String substring() method - Syntax and Examples

Learn about the Java String substring() method, how to use it with different parameters, and see practical examples.

This reference explains String substring() method - Syntax and Examples through its syntax, examples and boundaries. Use the linked course lessons when the surrounding Java concepts are unfamiliar.

Sometimes you need to extract a portion of a string. The substring() method in Java is your tool for exactly that! It allows you to create a new String consisting of characters from the original String, starting at a specified index and optionally ending before another specified index.

public String substring(int beginIndex) 

public String substring(int beginIndex,
                         int endIndex)

The substring() method returns a new String that is a substring of the original string.

Example 1: Extracting from a single index

This example demonstrates using substring() with only the starting index. It extracts characters from the beginning of the string up to (but not including) the specified index.

String str = "Hello World";
String sub = str.substring(5); // Extract substring starting at index 5
System.out.println(sub);

World

Here, we start the substring from index 5 of "Hello World". Index 5 points to the character 'W'. The returned substring includes characters at indices 5, 6, 7, 8, 9, and 10 ('W', 'o', 'r', 'l', 'd').

Example 2: Extracting between two indexes

This example shows how to specify both a starting and ending index for the substring.

String str = "Hello World";
String sub = str.substring(0, 5); // Extract substring from index 0 up to (but not including) index 5
System.out.println(sub);

Hello

In this case, we're extracting a substring that starts at index 0 and ends just before index 5. This includes the characters at indices 0, 1, 2, 3, and 4 ('H', 'e', 'l', 'l', 'o'). The character at index 5 (' ') is *not* included.

Example 3: Handling Index Out of Bounds

It's important to be aware that if your indices are out of bounds, you will get a StringIndexOutOfBoundsException. Let’s look at an example where we try to extract from an invalid index.

String str = "Hello";
// This line will throw StringIndexOutOfBoundsException because endIndex is out of bounds
//String sub = str.substring(0, 10); 
//System.out.println(sub);

The above code would result in a runtime error. When using the two-argument version of substring(), ensure that both beginIndex is non-negative and less than the length of the string, and that endIndex is greater or equal to beginIndex and less than the length of the string.

Example 4: Substring with start index equals string length

What happens when we specify a begin index equal to the string's length?

String str = "Hello";
String sub = str.substring(5); //beginIndex is equal to string length
System.out.println(sub);

When the begin index equals the string's length, an empty string is returned.

Example 5: Substring with start and end indexes equal

Let’s see what happens when we set both indexes to be equal in value

String str = "Hello";
String sub = str.substring(2, 2); //start index equals end index 
System.out.println(sub);

When the start and end indexes are equal an empty string is returned.

Check the example’s types, inputs and documented behavior before copying it into an application. Some APIs require external libraries, files or platform services; consult their setup notes.

Related explanations: Java Data Types - Primitive and Non-Primitive, Java Class - Blueprint of Objects, Exceptions in Java Introduction.

Supporting documentation: Official Java learning guide.