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Strings > Special String Operations >
Siva Nookala - 06 Oct 2016
A string can be concatenated with various data types. They include int, float, double, boolean, references etc.,

String concatenation can be done in two ways
  • Using concat() method.
  • Using the concatenation operator '+'.
As Stringsare immutable objects a new Stringobject is created for every concatenation operation. The concat() method accepts only strings as arguments i.e. using concat() only strings can be concatenated while the '+' operator converts the given argument to a string (using toString() method for objects).

How the '+' operator works:
String str1 = "Merit", str2 = "Campus";
String str = str1 + str2;
  • If any of the operands of '+' operator is a Stringit works as a concatenation operator otherwise as an arithmetic operator.
  • Initially a new StringBuilderobject is created for one of the operand, assume str1, the other operand is appended to this StringBuilder object and finally converted to String object using toString() method and is assigned.
String Concatenation Demo
class StringConcatDemo
    public static void main(String arg[])
        String str1 = "Welcome ", str2 = " Merit", str3 = " Campus";
        int number = 2;
        str1 = str1 + number + str2 + str3; // LINE A
        str2 = str2.concat(str3); // LINE B
        boolean east = true;
        System.out.println("Sun rises in the east." + "\nThis statement is " + east); // LINE C
        str1 = null;
        // str1 = str1.concat(str2 + str3); // LINE D
        // str1 += number; // LINE E
        // System.out.println(str1);
        // System.out.println("Value=" + number + number); // LINE F
        // System.out.println("Value=" + ( number + number) ); // LINE G    

Welcome 2 Merit Campus
Merit Campus
Sun rises in the east.
This statement is true


At LINE A, an integeris concatenated to a Stringand a Stringis concatenated to another String, the result of the operation is Welcome 2 Merit Campus.
At LINE B, concatenation is done using concat() method. Now the str2 contains Merit Campus.
At LINE C, a booleanis concatenated to the String.

  • Uncomment LINE D. It produces a NullPointerException as str1 is null.
  • Uncomment LINE E. str1 contains null, it is concatenated to number without any exception. Now str1 contains null2.
  • Uncomment LINE F. Here the expression is evaluated from left to right. The expression is parsed as ( ("Value=" + number) + number). Two operations are concatenation operations, producing the result Value=22
  • Uncomment LINE G. As parenthesis has the highest priority, arithmetic operation is performed initially and then concatenation is done. This results in Value=4.
  • Try to concatenate an object reference or any other data type to a String object. For example define a new class A and create an object using new and add that object reference to the string. Don't forget to print after adding the reference.

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