The substring() method in Java is used to extract a portion of a string. It is part of the String class and returns a new string that is a subset of the original string.

Syntax

Java provides two overloaded versions of the substring() method:

• public String substring(int beginIndex)
  Returns a substring starting from beginIndex (inclusive) to the end of the string.

• public String substring(int beginIndex, int endIndex)
  Returns a substring from beginIndex (inclusive) to endIndex (exclusive).

Key Points

• Indexing starts at 0.

• The beginIndex is inclusive, while the endIndex is exclusive.

• The method always returns a new string; the original string remains unchanged because String is immutable in Java.

• If indices are invalid, it throws a StringIndexOutOfBoundsException.

• If the string is null, it throws a NullPointerException.

Examples

String str = "Hello, World!";

// Extract from index 7 to end
String part1 = str.substring(7); // "World!"

// Extract from index 0 to 5 (exclusive of index 5)
String part2 = str.substring(0, 5); // "Hello"

// Extract domain from URL
String url = "https://www.digitalocean.com";
String domain = url.substring(8, url.indexOf("/", 8)); // "www.digitalocean.com"

Common Use Cases

• Extracting file extensions: filename.substring(filename.lastIndexOf(".") + 1)

• Parsing dates: date.substring(0, 4) for year.

• Extracting parts of URLs or email addresses.

• Checking for palindromes or comparing string segments.

Best Practices

• Always validate beginIndex and endIndex to avoid exceptions.

• Check for null strings before calling substring().

• Be mindful of memory usage: in older Java versions (pre-7), substrings shared the underlying character array, potentially causing memory leaks if large strings were retained.

For more details, refer to the Java documentation.