STRINGS AND STRINGBUILDERS. Spring 2019

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1 STRINGS AND STRINGBUILDERS Spring 2019

2 STRING BASICS In Java, a string is an object. Three important pre-built classes used in string processing: the String class used to create and store immutable strings the StringBuilder class strings that can be modified the StringTokenizer class a utility class used to extract tokens from a string

3 THE String CLASS Part of the java.lang package 13 constructors and nearly 50 methods API from Oracle full listing of class features Strings are immutable, which means that once you build a String object, it is fixed it cannot be changed The only methods that can alter or set the instance variables are the constructors. Any non-constructor method that seems to change a string actually returns a new String object You can assign a String reference variable to a new string, discarding the old one

4 CONSTRUCTING A STRING One constructor allows you to use a string literal as a parameter. Examples: String greeting = new String( Hello, World! ); String subject = new String( Math ); You can also use the following shorthand notation: String sentence = The quick brown fox sat around for a while ;

5 COMMON String CLASS METHODS equals() returns true if two strings are the same, false otherwise if (str1.equals(str2)) System.out.print( The strings are the same ); compareto() good for comparing two strings, good for sorting (similar to strcmp in C) if (str1.compareto(str2) < 0) System.out.print("str1 comes before str2 in lexicographic ordering"); else if (str1.compareto(str2) == 0) System.out.print("str1 is the same as str2"); else if (str1.compareto(str2) > 0) System.out.print("str2 comes before str1 in lexicographic ordering"); equalsignorecase() just like equals()except the case doesn t matter in making a match. There are other comparison methods such as regionmatches, startswith, and endswith. See the API for details. Note: Don t try to compare strings using ==, <, >, etc. This only compares their reference variables, not the objects themselves Here is an example of string comparisons (note: comparisons are lexicographic ordering, not alphabetic)

6 COMMON String CLASS METHODS concat() Returns a concatenation of two strings String s1 = "Dog"; String s2 = "food"; String s3 = s1.concat(s2); // s3 now stores the string //"Dogfood" // note: s1 and s2 are NOT changed The + symbol also performs concatenation String s1 = "Cat"; String s2 = "nap"; String s3 = s1 + s2; // s3 now stores "Catnap //(s1 and s2 unchanged)

7 COMMON String CLASS METHODS substring() extracts part of a string and returns it. Takes two parameters (begin index and end index) or 1 parameter (begin index). Strings are 0 -indexed. Substring returned is [begin, end). String s1 = "Hello, World"; String s2 = s1.substring(0,5); // s2 is now "Hello". // picks up indices 0 4 String s3 = s1.substring(0,7) + "Dolly"; System.out.print(s3); // prints "Hello, Dolly" System.out.print(s3.substring(4)); // prints "o, Dolly" // can even use substring on string literals String s4 = "What's up doc?".substring(10,13); // s4 = "doc" length() return s a string s length (number of characters)

8 COMMON String CLASS METHODS charat() returns the character at a given index String s1 = "Rumplestiltskin"; System.out.print(s1.charAt(5)); // output: e Some conversion methods tolowercase() returns lower case version of string touppercase() returns upper case version of string trim() returns a string that eliminates leading and trailing blank characters replace() returns a string with one character replaced with a new one String s1 = "Zebra"; String s2 = s1.tolowercase(); // s2 is "zebra" String s3 = s1.touppercase(); // s3 is "ZEBRA" String s4 = " Apple "; String s5 = s4.trim(); // s5 is "Apple" String s6 = s5.replace('e', 'y'); // s6 is "Apply"

9 COMMON String CLASS METHODS valueof() There are several overloaded versions of this. They are static methods used for converting other values to String objects int x = 12345; String s7 = String.valueOf(4.56); // s7 is "4.56" String s8 = String.valueOf(16); // s8 is "16" String s9 = String.valueOf(x); // s9 is "12345" format() Static method that takes 1 or more parameters that returns a String formatted as if you had passed the parameters to a call to printf() isempty() Returns true if the string is the empty string (length of 0) isblank() Returns true if the string contains only whitespace characters This example has some String class method calls

