Strings, which are widely used in Java programming, are a sequence of characters. In the Java programming language, strings are objects.The Java platform provides the
String
class to create and manipulate strings.Creating Strings
The most direct way to create a string is to write:In this case, "Hello world!" is a string literala 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 greeting = "Hello world!";String
object with its value—in this case,Hello world!
.As with any other object, you can create
String
objects by using thenew
keyword and a constructor. TheString
class has 11 constructors that allow you to provide the initial value of the string using different sources, such as an array of characters:The last line of this code snippet displayschar[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.'}; String helloString = new String(helloArray); System.out.println(helloString);hello
.
Note: TheString
class is immutable, so that once it is created aString
object cannot be changed. TheString
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.String Length
Methods used to obtain information about an object are known as accessor methods. One accessor method that you can use with strings is thelength()
method, which returns the number of characters contained in the string object. After the following two lines of code have been executed,len
equals 17:A palindrome is a word or sentence that is symmetric—it 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 palindrome = "Dot saw I was Tod"; int len = palindrome.length();String
methodcharAt(i)
, which returns the ith character in the string, counting from 0.Running the program produces this output:public class StringDemo { public static void main(String[] args) { String palindrome = "Dot saw I was Tod"; int len = palindrome.length(); char[] tempCharArray = new char[len]; char[] charArray = new char[len]; // put original string in an array of chars for (int i = 0; i < len; i++) { tempCharArray[i] = palindrome.charAt(i); } // reverse array of chars for (int j = 0; j < len; j++) { charArray[j] = tempCharArray[len - 1 - j]; } String reversePalindrome = new String(charArray); System.out.println(reversePalindrome); } }To accomplish the string reversal, the program had to convert the string to an array of characters (firstdoT saw I was toDfor
loop), reverse the array into a second array (secondfor
loop), and then convert back to a string. TheString
class includes a method,getChars()
, to convert a string, or a portion of a string, into an array of characters so we could replace the firstfor
loop in the program above withpalindrome.getChars(0, len - 1, tempCharArray, 0);Concatenating Strings
TheString
class includes a method for concatenating two strings:This returns a new string that is string1 with string2 added to it at the end.string1.concat(string2);You can also use the
concat()
method with string literals, as in:Strings are more commonly concatenated with the"My name is ".concat("Rumplestiltskin");+
operator, as inwhich results in"Hello," + " world" + "!"The"Hello, world!"+
operator is widely used inwhich printsString string1 = "saw I was "; System.out.println("Dot " + string1 + "Tod");Such a concatenation can be a mixture of any objects. For each object that is not aDot saw I was TodString
, itstoString()
method is called to convert it to aString
.
Note: The Java programming language does not permit literal strings to span lines in source files, so you must use the+
concatenation operator at the end of each line in a multi-line string. For example,Breaking strings between lines using theString quote = "Now is the time for all good " + "men to come to the aid of their country.";+
concatenation operator is, once again, very common inCreating Format Strings
You have seen the use of theprintf()
andformat()
methods to print output with formatted numbers. TheString
class has an equivalent class method,format()
, that returns aString
object rather than aPrintStream
object.Using
String's
staticformat()
method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead ofyou can writeSystem.out.printf("The value of the float variable is %f, while the value of the " + "integer variable is %d, and the string is %s", floatVar, intVar, stringVar);String fs; fs = String.format("The value of the float variable is %f, while the value of the " + "integer variable is %d, and the string is %s", floatVar, intVar, stringVar); System.out.println(fs);