String Concatenation in Java

String Concatenation in java is very common. Being a Java developer, you are aware that every once in a while we write code for String concatenation.

String Concatenation in Java

I am sure that you might have heard that we should not use String “+” operator for string concatenation in java. Also we should use StringBuffer or StringBuilder for this purpose. It’s a very common java interview questions and you should be prepared for it.

If you have dug deeper, you might know that String internally uses StringBuffer (till Java 1.4) or StringBuilder (Java 1.5 onwards) for String “+” operator calls.

Let’s write a very simple String concatenation java program to prove this.

package com.journaldev.string.concatenation;

public class StringConcatenationExample {

	public static void main(String args[]){
		String str = new String ("Journal  ");
		str += "Dev!!";
	}

}

Now we will edit the same program to use StringBuffer.

package com.journaldev.string.concatenation;

public class StringConcatenationExample {

	public static void main(String args[]){
	     StringBuffer str = new StringBuffer ("Journalbytecode");
	     str.append("Dev!!");
	}

}

I have kept aside the byte code for both these programs, as you can see the only difference between both these programs are in line no 6 and 7. Let’s check out the byte code now.

String Concatenation Byte Code

˛∫æ2.
com/journaldev/string/concatenation/StringConcatenationExamplejava/lang/Object
<init>
()V
Code
	
LineNumberTable
LocalVariableTable
this
Lcom/journaldev/string/concatenation/StringConcatenationExample;
main
([Ljava/lang/String;)V
java/lang/String
	Journal

(Ljava/lang/String;)V
java/lang/StringBuilder

valueOf
&(Ljava/lang/Object;)Ljava/lang/String;

Dev!!
!"#
append
-(Ljava/lang/String;)Ljava/lang/StringBuilder;
%&'
toString
()Ljava/lang/String;
args
[Ljava/lang/String;
str
Ljava/lang/String;

SourceFile
TTT.java!


/

*∑±



	
[ªY∑LªY+∏∑∂ ∂$L±

()
*+

,-

StringBuffer append byte code

˛∫æ2#
com/journaldev/string/concatenation/StringConcatenationExamplejava/lang/Object
<init>
()V
Code
	
LineNumberTable
LocalVariableTable
this
Lcom/journaldev/string/concatenation/StringConcatenationExample;
main
([Ljava/lang/String;)V
java/lang/StringBuffer
Journal

(Ljava/lang/String;)V
Dev!!

append
,(Ljava/lang/String;)Ljava/lang/StringBuffer;
args
[Ljava/lang/String;
str
Ljava/lang/StringBuffer;

SourceFile
TTT.java!


/

*∑±



	
NªY∑L+∂W±


 

!"

From the byte code, it’s very clear that internally String + is using StringBuilder append method for string concatenation in Java.

String Concatenation in java using + operator explained

Here are the steps involved in String concatenation using + operator:

1. A new StringBuilder object is created
2. String “Journal” is copied to the newly created StringBuilder object
3. StringBuilder append() method is called to append “Dev !!” to the object
4. StringBuilder toString() method is called to get the String object
5. The new String object reference is assigned to str and older string “Journal” is now available for garbage collection.

String concatenation using append explained

If we are using StringBuffer or StringBuilder object, it’s done in following steps:

1. A new StringBuffer object is created with value “Journal “
2. append() method is called to to append “Dev !!” to the object
3. StringBuffer toString() method is called to get the String object

Clearly second way is less time consuming and use less resources and produces less garbage collection.

If you are interested in learning more about these, read String vs StringBuffer vs StringBuilder.

Read this post to see how much StringBuilder is faster than StringBuffer in java.

Reference: String API Doc