Core Java

Java String Concatenation Methods

In Java, String concatenation refers to the process of combining multiple strings into a single string. Let us explore various Java string concatenation methods.

1. Introduction

1.1 Immutable String Concatenation

In Java, strings are immutable, which means that once a string object is created, it cannot be changed. When you perform concatenation on strings using immutable operations, you create new string objects to hold the combined content. This approach has some advantages and disadvantages:

1.1.1 Advantages

  • Safety: Immutable strings are thread-safe because they cannot be modified once created, which helps prevent data corruption in multi-threaded applications.
  • Predictable: Immutable strings ensure that the original string values remain unchanged, making it easier to reason about code.

1.1.2 Disadvantages

  • Performance: Immutable concatenation can be inefficient for large-scale concatenations or in loops because it creates a new string object each time, resulting in unnecessary memory overhead.
  • Complexity: Code that performs a lot of string concatenation using the + operator, concat(), etc. method may lead to performance bottlenecks.

1.2 Mutable String Concatenation

To address the performance drawbacks of immutable concatenation, Java provides mutable alternatives like StringBuilder and StringBuffer. These classes allow you to modify the content of a string without creating a new object each time you perform concatenation. The primary difference between them is that StringBuilder is not thread-safe (not synchronized), while StringBuffer is thread-safe (synchronized)

1.2.1 Advantages

  • Performance: Mutable concatenation is more efficient, especially for large-scale or iterative concatenations, as it avoids creating unnecessary string objects.
  • Flexibility: You can easily modify the content of a mutable string during operations.

1.2.2 Disadvantages

  • Thread-safety: StringBuilder is not thread-safe, so you need to synchronize access explicitly if used in a multi-threaded context.

2. Immutable String Concatenation

Immutable String Concatenation in Java involves combining multiple strings into a new string without modifying the original strings. Java strings are immutable, meaning that once a string is created, it cannot be changed. Therefore, when you perform concatenation using immutable operations, you create new string objects to hold the combined content. This approach ensures that the original strings remain unchanged. Here’s a detailed explanation of five common methods for immutable string concatenation in Java:

2.1 Using Addition (+) Operator

The + operator is one of the most common ways to concatenate strings in Java. When you use the + operator with strings, Java automatically converts the operands to strings and combines them into a new string.


String str1 = "Hello, ";
String str2 = "world!";
String result = str1 + str2; // Creates a new string object
System.out.println(result); // Output: Hello, world!

2.2 Using concat() Method

The concat() method is provided by the String class for string concatenation. It creates a new string by appending the argument string to the end of the original string.


String str1 = "Hello, ";
String str2 = "world!";
String result = str1.concat(str2); // Creates a new string object
System.out.println(result); // Output: Hello, world!

2.3 Using String.join() Method

The String.join() method is used to concatenate multiple strings with a delimiter. It takes an iterable of strings and joins them using the specified delimiter.


String[] words = {"Hello", "world"};
String result = String.join(", ", words); // Creates a new string object
System.out.println(result); // Output: Hello, world

2.4 Using String.format() Method

The String.format() method allows you to create formatted strings by specifying a format string and supplying the values to be inserted into the format placeholders.


String name = "Alice";
int age = 30;
String message = String.format("My name is %s, and I am %d years old.", name, age); // Creates a new string object
System.out.println(message); // Output: My name is Alice, and I am 30 years old.

2.5 Using Java Stream API

You can use the Java Stream API to concatenate strings from a collection or array. Here’s an example using a list of strings:


List<String> words = Arrays.asList("Hello", "world");
String result =", ")); // Creates a new string object
System.out.println(result); // Output: Hello, world

In all these examples, the original strings (str1, str2, words, etc.) remain unchanged, and new string objects are created to hold the concatenated result. Immutable string concatenation is safe, predictable, and suitable for most string manipulation scenarios. However, for extensive or performance-critical concatenations, consider using mutable alternatives like StringBuilder or StringBuffer to avoid unnecessary object creation.

