Difference between concat() and concat operator ‘+’ in java

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When there is a need to concatenate two or more strings in java we generally use the concatenation operator ‘+’. But, for the same purpose we should prefer using concat() method from String class. After a closer look at the size of .class files which is generated after compiling the code, it is revealed that the size of the .class file containing ‘+’ operator exceeds by a greater margin than the .class file performing the same concatenation but by using concat() method.

ConcatFunDemo uses the concat() of String class:

class ConcatFunDemo {
	public static void main(String[] s) {
		String s1 = "www.";
		String s2 = "pcsalt.";
		String s3 = "com";

		System.out.println(s1.concat(s2).concat(s3));
	}
}

ConcatOpDemo uses the concatenation operator (+):

class ConcatOpDemo {
	public static void main(String[] s) {
		String s1 = "www.";
		String s2 = "pcsalt.";
		String s3 = "com";

		System.out.println(s1+s2+s3);
	}
}

Compile it:

$ javac ConcatFunDemo.java
$ javac ConcatOpDemo.java

The size of the .class file generated after compilation is:

ConcatFunDemo.class : 556 bytes
ConcatOpDemo.class  : 630 bytes

The above example contains only three strings which were concatenated in order to print the result. Just imagine the size of ByteCode of a project with hundred lines of code and multiple string concatenations. The revelation is that a concat() method is a lighter weight tool than the ‘+’ operator. Hence preferring concat() method over ‘+’ operator is a smart move.