1. Modifying strings

In Java, strings are immutable objects. This was done to make the String class highly optimized and to allow it to be used everywhere. For example, only immutable types are recommended for use as keys in the HashMap collection.

However, situations often arise when programmers would find it more convenient for the String class to be mutable. They want a class that doesn't create a new substring every time one of its methods is called.

Well, suppose we have a very large string and we need to frequently add something to the end of it. In this case, even a collection of characters (ArrayList<Character>) can be more efficient than constantly recreating and concatenating String objects.

That's precisely why a String-like type that can be changed was added to the Java language. It is called StringBuilder.

Creating an object

To create a StringBuilder object based on an existing string, you need to execute a statement like:

StringBuilder name = new StringBuilder(string);

To create an empty mutable string, you need to use a statement like this:

StringBuilder name = new StringBuilder();

List of methods

The StringBuilder class has two dozen helpful methods. Here are the most important ones:

Method Description
StringBuilder append(obj)
Converts the passed object to a string and appends it to the current string
StringBuilder insert(int index, obj)
Converts the passed object to a string and inserts it into the current string
StringBuilder replace(int start, int end, String str)
Replaces the part of the string specified by the start..end interval with the passed string
StringBuilder deleteCharAt(int index)
Removes the character with the specified index from the string
StringBuilder delete(int start, int end)
Removes characters within the specified interval from the string
int indexOf(String str, int index)
Searches for a substring in the current string
int lastIndexOf(String str, int index)
Searches for a substring in the current string, starting from the end
char charAt(int index)
Returns the character in the string at the passed index
String substring(int start, int end)
Returns the substring defined by the specified interval
StringBuilder reverse()
Reverses the current string.
void setCharAt(int index, char)
Changes the character at the specified index to the passed character
int length()
Returns the length of the string in characters

Here is a brief description of each method

2. Description of methods:

Appending to a string

To add something to a mutable string (StringBuilder), use the append() method. Example:

Code Description
StringBuilder builder = new StringBuilder("Hi");

Converting to a standard string

To convert a StringBuilder object to a String object, you just need to call its toString() method. Example

Code Output
StringBuilder builder = new StringBuilder("Hi");
String result = builder.toString();

How do I delete a character?

To delete a character in a mutable string, you need to use the deleteCharAt() method. Example:

Code Output
StringBuilder builder = new StringBuilder("Hello");
String result = builder.toString();

How do I replace part of a string with another string?

For this there is the replace(int begin, int end, String str) method. Example:

Code Output
StringBuilder builder = new StringBuilder("Mellow");
builder.replace(2, 5, "Hello!");
String result = builder.toString();

3. Useful examples of working with strings

How do I reverse a string?

There is a special method for doing this — reverse(); Example:

Code Output
String str = "Hello";
StringBuilder builder = new StringBuilder(str);
String result = builder.toString();

StringBuffer class

There is another class — StringBuffer, which is an analogue of the StringBuilderclass, but its methods are marked with the synchronized modifier. It means that the StringBuffer object can be accessed simultaneously from multiple threads.

But it is much slower than StringBuilder. You may need this class when you start to actively explore multithreading in the Java Multithreading quest.