3. Immutable String Concatenation Comparison

MethodPerformanceMemory Usage
Using Addition (+) OperatorMay be inefficient for large-scale concatenations or loops due to frequent object creation.Higher memory usage as it creates new string objects for each concatenation operation.
Using concat() MethodSimilar to the + operator, it may be inefficient for large-scale concatenations.Higher memory usage as it creates new string objects for each concatenation operation.
Using String.join() MethodGenerally efficient for joining strings with a delimiter.Memory-efficient as it minimizes unnecessary string object creation.
Using String.format() MethodEfficient for creating formatted strings, especially for complex formatting.Higher memory usage due to the creation of formatted string objects.
Using Java Stream APIEfficient for joining strings from collections or arrays.Memory-efficient as it avoids creating intermediate string objects.

4. Mutable String Concatenation

Mutable string concatenation in Java involves the process of efficiently combining multiple strings into a single string while allowing modifications to the content during the concatenation process. Unlike immutable string concatenation, where new string objects are created for each concatenation, mutable concatenation is performed using classes like StringBuffer, StringBuilder, and StringJoiner, which provide mutable string manipulation capabilities. Let’s delve into each of these topics in detail with examples:

4.1 Using StringBuffer

StringBuffer is a class in Java that provides mutable string manipulation and is thread-safe (synchronized). It is suitable for scenarios where thread safety is a concern, but it may be slightly slower than StringBuilder due to synchronization.


StringBuffer stringBuffer = new StringBuffer();
stringBuffer.append("Hello, ");
String result = stringBuffer.toString(); // Convert StringBuffer to a string
System.out.println(result); // Output: Hello, world!

4.2 Using StringBuilder

StringBuilder is similar to StringBuffer but is not thread-safe. It is more efficient than StringBuffer in single-threaded scenarios because it does not incur the synchronization overhead.


StringBuilder stringBuilder = new StringBuilder();
stringBuilder.append("Hello, ");
String result = stringBuilder.toString(); // Convert StringBuilder to a string
System.out.println(result); // Output: Hello, world!

In practice, StringBuilder is often preferred over StringBuffer unless you specifically need thread safety.

4.3 Using StringJoiner

StringJoiner is a class introduced in Java 8 that provides a way to join strings with a delimiter and an optional prefix and suffix. It is especially useful when you want to concatenate elements from collections or arrays.


StringJoiner joiner = new StringJoiner(", ", "Names: [", "]");
String result = joiner.toString();
System.out.println(result); // Output: Names: [Alice, Bob, Charlie]

In this example, StringJoiner allows you to join names with a comma and enclose them in square brackets.

In summary, mutable string concatenation in Java is a powerful technique that allows you to efficiently concatenate and modify strings. The choice between StringBuilder, StringBuffer, and StringJoiner depends on factors such as performance requirements and the need for thread safety in your application.

5. Mutable String Concatenation Comparison

MethodPerformanceMemory UsageThread Safety
Using StringBufferEfficient for single-threaded scenarios, slightly slower due to synchronization.Higher memory usage due to synchronization overhead.Thread-safe (synchronized).
Using StringBuilderMost efficient for single-threaded scenarios due to no synchronization overhead.Lower memory usage compared to StringBuffer.Not thread-safe.
Using StringJoinerEfficient for joining elements from collections or arrays.Memory is efficient, as it avoids unnecessary object creation.Not thread-safe; primarily used for joining strings with a delimiter.

6. Conclusion

In the realm of Java string manipulation, the choice between immutable and mutable concatenation techniques holds significant implications for code efficiency and functionality.

Immutable string concatenation is achieved through methods like the + operator and concat(), which can be less efficient for extensive operations, leading to higher memory usage and slower execution due to the creation of numerous new string objects. However, it excels in ensuring thread safety and preserving string immutability.

On the other hand, mutable concatenation with StringBuffer or, preferably, StringBuilder is notably more efficient, especially in single-threaded applications, as it minimizes memory overhead by allowing in-place modifications. This approach offers superior performance and memory control, making it well-suited for tasks involving large or complex strings and extensive manipulations.

Additionally, StringJoiner provides an efficient and structured method for joining strings from collections or arrays. Ultimately, the choice between these techniques hinges on specific use cases, with immutable concatenation favoring thread safety and simplicity, and mutable concatenation or StringJoiner tailored for performance-critical tasks and memory efficiency.


An experience full-stack engineer well versed with Core Java, Spring/Springboot, MVC, Security, AOP, Frontend (Angular & React), and cloud technologies (such as AWS, GCP, Jenkins, Docker, K8).
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