Assembling Strings with String Concatenation

Java allows you to assemble String objects into larger strings by using operators + or +=. This is known as string concatenation. When both operands of operator + are String objects, operator + creates a new String object in which the characters of the right operand are placed at the end of those in the left operand—e.g., the expression "hello " + "there" creates the String "hello there".

In line 24 of Fig. D.2, the expression "Maximum is: " + result uses operator + with operands of types String and double. Every primitive value and object in Java has a String representation. When one of the + operator’s operands is a String, the other is converted to a String, then the two are concatenated. In line 24, the double value is converted to its String representation and placed at the end of the String "Maximum is: ". If there are any trailing zeros in a double value, these will be discarded when the number is converted to a String—for example 9.3500 would be represented as 9.35.

Primitive values used in String concatenation are converted to Strings. A boolean concatenated with a String is converted to the String "true" or "false". All objects have a toString method that returns a String representation of the object. When an object is concatenated with a String, the object’s toString method is implicitly called to obtain the String representation of the object. ToString can be called explicitly.