Java CharSequence Interface

Last modified: April 13, 2025

The java.lang.CharSequence interface represents a readable sequence of character values, providing a consistent, immutable view of textual data. It serves as an abstraction for various types of character sequences, allowing uniform access to their contents. This interface is implemented by several classes, including String, StringBuilder, and StringBuffer, each offering different functionalities for handling and manipulating text.

Introduced in Java 1.4, CharSequence was designed to unify operations on different character sequence types. It provides several methods, such as:

length() - Returns the number of characters in the
sequence.
charAt(int index) - Retrieves the character at the
specified index.
subSequence(int start, int end) - Returns a portion of the
character sequence.
toString() - Converts the sequence into a
String representation.

Since CharSequence is an interface, it does not provide a concrete implementation but rather defines a contract that implementing classes must follow. The String class, for example, represents immutable character sequences, while StringBuilder and StringBuffer allow mutable string operations with different threading behaviors.

Many Java APIs accept CharSequence as a parameter, enabling flexibility when working with different types of character sequences. This abstraction makes it easier to process text without being constrained to a specific underlying implementation.

CharSequence Interface Methods

The CharSequence interface defines several methods for examining characters. These methods must be implemented by all classes that implement the interface. The main methods include length, charAt, subSequence, and toString.

Basic CharSequence Usage

This example demonstrates basic usage of CharSequence with different implementations. We show how to use the interface methods with String, StringBuilder, and StringBuffer.

Main.java

void main() {

// Different CharSequence implementations
CharSequence str = "Hello World";
CharSequence sb = new StringBuilder("Java Programming");
CharSequence sbf = new StringBuffer("CharSequence Demo");

// Common operations
System.out.println("String length: " + str.length());
System.out.println("5th char in StringBuilder: " + sb.charAt(4));
System.out.println("Subsequence of StringBuffer: " + 
                    sbf.subSequence(0, 11));

// toString() usage
String s = str.toString();
System.out.println("Converted to String: " + s);

}

This example shows how CharSequence provides a common interface for different character sequence types. We can call the same methods regardless of the underlying implementation. The toString method converts any CharSequence to a String when needed.

CharSequence with String

Strings are the most common CharSequence implementation. This example shows specific String operations through the CharSequence interface.

Main.java

void processSequence(CharSequence seq) {

System.out.println("Processing sequence: " + seq);
System.out.println("Length: " + seq.length());
System.out.println("First char: " + seq.charAt(0));
System.out.println("Last char: " + seq.charAt(seq.length() - 1));

}

void main() {

String text = "Programming in Java";
processSequence(text);
processSequence(text.subSequence(0, 11));

}

Here we demonstrate how String works as a CharSequence. The processSequence method accepts any CharSequence, allowing it to work with both the full String and its subsequence. This shows the flexibility of programming to interfaces.

CharSequence with StringBuilder

StringBuilder is a mutable CharSequence implementation. This example shows how to use StringBuilder through the CharSequence interface while maintaining mutability.

Main.java

void main() {

StringBuilder sb = new StringBuilder("Initial Value");
CharSequence cs = sb;

System.out.println("Original: " + cs);
System.out.println("Length: " + cs.length());
System.out.println("Char at 3: " + cs.charAt(3));

// Mutate the underlying StringBuilder
sb.append(" Appended");
System.out.println("After mutation: " + cs);

// Create subsequence
CharSequence sub = cs.subSequence(8, 13);
System.out.println("Subsequence: " + sub);

}

This example demonstrates that while CharSequence itself is read-only, the underlying StringBuilder remains mutable. Changes to the StringBuilder are visible through the CharSequence reference. The subsequence operation creates a new CharSequence view of part of the original.

CharSequence in Regular Expressions

Java’s regular expression API extensively uses CharSequence. This example shows pattern matching with different CharSequence implementations.

Main.java

void main() {

Pattern pattern = Pattern.compile("\\d{3}-\\d{2}-\\d{4}");

CharSequence[] inputs = {
    "123-45-6789", // String
    new StringBuilder("987-65-4321"),
    new StringBuffer("Invalid-12-3456"),
    "Another invalid 12-34-5678"
};

for (CharSequence input : inputs) {
    Matcher matcher = pattern.matcher(input);
    System.out.println(input + ": " + 
                        (matcher.matches() ? "Valid" : "Invalid"));
}

}

This example demonstrates how Java’s regex API accepts any CharSequence. We test different implementations against a social security number pattern. The and Matcher classes work with all CharSequence types, showing the interface’s utility in API design.

Custom CharSequence Implementation

This example shows how to create a custom CharSequence implementation. We’ll build a simple sequence that reverses another CharSequence.

Main.java

public class ReverseCharSequence implements CharSequence {

private final CharSequence original;

public ReverseCharSequence(CharSequence original) {
    this.original = original;
}

@Override
public int length() {
    return original.length();
}

@Override
public char charAt(int index) {
    return original.charAt(original.length() - 1 - index);
}

@Override
public CharSequence subSequence(int start, int end) {
    return new ReverseCharSequence(
        original.subSequence(original.length() - end, 
                           original.length() - start));
}

@Override
public String toString() {

    StringBuilder sb = new StringBuilder(length());
    for (int i = 0; i < length(); i++) {
        sb.append(charAt(i));
    }
    return sb.toString();
}

}

void main() {

CharSequence original = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
CharSequence reversed = new ReverseCharSequence(original);

System.out.println("Original: " + original);
System.out.println("Reversed: " + reversed);
System.out.println("5th char: " + reversed.charAt(4));
System.out.println("Subseq(5-10): " + 
                    reversed.subSequence(5, 10));

}

This example demonstrates creating a custom CharSequence that reverses another sequence. The ReverseCharSequence class implements all required methods, providing a view of the original sequence in reverse order. This shows how CharSequence can be used to create specialized sequence views.

CharSequence Performance Considerations

This example compares performance characteristics of different CharSequence implementations for common operations.

Main.java

public static void measurePerformance(CharSequence seq, String type) {

final int ITERATIONS = 1000000;

// Measure charAt performance
long start = System.nanoTime();
for (int i = 0; i < ITERATIONS; i++) {
    seq.charAt(i % seq.length());
}
long duration = System.nanoTime() - start;
System.out.printf("%s charAt: %d ns%n", type, duration/ITERATIONS);

// Measure subSequence performance
start = System.nanoTime();
for (int i = 0; i < ITERATIONS; i++) {
    seq.subSequence(0, seq.length() / 2);
}
duration = System.nanoTime() - start;
System.out.printf("%s subSequence: %d ns%n", type, duration/ITERATIONS);

}

void main() {

CharSequence str = "PerformanceTestString";
CharSequence sb = new StringBuilder("PerformanceTestString");
CharSequence sbf = new StringBuffer("PerformanceTestString");

measurePerformance(str, "String");
measurePerformance(sb, "StringBuilder");
measurePerformance(sbf, "StringBuffer");

}

This example measures the performance of basic CharSequence operations across different implementations. String typically offers the fastest read operations, while StringBuilder and StringBuffer may have different performance characteristics. Results may vary based on JVM implementation and version.

Source

Java CharSequence Interface Documentation

In this article, we’ve covered the Java CharSequence interface with practical examples. Understanding CharSequence is valuable for writing flexible APIs that can work with different character sequence types efficiently.

Author

My name is Jan Bodnar, and I am a dedicated programmer with many years of experience in the field. I began writing programming articles in 2007 and have since authored over 1,400 articles and eight e-books. With more than eight years of teaching experience, I am committed to sharing my knowledge and helping others master programming concepts.

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