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Computational Problem Solving with Java and Interactive Object Graphics

Mark F. Russo, PhD

Section 4.1 StringBuilder Class

Recall that String objects are immutable. This implies that every time two String objects are concatenated, a new String object is created that holds a sequence of characters copied from the two original Strings. This is true whether we use the .concat(...) method of String or the + operator.
String immutability is not a problem when concatenating a few small Strings, but when large Strings are concatenated over and over to build even larger String objects, your program can slow down because larger and larger chunks of memory are repeatedly allocated and deallocated. A more efficient approach to concatenating large and numerous String objects is a dedicated class that manages memory more efficiently, inflating an internal memory store less frequently, in larger increments, and only when necessary. This is what Java’s built-in StringBuilder class does for us.
Like other Java classes, instantiating a StringBuilder object involves invoking its constructor using the new keyword, as in the following example.
// StringBuilders.java
public class StringBuilders {
  public static void main(String[] args) {

    StringBuilder sb1;          // Declare variable
    sb1 = new StringBuilder();  // Initialize
    sb1.append("Hello");        // Add String and print
    System.out.println( "sb1: " + sb1.toString() );

    StringBuilder sb2;          // Declare another
    sb2 = new StringBuilder();  // Initialize
    sb2.append("Goodbye");      // Add String and print
    System.out.println( "sb2: " + sb2.toString() );

    sb2 = sb1;
    sb1.append(" there");       // Add to sb1
    System.out.println( "sb2: " + sb2.toString() );
  }
}
Listing 4.1.1. StringBuilder example
Unlike String objects, the content of a StringBuilder may be modified. If you are interested in working with a modifiable data structure holding a sequence of characters, StringBuilder is a good option. You might need this type of capability when programmatically building or modifying the contents of a large text file, such as an HTML document which is composed of a sequence of characters. StringBuilder objects have several useful methods that enable this kind of work. See Table 4.1.2.
Table 4.1.2. Useful StringBuilder Methods
Method Returns Description
append(...) StringBuilder Converts several types to a String
format and then appends it to the
end of the StringBuilder.
Returns a reference to itself,
allowing method chaining.
delete(int start, int end) StringBuilder Removes chars beginning at index
start and extending through index
end - 1. Returns a reference to
itself, allowing method chaining.
replace(int start, int end, String str) StringBuilder Replace chars beginning at index start
through index end - 1 with str.
Returns a reference to itself,
allowing method chaining.
insert(int offset, String substr) StringBuilder Inserts substr beginning at
offset.
Returns a reference to itself,
allowing method chaining.
indexOf(String substr) int Return index where substr
is found.
Returns -1 if not found.
charAt(int index) char Returns char at index index.
substring(int start, int end) String Returns a String of characters
from start through end - 1.
length() int Returns the length of the current
character sequence.
setLength(int newLength) void Sets the total size of the
StringBuilder to newLength.
setLength(0) removed all
characters from the
StringBuilder.
toString() String Returns the length of the current
character sequence.
Compiling and running the program in Listing 4.1.1 produces the following.
javac StringBuilders.java
java StringBuilders
sb1: Hello
sb2: Goodbye
sb2: Hello there
Does it surprise you that the first time sb2 was printed the output was Goodbye and the second time the output was Hello there? The String " there" was appended to sb1, not sb2. No StringBuilder methods were invoked on sb2. What happened?
The next section will help you understand why the printed value of sb2 changed so abruptly.
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