Are Java's `concat()` Method and the ' ' Operator for String Concatenation Truly Equivalent?

String Concatenation: concat() and " " Operator

Question:

Are the concat() method and the " " operator equivalent in terms of string concatenation in Java?

Background:

The " " operator and the concat() method are both used to concatenate strings in Java. Both methods appear to perform similar tasks, but under the hood, there might be some differences.

Answer:

No, the concat() method and the " " operator are not strictly equivalent. Here are a few key distinctions:

1. Semantics:

• If one of the operands in the " " operator is null, it will be treated as an empty string. In contrast, concat() throws a NullPointerException if one of its arguments is null.
• The " " operator can silently convert its arguments to strings if necessary (using toString() method), while concat() only accepts String values.

2. Implementation:

Using javap -c to disassemble code, we can observe the bytecode for "a = b;":

// ...
java.lang.String cat(java.lang.String, java.lang.String);
  Code:
   0:   new     #2; //class java/lang/StringBuilder
   3:   dup
   4:   invokespecial   #3; //Method java/lang/StringBuilder."<init>":()V
   7:   aload_1
   8:   invokevirtual   #4; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   11:  aload_2
   12:  invokevirtual   #4; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   15:  invokevirtual   #5; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   18:  astore_1
 // ...

This bytecode demonstrates that "a = b;" is equivalent to the following:

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

On the other hand, the concat() method has its own dedicated implementation:

public String concat(String s) {
    // ...
}

3. Performance:

Traditionally, concat() was expected to be faster than the " " operator, as it operates directly on the char array representation of a string. However, with advancements in HotSpot JVM, the " " operator has become more competitive in terms of performance. In most cases, there isn't a significant difference in speed between the two methods.

Conclusion:

While both concat() and the " " operator can be used for string concatenation, they do have subtle differences in their semantics, implementation, and potential performance. Understanding these differences can help you make informed decisions when working with strings in Java.

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