2.11. Getting Input from Input Dialogs

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2.10. The String Type

The char type only represents one character. To represent a string of characters , use the data type called String . For example, the following code declares the message to be a string that has an initial value of "Welcome to Java".

 String message =   "Welcome to Java"   ; 

String is actually a predefined class in the Java library just like the System class and JOptionPane class. The String type is not a primitive type. It is known as a reference type . Any Java class can be used as a reference type for a variable. Reference data types will be thoroughly discussed in Chapter 7, "Classes and Objects." For the time being, you only need to know how to declare a String variable, how to assign a string to the variable, and how to concatenate strings.

As first shown in Listing 2.1, two strings can be concatenated . The plus sign ( + ) is the concatenation operator if one of the operands is a string. If one of the operands is a non-string (e.g., a number), the non-string value is converted into a string and concatenated with the other string. Here are some examples:

  // Three strings are concatenated  String message =   "Welcome"   +   "to"   +   "Java"   ;  // String Chapter is concatenated with number 2  String s =   "Chapter"   +   2   ;  // s becomes Chapter2  

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  // String Supplement is concatenated with character B  String s1 =   "Supplement"   +   'B'   ;  // s becomes SupplementB  

If neither of the operands is a string, the plus sign ( + ) is the addition operator that adds two numbers .

The shorthand += operator can also be used for string concatenation. For example, the following code appends the string "and Java is fun" with the string "Welcome to Java" in message .

 message +=   " and Java is fun"   ; 

So the new message is "Welcome to Java and Java is fun" .

Suppose that i = 1 and j = 2 , what is the output of the following statement?

 System.out.println(   "i + j is "   + (i + j); 

The output is "i + j is 12" because "i + j is " is concatenated with the value of i first. To force i + j to be executed first, enclose i + j in the parentheses, as follows :

 System.out.println(   "i + j is "   +  (  i + j  )  );