String concatenation: concat() vs “+” operator

Question

Assuming String a and b:

a += b a = a.concat(b)

Under the hood, are they the same thing?

Here is concat decompiled as reference. I'd like to be able to decompile the + operator as well to see what that does.

public String concat(String s) { int i = s.length(); if (i == 0) { return this; } else { char ac[] = new char[count + i]; getChars(0, count, ac, 0); s.getChars(0, i, ac, count); return new String(0, count + i, ac); } }

Answer 1

No

If a is null, then a.concat(b) throws a NullPointerException but a+=b will treat the original value of a as if it were null.

The concat() method only accepts String values while the + operator will silently convert the argument to a String (using the toString() method for objects). So the concat() method is more strict in what it accepts.

write a simple class with a += b;

public class Concat { String cat(String a, String b) { a += b; return a; } }

Now disassemble with javap -c (included in the Sun JDK). You should see a listing including:

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 19: aload_1 20: areturn

So, a += b is the equivalent of

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

The concat method should be faster. However, with more strings the StringBuilder method wins, at least in terms of performance.

The source code of String and StringBuilder (and its package-private base class) is available in src.zip of the Sun JDK. You can see that you are building up a char array (resizing as necessary) and then throwing it away when you create the final String. In practice memory allocation is surprisingly fast.

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