Difference Between Concat() and + (String Concatenation Operator) in Java

1. concat()
   
   This method is of java.lang.String class which takes only one parameter as String.
   

   public java.lang.String concat(java.lang.String);

2. +
   
   If on either side of + operator, there is a String, then + operator is called as concatenation operator.

Both concat() and + are used to concatenate the strings but they have some differences as described below.

1. java.lang.NullPointerException
   
   concat() method if called on null String reference variable, will throw NullPointerException; while this is not the case with + operator.
   
   Example
   

   String str = null;
   str.concat(“abc”);
   will throw java.lang.NullPointerException.
   While
   String str = null;
   System.out.println(str+”abc”); // Output:- nullabc

   While concatenating strings with + operator, a null reference variable changes to “null” (i.e. a String containing null as its contents.). That’s why str becomes “null” and output is nullabc.
   
   
2. Number of arguments
   
   concat() takes only one argument while + can take any number of arguments.
   
   Example
   

   “Hello”+”James”+”Gosling”

3. Type of Argument
   
   Concat() only takes String type as argument. If any other type of argument is passed to concat() it will give compile time error.
   
   Example
   

   s.concat(2); // error as 2 is a non string value
   
   s.concat(“2”); // no error as here 2 is string (enclosed in “”)

   While + can take any type of argument, while doing concatenation non-string type argument is converted to String by using its toString() method.
   
   Example
   

   String str=”abc”;
   int i=2;
   
   System.out.println(str+i); // no error, Output:- abc2.

   Here int value 2 is a primitive value. It will be first converted to a string value as,
   
   int------>Integer.toString(2)-------> a String representation of int value 2.
   
   Note
   
   At least one argument of + must be of string type otherwise + will behave like mathematical addition operator instead of string concatenation operator.
   
   
4. Creation of new String object
   
   concat() returns new String object only when the length of argument string is > 0.
   
   Example
   

   String s1=”Hello”;
   String s2=”James”;
   String s3=s1.concat(s2);

   Here concat() returns a new String object whose reference is stored in s3.
   
   But if the length of argument string is 0, then concat() returns same original string instead of returning a new String object.
   
   Example
   

   String s1=”Hello”;
   String s2=s1.concat(“”);

   Here “” (an empty String with length 0) is passed as an argument to concat(). So no new String object is created but reference to original string s1 is returned and strored in s2.
   
   Therefore, s1==s2 will give true here
   i.e. both s1 and s2 are pointing to same objects.
   
   While,
   
   + creates a new String object every time it concatenates something, except when the concatenation is done at compile time.
   
   Example
   

   String s1=”Hello”;
   
   String s2=s1+”James”;

   Here a new String object s2 is created with concatenated result (Hello James).
   
   Similarly,
   

   String s1=”Hello”;
   
   String s2=”James”;
   
   String s3=s1+s2;

   Will create a new String object s3.
   
   In both above examples concatenation happens at run time.
   
   But,
   

   String s4=”Hello”+”James”;

   Will not create a new String object because here concatenation will happen at compile time.
   
   There will be only one String object with contents “Hello James”. Both String literals “Hello” and “James” will be discarded.
   
   
5. Performance
   
   Concat() is consider better than + operator because concat() returns a new String object only when length of its arguments is > 0.