21. Extracting Substring 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) 10 11 12 13 14 J a v a message.substring(11)
  22. 22. String Comparisons • equals String s1 = new String("Welcome“); String s2 = "welcome"; if (s1.equals(s2)){ // s1 and s2 have the same contents } if (s1 == s2) { // s1 and s2 have the same reference }
  23. 23. 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
  24. 24. String C0nversi0ns The contents of a string cannot be changed once the string is created. But you can convert a string to a new string using the following methods: • • • • toLowerCase toUpperCase trim replace(oldChar, newChar)
  25. 25. Finding a Character or a Substring in a String "Welcome "Welcome "Welcome "Welcome "Welcome "Welcome "Welcome 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.
  26. 26. The StringBuffer Class • The StringBuffer class is an alternative to the String class. In general, a string buffer can be used wherever a string is used. StringBuffer is more flexible than String. You can add, insert, or append new contents into a string buffer. However, the value of a String object is fixed once the string is created
  27. 27. Example • Count the number of words in a given String Example: Input: National Polytechnic Institute of Cambodia Output: Word Count: 5 words
  28. 28. java.lang.StringBuffer +StringBuffer() Constructs an empty string buffer with capacity 16 +StringBuffer(capacity: int) Constructs a string buffer with the specified capacity +StringBuffer(str: String) Constructs a string buffer with the specified string +append(data: char[]): StringBuffer Appends a char array into this string buffer +append(data: char[], offset: int, len: int): StringBuffer Appends a subarray in data into this string buffer +append(v: aPrimitiveType): StringBuffer Appends a primitive type value as string to this buffer +append(str: String): StringBuffer Appends a string to this string buffer +capacity(): int Returns the capacity of this string buffer +charAt(index: int): char Returns the character at the specified index +delete(startIndex: int, endIndex: int): StringBuffer Deletes characters from startIndex to endIndex +deleteCharAt(int index): StringBuffer Deletes a character at the specified index +insert(index: int, data: char[], offset: int, len: int): StringBuffer Inserts a subarray of the data in the array to the buffer at the specified index +insert(offset: int, data: char[]): StringBuffer Inserts data to this buffer at the position offset +insert(offset: int, b: aPrimitiveType): StringBuffer Inserts a value converted to string into this buffer +insert(offset: int, str: String): StringBuffer Inserts a string into this buffer at the position offset +length(): int Returns the number of characters in this buffer +replace(int startIndex, int endIndex, String str): StringBuffer Replaces the characters in this buffer from startIndex to endIndex with the specified string +reverse(): StringBuffer Reveres the characters in the buffer +setCharAt(index: int, ch: char): void Sets a new character at the specified index in this buffer +setLength(newLength: int): void Sets a new length in this buffer +substring(startIndex: int): String Returns a substring starting at startIndex +substring(startIndex: int, endIndex: int): String Returns a substring from startIndex to endIndex
  29. 29. StringBuffer Constructors • public StringBuffer() No characters, initial capacity 16 characters. • public StringBuffer(int length) No characters, initial capacity specified by the length argument. • public StringBuffer(String str) Represents the same sequence of characters as the string argument. Initial capacity 16 plus the length of the string argument.
  30. 30. Appending New Contents into a String Buffer StringBuffer strBuf = new StringBuffer(); strBuf.append("Welcome"); strBuf.append(' '); strBuf.append("to"); strBuf.append(' '); strBuf.append("Java");
  31. 31. The StringTokenizer Class java.util.StringTokenizer +StringTokenizer(s: String) Constructs a string tokenizer for the string. +StringTokenizer(s: String, delimiters: Constructs a string tokenizer for the string with the specified delimiters. String) +StringTokenizer(s: String, delimiters: Constructs a string tokenizer for the string with the delimiters and returnDelims. String, returnDelimiters: boolean) +countTokens(): int Returns the number of remaining tokens. +hasMoreTokens(): boolean Returns true if there are more tokens left. +nextToken(): String Returns the next token. +nextToken(delimiters: String): String Returns the next token using new delimiters.
  32. 32. Example String s = "Java is cool."; StringTokenizer tokenizer = new StringTokenizer(s); System.out.println("The total number of tokens is " + tokenizer.countTokens()); The total number of tokens is while (tokenizer.hasMoreTokens()) 3 System.out.println(tokenizer.nextToken()); Java The code displays is cool.

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