Java课件第二章补充字符串

1、1,I Constructing a String,II String Methods,2,The String Class,Constructing a String: Obtaining String length and Retrieving Individual Characters in a string String String Concatenation (concat) Substrings (substring(index), substring(start, end) Comparisons (equals, compareTo) String Conversions F

2、inding a Character or a Substring in a String Conversions between Strings and Arrays Converting Characters and Numeric Values to Strings,3,Constructing Strings,Format: String newString = new String(stringLiteral); String message = “stringLiteral;,Example: String message = new String(“Hello); String

3、message = Welcome to Java;,4,Storage String pool ,Difference between new and shorthand initializer,Question: How many String objects are created?,Two,Constructor: publicString(Stringoriginal) /othercode. ,Note:The parameter is a String object,6,Stringa= abc; String b= abc;,Difference between new and

4、 shorthand initializer,Question: How many String objects are created?,One,7,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.,8,Strings Are Immutable,A String object is immutable; its contents c

5、annot be changed.,Does the following code change the contents of the string String s = Java; s = HTML;,？,9,0 x1245,s,Trace Code,String s = Java; s = HTML;,After executing s=“java”;,After executing s=“HTML”;,10,Canonical Strings（规范字符串）,canonical string: if two String objects were created with the sam

6、e string literal using the shorthand initializer, then the JVM stores them in the same object. This string literal is called canonical string.,improve efficiency and save memory,Using String objects intern method to return a canonical string,11,intern: public native String intern() Returns a canonic

7、al representation for the string object. If s and t are strings such that s.equals(t), it is guaranteed that ern() = ern().,Canonical Strings,12,String s = Welcome to Java; String s1 = new String(Welcome to Java); String s2 = ern(); String s3 = Welcome to Java; System.out.println(s1

8、= s is + (s1 = s); System.out.println(s2 = s is + (s2 = s); System.out.println(s = s3 is + (s = s3);,Examples,display s1 = s is false s2 = s is true s = s3 is true,13,Finding String Length,Finding string length using the length() method:,Example：message = Welcome; message.length() (returns 7),14,Ret

9、rieving Individual Characters in a String,Do not use message0 Use message.charAt(index) public char charAt(int index) Returns the character at the specified index. An index ranges from 0 to length() - 1. Index starts from 0,charAt方法的使用,15,public String concat(String str) Concatenates the specified s

10、tring to the end of this string. If the length of the argument string is 0, then this object is returned. Parameters: the String that is concatenated to the end of this String. Returns: a string that represents the concatenation of this objects characters followed by the string arguments characters.

11、,String Concatenation,16,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);,17,public String substring(int beginIndex) Returns a new string that is a substring of this string. The substring begins at th

12、e specified index and extends to the end of this string. public String substring(int beginIndex, int endIndex) Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Parameters: the beginning index

13、, inclusive. Returns: the specified substring. Throws: StringIndexOutOfBoundsException if the beginIndex is out of range.,Extracting Substrings,18,Extracting Substrings,String is an immutable class; its valuescannot be changed individually. String s1 = Welcome to Java; String s2 = s1.substring(0, 11

14、) + HTML;,19,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 ,20,String Comparisons, cont.,compareTo(Object object) String s1 = new String(Welcome“); String s2 = we

15、lcome; if (pareTo(s2) 0) / s1 is greater than s2 else if (pareTo(s2) = 0) / s1 and s2 have the same contents else / s1 is less than s2,21,String Conversions,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

16、: toLowerCase toUpperCase trim replace(oldChar, newChar),22,Finding a Character or a Substring in a String,Welcome to Java.indexOf(W) returns 0. Welcome to Java.indexOf(x) returns -1. Welcome to Java.indexOf(o, 5) returns 9. Welcome to Java.indexOf(come) returns 3. Welcome to Java.indexOf(Java, 5) r

17、eturns 11. Welcome to Java.indexOf(java, 5) returns -1. Welcome to Java.lastIndexOf(a) returns 14.,23,Convert Character and Numbers to Strings,The String class provides several static valueOf methods for converting a character, an array of characters, and numeric values to strings. These methods hav

