The StringBuilder Class
Trail: Learning the Java Language
Lesson: Numbers and Strings
Section: Strings

The StringBuilder Class

StringBuilder objects are like String objects, except that they can be modified. Internally, these objects are treated like variable-length arrays that contain a sequence of characters. At any point, the length and content of the sequence can be changed through method invocations.

Strings should always be used unless string builders offer an advantage in terms of simpler code (see the sample program at the end of this section) or better performance. For example, if you need to concatenate a large number of strings, appending to a StringBuilder object is more efficient.

Length and Capacity

The StringBuilder class, like the String class, has a length() method that returns the length of the character sequence in the builder.

Unlike strings, every string builder also has a capacity, the number of character spaces that have been allocated. The capacity, which is returned by the capacity() method, is always greater than or equal to the length (usually greater than) and will automatically expand as necessary to accommodate additions to the string builder.

StringBuilder Constructors
Constructor Description
StringBuilder() Creates an empty string builder with a capacity of 16 (16 empty elements).
StringBuilder(CharSequence cs) Constructs a string builder containing the same characters as the specified CharSequence, plus an extra 16 empty elements trailing the CharSequence.
StringBuilder(int initCapacity) Creates an empty string builder with the specified initial capacity.
StringBuilder(String s) Creates a string builder whose value is initialized by the specified string, plus an extra 16 empty elements trailing the string.

For example, the following code

// creates empty builder, capacity 16
StringBuilder sb = new StringBuilder();
// adds 9 character string at beginning

will produce a string builder with a length of 9 and a capacity of 16:

A string builder's length is the number of characters it contains; a string builder's capacity is the number of character spaces that have been allocated.

The StringBuilder class has some methods related to length and capacity that the String class does not have:

Length and Capacity Methods
Method Description
void setLength(int newLength) Sets the length of the character sequence. If newLength is less than length(), the last characters in the character sequence are truncated. If newLength is greater than length(), null characters are added at the end of the character sequence.
void ensureCapacity(int minCapacity) Ensures that the capacity is at least equal to the specified minimum.

A number of operations (for example, append(), insert(), or setLength()) can increase the length of the character sequence in the string builder so that the resultant length() would be greater than the current capacity(). When this happens, the capacity is automatically increased.

StringBuilder Operations

The principal operations on a StringBuilder that are not available in String are the append() and insert() methods, which are overloaded so as to accept data of any type. Each converts its argument to a string and then appends or inserts the characters of that string to the character sequence in the string builder. The append method always adds these characters at the end of the existing character sequence, while the insert method adds the characters at a specified point.

Here are a number of the methods of the StringBuilder class.

Various StringBuilder Methods
Method Description
StringBuilder append(boolean b)
StringBuilder append(char c)
StringBuilder append(char[] str)
StringBuilder append(char[] str, int offset, int len)
StringBuilder append(double d)
StringBuilder append(float f)
StringBuilder append(int i)
StringBuilder append(long lng)
StringBuilder append(Object obj)
StringBuilder append(String s)
Appends the argument to this string builder. The data is converted to a string before the append operation takes place.
StringBuilder delete(int start, int end)
StringBuilder deleteCharAt(int index)
The first method deletes the subsequence from start to end-1 (inclusive) in the StringBuilder's char sequence. The second method deletes the character located at index.
StringBuilder insert(int offset, boolean b)
StringBuilder insert(int offset, char c)
StringBuilder insert(int offset, char[] str)
StringBuilder insert(int index, char[] str, int offset, int len)
StringBuilder insert(int offset, double d)
StringBuilder insert(int offset, float f)
StringBuilder insert(int offset, int i)
StringBuilder insert(int offset, long lng)
StringBuilder insert(int offset, Object obj)
StringBuilder insert(int offset, String s)
Inserts the second argument into the string builder. The first integer argument indicates the index before which the data is to be inserted. The data is converted to a string before the insert operation takes place.
StringBuilder replace(int start, int end, String s)
void setCharAt(int index, char c)
Replaces the specified character(s) in this string builder.
StringBuilder reverse() Reverses the sequence of characters in this string builder.
String toString() Returns a string that contains the character sequence in the builder.

Note: You can use any String method on a StringBuilder object by first converting the string builder to a string with the toString() method of the StringBuilder class. Then convert the string back into a string builder using the StringBuilder(String str) constructor.

An Example

The StringDemo program that was listed in the section titled "Strings" is an example of a program that would be more efficient if a StringBuilder were used instead of a String.

StringDemo reversed a palindrome. Here, once again, is its listing:

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] = 
        // reverse array of chars
        for (int j = 0; j < len; j++) {
            charArray[j] =
                tempCharArray[len - 1 - j];
        String reversePalindrome =
            new String(charArray);

Running the program produces this output:

doT saw I was toD

To accomplish the string reversal, the program converts the string to an array of characters (first for loop), reverses the array into a second array (second for loop), and then converts back to a string.

If you convert the palindrome string to a string builder, you can use the reverse() method in the StringBuilder class. It makes the code simpler and easier to read:

public class StringBuilderDemo {
    public static void main(String[] args) {
        String palindrome = "Dot saw I was Tod";
        StringBuilder sb = new StringBuilder(palindrome);
        sb.reverse();  // reverse it

Running this program produces the same output:

doT saw I was toD

Note that println() prints a string builder, as in:


because sb.toString() is called implicitly, as it is with any other object in a println() invocation.

Note: There is also a StringBuffer class that is exactly the same as the StringBuilder class, except that it is thread-safe by virtue of having its methods synchronized. Threads will be discussed in the lesson on concurrency.

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