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Chapter 9 Characters and Strings. Topics Character primitives Character Wrapper class More String Methods String Comparison String Buffer String Tokenizer.

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Presentation on theme: "Chapter 9 Characters and Strings. Topics Character primitives Character Wrapper class More String Methods String Comparison String Buffer String Tokenizer."— Presentation transcript:

1 Chapter 9 Characters and Strings

2 Topics Character primitives Character Wrapper class More String Methods String Comparison String Buffer String Tokenizer

3 Character Data In Java, single characters are represented using the primitive data type char. Character constants are written as symbols enclosed in single quotes: char ch1 = 'X';

4 Character Encoding Characters are stored in memory as integer values Each character has a unique "code" that is used to represent it. Having a standard encoding allows different computers to share information easily. Several encoding schemes have been used –ASCII –EBCDIC (not used any more) –Unicode

5 ASCII Codes ASCII stands for American Standard Code for Information Interchange. ASCII is one of the document coding schemes widely used today. –each character represented by 8 bits ( values 0-255) ASCII works well for English-language documents because all characters and punctuation marks are included in the ASCII codes. ASCII does not represent the full character sets of other languages.

6 Unicode The Unicode Worldwide Character Standard (Unicode) supports the interchange, processing, and display of the written texts of diverse languages. Java uses the Unicode standard for representing char constants. Each character is represented with 2 bytes (16 bits).

7 Character Class Like the other primitive types, there is a wrapper class for the char type. Useful Methods –charValue( ) returns a char that is the value stored in the Character –getNumericValue( ) returns the Unicode value for the char

8 Static CharacterMethods boolean methods –isDigit( char) –isLetter( char) –isSpace( char) –isUpperCase( char) –isLowerCase(char) other methods –char toLowerCase( char) –toUpperCase( char)

9 Strings A string is a sequence of characters that is treated as a single value. Instances of the String class are used to represent strings in Java. We access individual characters of a string by calling the charAt method of the String object.

10 Strings Each character in a string has an index that we can use to access the character. We know for learning about arrays that Java uses zero-based indexing –the first character’s index is 0, the second is 1, and so on. To refer to the first character of the word name, we say name.charAt(0)

11 String Indexing An indexed expression is used to refer to individual characters in a string.

12 Strings Since String is a class, we can create an instance of a class by using the new method. The statements we have used so far, such as String name1 = “Kona”; works as a shorthand for String name1 = new String(“Kona”); But this shorthand works for the String class only.

13 Other String Methods String toLowerCase() String toUpperCase() String replace( char oldChar, char newChar); and many more

14 String Comparison String comparison may be done in several ways. –The methods equals and equalsIgnoreCase compare string values; one is case-sensitive and one is not. –The method compareTo returns an int value: Zero (0) if the strings are equal. A negative integer if the first string is less than the second. A positive integer if the first string is greater than the second.

15 String Equality Comparing String objects is similar to comparing other objects. The equality test (==) is true if the contents of the variables are the same. –For a reference data type, the equality test is true if both variables refer to the same object, because they both contain the same address. The equals method is true if the String objects to which the two variables refer contain the same string value. –the same sequence of characters

16 Equality Testing The difference between the equality test and the equals method.

17 Equality Testing

18 Equality Testing for Strings As long as a new String object is created using the new operator, the rule for comparing objects applies to comparing strings. String str = new String (“Java”); If the new operator is not used, a String reference refers to the literal string constant. String str = “Java”; –There will be only one copy of any literal string

19 Memory Diagram The difference between using and not using the new operator for String.

20 Immutability When a String object is created, it cannot be changed. –Look at the documentation - there are no mutator methods –We say Strings are immutable. Manipulating the content of a String requires the creation of a new String object. –If you are doing a lot of manipulation this is not very efficient.

21 StringBuffer Class Manipulating the content of a string, such as replacing a character, appending a string with another string, deleting a portion of a string, and so on, may be accomplished more efficiently using the StringBuffer class.

22 StringBuffer Example StringBuffer word = new StringBuffer(“Java”); word.setCharAt(0, ‘D’); word.setCharAt(1, ‘i’ ); changes the string from “Java” to “Diva.”

23 This example reads a sentence and replaces all vowels in the sentence with the character X. tempStringBuffer= new StringBuffer(inSentence); numberOfCharacters = tempStringBuffer.length(); for (int index = 0; index < numberOfCharacters; index++) { if ( isVowel( letter) { tempStringBuffer.setCharAt(index,'X');

24 StringBuffer Operations We cannot input StringBuffer objects. –We must input String objects and convert them to StringBuffer objects. We use the append method to append a String or StringBuffer object to the end of a StringBuffer object. –The method append can also take an argument of the primitive data type. –Any primitive data type argument is converted to a string before it is appended to a StringBuffer object.

25 StringBuffer Insertion We can insert a string at a specified position by using the insert method..insert(, ); where must be greater than or equal to 0 and less than or equal to the length of, and the can be an object or a primitive. For example, executing StringBuffer str = new StringBuffer(“Java is great”); str.insert(8, “really”); changes the string " Java is great" to "Java is really great".

26 String Tokenizer A StringTokenizer object can be used to break a string up into smaller substrings (tokens) By default, the tokenizer separates the string at white space The default StringTokenizer constructor takes the original string to be separated as a parameter StringTokenizer st = new StringTokenizer( text); An alternate constructor allows you to specify different separators (delimiters). StringTokenizer st = new StringTokenizer( text, delims);

27 String Tokenizer Methods The hasMoreTokens method returns true if there are more tokens in the string. Each call to the nextToken method returns the next token in the string. If you need to know how many tokens there are, you can call countTokens

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