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Method concat() of String class is used to concat two String objects or two String literals.

Signature of concat()

public String concat(String st)

• Access modifier - concat() method is a public method.
• Parameter passed - A String value is passed to it, which is added at the end of the value of invoked String object.
• Value returned - It returns the concatenated String value. This concatenated String value if not stored or used, is lost.

Calling concat() method on a String object


In the upcoming code, we are calling concat() method on a String object, to add its content with the contents of another String object (which is passed as a parameter to concat() method).

// Program to concatenate values in two String Objects.

class StringCon1
{
public static void main(String[] ar)
{
String str1 = new String("Hello");
String str2 = new String("World");
System.out.println("First  String value : "+ str1);
System.out.println("Second String value : "+ str2);

System.out.println("Concatenated strings : "+ str1.concat(str2));
}
}





Output is -




First  String value : Hello
Second String value : World
Concatenated strings : HelloWorld


First String object is initialized with the value Hello and other String object initialized with the value World. Method concat() is called on the first String object and second String is passed as parameter to concat() method. This adds the content of second String object as the end of first String object.






Method concat() doesn't modify the content of the invoked String object.


Method concat() only returns the addition of the value invoked String object with the alue of other String object which was passed as a parameter to concat() method. Method concat() doesn't modify the content of invoked String object.

// Program to concatenate values in two String Objects.


class StringCon1
{
public static void main(String[] ar)
{
String str1 = new String("San");
String str2 = new String(" Franciso");
System.out.println("First  String value : "+ str1);
System.out.println("Second String value : "+ str2);

System.out.println("Concatenated strings : "+ str1.concat(str2));

System.out.println("Value in the first String, post concat() : " + str1);
}

}





Output is :




First  String value : San
Second String value : Francisco
Concatenated String value : San Francisco
Value in the first String, post concat() : San


Method concat() is called on first String and the second String is passed to this method as parameter and their concatenated result is displayed. Post the call to concat() method, the first String on which concat() method was called, still contains its original value.




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Strings are immutable but reference variables are mutable.


String objects are immutable, but the reference variables pointing to them are not, therefore in order to display the result of calling the concat() method on the original object, we can make the reference variable to the original string, point to the concatenated result calling the concat() method(but in this case, the original string will be lost).



// Program to concatenate values in two String Objects.

class StringCon2
{
public static void main(String[] ar)
{
String str1 = new String("South");
String str2 = new String(" Africa");

System.out.println("First String value is: "+ str1);
System.out.println("Second String value is: "+ str2);

str1 = str1.concat(str2);
System.out.println("Value in the first String after concatenation with the second String : "+ str1);
}


}





Output is :




First  String value is: South
Second String value is:  Africa
Value in the first String after concatenation with the second String : South Africa








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