Java Strings

  • View
    644

  • Download
    2

Embed Size (px)

Transcript

  • 1.Java Strings Presented By: Rabia Saeed 301

2. Strings Intr0duction The String class The Character class The StringBuffer class 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. The String Class(1) String is in java.lang package. Since java.lang.* is always imported automatically, we dont need to import the String class. Declaration: String s1; Initialization: s1=Information Technology; Or, short-cut: String s1=Information Technology; 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. 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. 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. 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()); Lets try this out: Public class TestString { public static void main(String[] args){ String s1=npic; 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. 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 changeds: String String object for "Java": String String object for "HTML"This string object is now unreferenced 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 changeds: String String object for "Java": String String object for "HTML"This string object is now unreferenced 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 objects intern method to return an interned string. For example, the following statements: 13. Examples String s = "Welcome to Java";s s2String s1 = new String("Welcome to Java"); String ds3: 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 trues1: 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. 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. Trace C0de String s = "Welcome to Java";sd: 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. Trace C0de String s = "Welcome to Java"; ds 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. Trace C0de String s = "Welcome to Java";sf 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. Finiding String Length Finding string length using the length() method: message = "Welcome"; message.length() (returns 7) 19. Retrieving Individual Characters in a String Do not use message[0] Use message.charAt(index) Index starts from 0 Indices0123456messageWelcomemessage.charAt(0)789tomessage.length() is 1510 11 12 13 14 Javamessage.charAt(14) 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. 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";Indices0123456messageWelcome789tomessage.substring(0, 11)10 11 12 13 14 Javamessage.substring(11) 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. 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. 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. Finding a Character or a Substring in a String "Welcome "Welcome "Welcome "Welcome "Welcome "Welcome "Welcometo to to to to to toJava".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. 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. Example Count the number of words in a given String Example: Input: National Polytechnic Institute of Cambodia Output: Word Count: 5 words 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[]): StringBufferAppends a char array into this string buffer+append(data: char[], offset: int, len: int): StringBufferAppends a subarray in data into this string buffer+append(v: aPrimitiveType): StringBufferAppends a primitive type value as string to this buffer+append(str: String): StringBufferAppends a string to this string buffer+capacity(): intReturns the capacity of this string buffer+charAt(index: int): charReturns the character at the specified index+delete(startIndex: int, endIndex: int): StringBufferDeletes characters from startIndex to endIndex+deleteCharAt(int index): StringBufferDeletes a character at the specified index+insert(index: int, data: char[], offset: int, len: int): StringBufferInserts a subarray of the data in the array to the buffer at the specified index+insert(offset: int, data: char[]): StringBufferInserts data to this buffer at the position offset+insert(offset: int, b: aPrimitiveType): StringBufferInserts a value converted to string into this buffer+insert(offset: int, str: String): StringBufferInserts a string into this buffer at the position offset+length(): intReturns the number of characters in this buffer+replace(int startIndex, int endIndex, String str): StringBufferReplaces the characters in this buffer from startIndex to endIndex with the specified string+reverse(): StringBufferReveres the characters in the buffer+setCharAt(index: int, ch: char): voidSets a new character at the specified index in this buffer+setLength(newLength: int): voidSets a new length in this buffer+substring(startIndex: int): StringReturns a substring starting at startIndex+substring(startIndex: int, endIndex: int): StringReturns a substring from startIndex to endIndex 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. 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. 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(): intReturns the number of remaining tokens.+hasMoreTokens(): booleanReturns true if there are more tokens left.+nextToken(): StringReturns the next token.+nextToken(delimiters: String): String Returns the next token using new delimiters. 32. Examp