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StringBuilder

Often we build text dynamically in our code resulting in a String made from other variables and string literals.

  • Repeated String concatenation can result in the creation of large numbers of String objects, wasting memory and increasing the need for expensive garbage collecion.
String userName = "bdobbs";         // "bdobbs" in String Pool.
String appName = "ThisApp";         // "ThisApp" in String Pool.
String message = "Welcome, "        // "Welcome, " in String Pool.
                 + userName         // "Welcome, bdobbs" on heap.
                 + "!\n\n";         // "Welcome, bdobbs!\n\n" on heap, "Welcome, bdobbs" discarded.
message += "You have logged into "; // New message on heap, previous version discarded.
message += appName;                 // New message on heap, previous version discarded.
message += ", please enjoy.\n";     // New message on heap, previous version discarded.
System.out.println(message);

The StringBuilder class lets us build dynamic text much more efficiently.

StringBuilder encapsulates a char array, but unlike String allows us to modify its contents, automatically adding more capacity as our text grows.

  • Unlike immutable Strings, a StringBuilder is mutable.
StringBuilder messageBuilder = new StringBuilder();
messageBuilder.append("Welcome, "); // "Welcome, " copied into messageBuilder's char array.
messageBuilder.append(userName);    // "bdobbs" copied into messageBuilder's char array
messageBuilder.append("!\n\n");     // messageBuilder automatically allocates a larger array,
                                    // copies "Welcome, bdobbs" and "!\n\n" into it.
messageBuilder.append("You have logged into "); // Copied into char array
messageBuilder.append(appName);                 // Copied into char array
messageBuilder.append(", please enjoy.");       // Copied into char array
message = messageBuilder.toString();            // Make a new String from char array on heap
System.out.println(message);

StringBuilder constructors

new StringBuilder() constructs an object with an initial capacity of 16 characters, but no actual content.

  • That is, the StringBuilder object contains a char array with length of 16, but no characters.

  • As text is added to fill the array, the StringBuilder automatically copies its contents to a new, larger array.

new StringBuilder("Some Text") constructs an object with an internal char array containing "Some Text", plus 16 empty char slots.

new StringBuilder(100) constructs an object with an empty char array of length 100.

  • If you have an idea how long your text will be, this preallocates enough capacity and saves your StringBuilder some work.

Practice Exercise

In Java's earlier days, programmers used the StringBuffer class to build dynamic text. StringBuffer is designed to be safe for use in multi-threaded programs, at a cost in performance. StringBuilder was created to work exactly like StringBuffer but is not thread-safe and is therefore faster. Java recommends using StringBuilder over StringBuffer wherever possible.

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