String Concatenation in Java: `concat()` vs. ` ` Operator – What's the Difference?

String Concatenation: Differences Between concat() and the " " Operator

Java offers two primary methods for concatenating strings: the concat() method and the " " operator. While both options perform string concatenation, there are subtle differences to consider.

Semantic Disparities:

• Null Handling: concat() throws a NullPointerException if the first string argument is null, while " =" silently treats it as an empty string.
• Argument Type: concat() requires String arguments, while " " can silently convert objects to strings using toString().

Implementation Details:

Under the hood, " " creates a StringBuilder object and appends the arguments before converting it back to a String, as revealed by the following decompiled code:

a = new StringBuilder()
    .append(a)
    .append(b)
    .toString();

In contrast, concat() directly creates a new char array, appends the arguments, and then constructs a new String.

Performance Considerations:

Traditionally, concat() was considered faster for short strings. However, in recent versions of the HotSpot JVM, the bytecode compiler optimizes the code generated by " ", resulting in comparable or even slightly better performance for longer strings.

Specific Scenarios:

• For short or infrequent concatenations, " " is generally more convenient.
• For repeated or large-scale concatenations, a StringBuilder should be used for optimal performance.
• Non-string arguments favor " " for automatic conversion to strings.
• Maintaining references to the original strings is best done with concat().

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