Strings, which are widely used in Java programming, are a sequence of characters. In the Java programming language, strings are objects.
\nThe Java platform provides the
String<\/CODE> <\/A>class to create and manipulate strings. <\/P>
\nCreating Strings<\/H3>The most direct way to create a string is to write:
\nString greeting = “Hello world!”;
\n<\/PRE><\/BLOCKQUOTE>In this case, “Hello world!” is a string literal<\/I>\u2014a series of characters in your code that is enclosed in double quotes. Whenever it encounters a string literal in your code, the compiler creates aString<\/CODE> object with its value\u2014in this case,Hello world!<\/CODE>.
\nAs with any other object, you can create
String<\/CODE> objects by using thenew<\/CODE> keyword and a constructor. TheString<\/CODE> class has 11 constructors that allow you to provide the initial value of the string using different sources, such as an array of characters: <\/P>
\nchar[] helloArray = { ‘h’, ‘e’, ‘l’, ‘l’, ‘o’, ‘.’};
\nString helloString = new String(helloArray);
\nSystem.out.println(helloString);
\n<\/PRE><\/BLOCKQUOTE>The last line of this code snippet displayshello<\/CODE>.
\n<\/P>
\n
\n
\nNote:<\/STRONG> TheString<\/CODE> class is immutable, so that once it is created aString<\/CODE> object cannot be changed. TheString<\/CODE> class has a number of methods, some of which will be discussed below, that appear to modify strings. Since strings are immutable, what these methods really do is create and return a new string that contains the result of the operation.
\n
\n<\/BLOCKQUOTE>
\nString Length<\/H3>Methods used to obtain information about an object are known as accessor methods<\/EM>. One accessor method that you can use with strings is the
length()<\/CODE> method, which returns the number of characters contained in the string object. After the following two lines of code have been executed,len<\/CODE> equals 17:
\nString palindrome = “Dot saw I was Tod”;
\nint len = palindrome.length();
\n<\/PRE><\/BLOCKQUOTE>A palindrome<\/I> is a word or sentence that is symmetric\u2014it is spelled the same forward and backward, ignoring case and punctuation. Here is a short and inefficient program to reverse a palindrome string. It invokes theString<\/CODE> methodcharAt(i)<\/CODE>, which returns the ith<\/FONT><\/SUP> character in the string, counting from 0.
\npublic class StringDemo {
\n public static void main(String[] args) {
\n String palindrome = “Dot saw I was Tod”;
\n int len = palindrome.length();
\n char[] tempCharArray = new char[len];
\n char[] charArray = new char[len];<\/p>\n\/\/ put original string in an array of chars
\n for (int i = 0; i < len; i++) {
\n tempCharArray[i] = palindrome.charAt(i);
\n } <\/p>\n\/\/ reverse array of chars
\n for (int j = 0; j < len; j++) {
\n charArray[j] = tempCharArray[len – 1 – j];
\n }<\/p>\nString reversePalindrome = new String(charArray);
\n System.out.println(reversePalindrome);
\n }
\n}<\/PRE><\/BLOCKQUOTE>Running the program produces this output:
\ndoT saw I was toD
\n<\/PRE><\/BLOCKQUOTE>To accomplish the string reversal, the program had to convert the string to an array of characters (firstfor<\/CODE> loop), reverse the array into a second array (secondfor<\/CODE> loop), and then convert back to a string. TheString<\/CODE> <\/A>class includes a method,getChars()<\/CODE>, to convert a string, or a portion of a string, into an array of characters so we could replace the firstfor<\/CODE> loop in the program above with
\npalindrome.getChars(0, len – 1, tempCharArray, 0);
\n<\/PRE><\/BLOCKQUOTE>
\nConcatenating Strings<\/H3>The
String<\/CODE> class includes a method for concatenating two strings:
\nstring1.concat(string2);
\n<\/PRE><\/BLOCKQUOTE>This returns a new string that is string1 with string2 added to it at the end.
\nYou can also use the
concat()<\/CODE> method with string literals, as in: <\/P>
\n“My name is “.concat(“Rumplestiltskin”);
\n<\/PRE><\/BLOCKQUOTE>Strings are more commonly concatenated with the+<\/B><\/CODE> operator, as in
\n“Hello,” + ” world” + “!”
\n<\/PRE><\/BLOCKQUOTE>which results in
\n“Hello, world!”
\n<\/PRE><\/BLOCKQUOTE>The+<\/B><\/CODE> operator is widely used inprint<\/CODE> statements. For example:
\nString string1 = “saw I was “;
\nSystem.out.println(“Dot ” + string1 + “Tod”);
\n<\/PRE><\/BLOCKQUOTE>which prints
\nDot saw I was Tod
\n<\/PRE><\/BLOCKQUOTE>Such a concatenation can be a mixture of any objects. For each object that is not aString<\/CODE>, itstoString()<\/CODE> method is called to convert it to aString<\/CODE>.
\n
\n
\nNote:<\/STRONG> The Java programming language does not permit literal strings to span lines in source files, so you must use the+<\/CODE> concatenation operator at the end of each line in a multi-line string. For example,
\nString quote = “Now is the time for all good ” +
\n “men to come to the aid of their country.”;
\n<\/PRE><\/BLOCKQUOTE>Breaking strings between lines using the+<\/CODE> concatenation operator is, once again, very common inprint<\/CODE> statements.
\n
\n<\/BLOCKQUOTE>
\nCreating Format Strings<\/H3>You have seen the use of the
printf()<\/CODE> andformat()<\/CODE> methods to print output with formatted numbers. TheString<\/CODE> class has an equivalent class method,format()<\/CODE>, that returns aString<\/CODE> object rather than aPrintStream<\/CODE> object.
\nUsing
String’s<\/CODE> staticformat()<\/CODE> method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of <\/P>
\nSystem.out.printf(“The value of the float variable is %f, while the value of the ” +
\n “integer variable is %d, and the string is %s”, floatVar, intVar, stringVar);
\n<\/PRE><\/BLOCKQUOTE>you can write
\nString fs;
\nfs = String.format(“The value of the float variable is %f, while the value of the ” +
\n “integer variable is %d, and the string is %s”, floatVar, intVar, stringVar);
\nSystem.out.println(fs);
\n<\/PRE><\/BLOCKQUOTE><\/BLOCKQUOTE>
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