18、e the same name valueOf with different argument types char, char, double, long, int, and float. For example, to convert a double value to a string, use String.valueOf(5.44). The return value is string consists of characters 5, ., 4, and 4.,24,The Character Class,25,Examples,Character charObject = ne

19、w Character(b); charOpareTo(new Character(a) returns 1 charOpareTo(new Character(b) returns 0 charOpareTo(new Character(c) returns -1 charOpareTo(new Character(d) returns 2 charObject.equals(new Character(b) returns true charObject.equals(new Character(d) returns false,26,The StringBuffer Class,The

20、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 contentsinto a string buffer. However, the value ofa String object is fixed once the string is cre

21、ated.,27,28,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 charactersas the string argumen

22、t. Initial capacity 16plus the length of the string argument.,29,Appending New Contents into a String Buffer,StringBuffer strBuf = new StringBuffer(); strBuf.append(Welcome); strBuf.append( ); strBuf.append(to); strBuf.append( ); strBuf.append(Java);,30,Example:,String s=“ABCD”; s.concat(“E”); s.rep

23、lace(C,F); System.out.println(s); What will be printed out? A. Compilation Error says that String is immutable B. ABFDE C. ABCDE D. ABCD,31,class Happy static StringBuffer sb1=new StringBuffer(A); static StringBuffer sb2=new StringBuffer(B); public static void main(String args) Happy h=new Happy();

24、h.methodA(sb1,sb2); System.out.println(sb1+ +sb2); public void methodA(StringBuffer x,StringBuffer y) y.append(x); What will be the output?,32,The StringTokenizer Class,33,Examples 1,String s = Java is cool.; StringTokenizer tokenizer = new StringTokenizer(s); System.out.println(The total number of

25、tokens is + tokenizer.countTokens(); while (tokenizer.hasMoreTokens() System.out.println(tokenizer.nextToken();,The total number of tokens is 3 Java is cool.,The code displays,34,Examples 2,String s = Java is cool.; StringTokenizer tokenizer = new StringTokenizer(s, ac); System.out.println(The total

26、 number of tokens is + tokenizer.countTokens(); while (tokenizer.hasMoreTokens() System.out.println(tokenizer.nextToken();,The total number of tokens is 4 J v is ool.,The code displays,35,Examples 3,String s = Java is cool.; StringTokenizer tokenizer = new StringTokenizer(s, ac, true); System.out.pr

27、intln(The total number of tokens is + tokenizer.countTokens(); while (tokenizer.hasMoreTokens() System.out.println(tokenizer.nextToken();,The total number of tokens is 7 J a v a is c ool.,The code displays,36,No no-arg Constructor in StringTokenizer,The StringTokenizer class does not have a no-arg c

28、onstructor. Normally it is a good programming practice to provide a no-arg constructor for each class. On rare occasions, however, a no-arg constructor does not make sense. StringTokenizer is such an example. A StringTokenizer object must be created for a string, which should be passed as an argumen

29、t from a constructor.,37,The Scanner Class,The delimiters are single characters in StringTokenizer. You can use the new JDK 1.5 java.util.Scanner class to specify a word as a delimiter.,JDK 1.5 Feature,String s = Welcome to Java! Java is fun! Java is cool!; Scanner scanner = new Scanner(s); scanner.

30、useDelimiter(Java); while (scanner.hasNext() System.out.println(scanner.next();,Creates an instance of Scanner for the string.,Sets “Java” as a delimiter.,hasNext() returns true if there are still more tokens left.,The next() method returns a token as a string.,Welcome to ! is fun! is cool!,38,Scann

31、ing Primitive Type Values,If a token is a primitive data type value, you can use the methods nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), or nextBoolean() to obtain it. For example, the following code adds all numbers in the string. Note that the delimiter is space by default.,String s = 1 2 3 4; Scanner scanner = new Scanner(s); int sum = 0; while (scanner.hasNext() sum += scanner.nextInt(); System.out.println(Sum is + sum);,JDK 1.5 Feature,39,Console Input Using Scanner,Another important application of the Scanner class is to read