Operators and Assignments - String Operators

• the + and += operators both work on Strings
• operators actually signfy concatenation
• the result of the operation is a new string
• Strings are objects, not primitive types, and are read-only and immutable; the contents never change
• String variables store references to a string object NOT the string itself

  String str  = "Hello";
  String str1 = "Universe!";
  String str2 = str + str1; // join the two strings together
    
  String str3 = "";
         str3 += str; // += only works with an initialized var
  String str4 = str2;

• in the above code a reference to the string "Hello" is stored in the variable str
• a reference to the string "Universe!" is stored in the variable str1
• a reference to a new string "Hello Universe!" is stored in the variable str2
• the reference for a new string "Hello" is stored in variable str3

  str3 == str  // false (ref to different String objects)

• the reference for str2 is stored in variable str4

    str4 == str2  // true (references are the same)

Where it can get confusing

• the String class creates a pool of Strings
• when you create a String by using the new operator or by using the + and += operators (the string is computed at runtime) you are implicitly telling the compiler to create a new String object
• when you create a String by assigning a string literal the compiler searches the existing string pool for an exact match. If it finds one, a new string is NOT created. Instead the variable is assigned a reference to the existing pooled string.

  String str5 = "Hello Universe!"; // created in the string pool
  String str6 = "Hello Universe!";    
   
  str5 == str2  // false (str2 is not part of the pool, created
                  // using '+' operator)
  str5 == str6  // true (matched an existing string found
                // in the pool)

• to actually compare the contents of String objects use the String method equals()

  str5.equals(str2);  // true (both objects hold the same string
                      // characters)

Strings and primitive types

• by the rules of String Conversion (see Conversion) any type can be converted to a string
• this includes the primitive types
• for primitive types, conversion occurs by the compiler calling Type.toString(x) behind the scenes.

int x = 10;
System.out.println("Result: " + x);

is the same as

System.out.println("Result: " + (Integer.toString(x)) );

Also see:

• String literals
  
• Sun Tech Tip: Interning Strings

Example Code

• TestStringOperators.java

Tips

• String operations whose result does not alter the original string (ie calling toUpperCase() on a String that is already in uppercase) return the original string reference; otherwise they return a reference to a new String
• Strings are immutable; the original String value can never be changed

Traps

• using == to compare the contents of two different String objects