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The Java™ Tutorials
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Numbers and Strings
Numbers
The Numbers Classes
Formatting Numeric Print Output
Beyond Basic Arithmetic
Summary of Numbers
Questions and Exercises
Characters
Strings
Converting Between Numbers and Strings
Manipulating Characters in a String
Comparing Strings and Portions of Strings
The StringBuilder Class
Summary of Characters and Strings
Autoboxing and Unboxing
Questions and Exercises
Trail: Learning the Java Language
Lesson: Numbers and Strings
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# Strings

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:

    String greeting = "Hello world!";

In this case, "Hello world!" is a string literal—a series of characters in your code that is enclosed in double quotes. Whenever it encounters a string literal in your code, the compiler creates a `String` object with its value—in this case, `Hello world!`.

As with any other object, you can create `String` objects by using the `new` keyword and a constructor. The `String` class has thirteen constructors that allow you to provide the initial value of the string using different sources, such as an array of characters:

    char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.' };
    String helloString = new String(helloArray);
    System.out.println(helloString);

The last line of this code snippet displays `hello`.

Note: The `String` class is immutable, so that once it is created a `String` object cannot be changed. The `String` 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 the `length()` 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:

    String palindrome = "Dot saw I was Tod";
    int len = palindrome.length();

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 the `String` method `charAt(i)`, which returns the ith character in the string, counting from 0.

    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);
        }
    }

Running the program produces this output:

    doT saw I was toD

To accomplish the string reversal, the program had to convert the string to an array of characters (first `for` loop), reverse the array into a second array (second `for` loop), and then convert back to a string. The `String` 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 first `for` loop in the program above with

    palindrome.getChars(0, len, tempCharArray, 0);

## Concatenating Strings

The `String` class includes a method for concatenating two strings:

    string1.concat(string2);

This returns a new string that is string1 with string2 added to it at the end.

You can also use the `concat()` method with string literals, as in:

    "My name is ".concat("Rumplestiltskin");

Strings are more commonly concatenated with the `+` operator, as in

    "Hello," + " world" + "!"

which results in

    "Hello, world!"

The `+` operator is widely used in `print` statements. For example:

    String string1 = "saw I was ";
    System.out.println("Dot " + string1 + "Tod");

which prints

    Dot saw I was Tod

Such a concatenation can be a mixture of any objects. For each object that is not a `String`, its `toString()` method is called to convert it to a `String`.

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:

    String quote =
        "Now is the time for all good " +
        "men to come to the aid of their country.";

Breaking strings between lines using the `+` concatenation operator is, once again, very common in `print` statements.

## Creating Format Strings

You have seen the use of the `printf()` and `format()` methods to print output with formatted numbers. The `String` class has an equivalent class method, `format()`, that returns a `String` object rather than a `PrintStream` object.

Using `String's` static `format()` method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of

    System.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);

you can write

    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);

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