10 SWITCH STATEMENTS Beginning with Java 7, you are allowed to use a String object in the expression of a switch statement This means you can also use String literals for the case labels Each case label is compared to the String object using the String.equals() method

11 THE StringBuilder CLASS Part of java.lang package API from Oracle full listing of class features A StringBuilder object is mutable (can be changed) StringBuilder objects have both length (number of characters) and capacity (current memory allocated) Some example constructions: // creates an empty string buffer with initial capacity of 16 // characters StringBuilder buf1 = new StringBuilder(); // creates empty string buffer with initial capacity given in // parameter StringBuilder buf2 = new StringBuilder(50); // creates string buffer filled with argument -- initial capacity // is length of given string plus 16 StringBuilder buf3 = new StringBuilder("Hello, World");

12 COMMON StringBuilder CLASS METHODS append() adds data to string in the buffer, several versions for different parameter types (see API for full set) StringBuilder buf1 = new StringBuilder(); buf1.append("hello"); buf1.append(','); buf1.append(" world!"); // buf1 is now "Hello, world!" buf1.append(' '); buf1.append(123.45); // buf1 is now "Hello, world! " insert() inserts data at a certain starting index. Multiple versions for different data types StringBuilder buf2 = new StringBuilder(); buf2.append("welcome home"); // buf2 now "Welcome home" buf2.insert(8,"to my humble "); // buf2 = "Welcome to my humble home"

13 COMMON StringBuilder CLASS METHODS delete(begin, end) deletes data from a string buffer StringBuilder buf3 = new StringBuilder("abcdefghijklm"); buf3.delete(4,9); // deletes indices 4-8. // buf3 is now "abcdjklm" deletecharat() delete a character at a specified index StringBuilder buf4 = new StringBuilder("abcdefg"); buf4.deletecharat(3); // buf4 is now "abcefg" buf4.deletecharat(1); // buf4 is now "acefg" Here is an example containing some inserts and deletes

14 COMMON StringBuilder CLASS METHODS reverse() reverses the contents of the string buffer setcharat() sets a character at the specified index (similar to deletecharat) capacity() returns the capacity of the buffer length() returns the length of the current string in buffer (<= capacity) setlength() sets the exact length of the string in the buffer to the new value (parameter). This is the string itself, not the capacity. If the new length is smaller than previous length, characters are truncated. If new length is bigger, null characters are appended. charat() returns character at a specified index (parameter)

15 THE StringTokenizer CLASS Part of java.util package API from Oracle full listing of class features The tokens are chunks of the string broken apart based on a set of delimiters Class contains methods that allow specifying the delimiter characters, separating a string into tokens, and counting the tokens

16 THE StringTokenizer CLASS Example usage: String s1 = "The quick brown fox jumped over the lazy dog."; StringTokenizer st = new StringTokenizer(s1); // default delimiters are space, tab, newline, carriage return System.out.print("Number of words = " + st.counttokens()); // prints "Number of words = 9" String s2 = st.nexttoken(); String s3 = st.nexttoken(); String s4 = st.nexttoken(); // s2 = "The" // s3 = "quick" // s4 = "brown You can also tokenize strings without using a tokenizer if you want to use the default delimiters Example of tokenizing a string without a tokenizer

17 COMMAND LINE ARGUMENTS Recall that the main method of a Java program looks like this: public static void main(string[] args) The String[] args part allows a set of arguments to be passed into the program from the command line. Suppose we execute the following command: java MyProgram file1.txt output lazy Inside the main program, the arguments are stored in an array of strings: args[0] is file1.txt args[1] is output args[2] is lazy args.length stores the number of parameters passed (3 here) More String examples (Deitel chapter 14)

Intro to Strings. Lecture 7 COP 3252 Summer May 23, 2017

Intro to Strings. Lecture 7 COP 3252 Summer May 23, 2017 Intro to Strings Lecture 7 COP 3252 Summer 2017 May 23, 2017 Strings in Java In Java, a string is an object. It is not a primitive type. The String class is used to create and store immutable strings.