CS124: Java, Section 2.8

Section 2.8
Details of Strings

JAVA HAS A BUILT-IN TYPE, String, to represent strings of characters. This type differs from the eight primitive types because a value of type String is an object. A String is not just a data value; it also has "methods." (A method is a subroutine that is part of an object.) For example, if str is a variable of type String, you can call the method str.length(), which is a function that returns the number of characters in the string. There's a lot more you can do with strings. This section covers some of the details.

One thing that you cannot do with strings is to use the relational operators <, >, <=, and <= to compare them. You can legally use == and != to compare Strings, but because of peculiarities in the way objects behave, they won't give the results you want. (I'll get back to this in a later chapter.) Instead, you should use methods that are provided for comparing Strings.

The String class defines a lot of methods. Here are just a few that you might find useful. Assume that s1 and s2 are Strings:

s1.equals(s2) is a boolean-valued function that returns true if s1 consists of exactly the same sequence of characters as s2.
s1.equalsIgnoreCase(s2) is another boolean-valued function that checks whether s1 is the same string as s2, but this function considers upper and lower case letters to be equivalent. Thus, if s1 is "cat", then s1.equals("Cat") is false, while s1.equalsIgnoreCase("Cat") is true.
s1.compareTo(s2) is an integer-valued function that compares the two strings. If the strings are equal, the value returned is zero. If s1 is less than s2, the value returned is -1, and if s1 is greater than s2, the value returned is 1. (If the strings consist entirely of lowercase letters, then "less than" and "greater than" refer to alphabetical order.)
s1.charAt(N), where N is an integer, is a char-valued function that returns the N-th character in the string. Positions are numbered starting with 0, so s1.charAt(0) is the actually the first character, s1.charAt(1) is the second, and so on.
s1.length(), as mentioned above, is an integer-valued function that gives the number of characters in s1.
s1.toUpperCase() is a String-valued function that returns a new string that is equal to s1, except that any lower case letters in s1 have been converted to upper case. There is also a method s1.toLowerCase().
s1.trim() is a String-valued function that returns a new string that is equal to s1 except that any non-printing characters such as spaces and tabs have been trimmed from the beginning and from the end of the string. Thus, if s1 has the value "fred ", then s1.trim() is the string "fred".

For the methods s1.toUpperCase(), s1.toLowerCase() and s1.trim(), note that the value of s1 is not changed. Instead a new string is created and returned as the value of the function. The returned value could be used, for example, in an assignment statement such as "s2 = s1.toLowerCase();".

Also useful is the fact that you can use the operator + to concatenate two strings. The concatenation of two strings is a new string consisting of all the characters of the first string followed by all the characters of the second string. For example, "Hello" + "World" evaluates to "HelloWorld". (Gotta watch those spaces, of course.)

Even more surprising is that you can concatenate values belonging to one of the primitive types onto a String using the + operator. The value of primitive type is converted to a string, just as it would be if you printed it to the console. For example, the expression "Number" + 42 evaluates to the string "Number42". And the statements
        console.put("After ");
        console.put(years);
        console.put(" years, the value is ");
        console.put(principal);
can be replaced by the single statement:
        console.put("After " + years + 
                            " years, the value is " + principal);
Obviously, this is very convenient. You can even use the += operator to add something onto the end of a String. For example, if you want to read a sequence of letters from the console and store them in a String, you could do it like this:
             String str = "";              // start with an empty string
             char ch = console.getChar();  // read one character
             while ( (ch >= 'a' && ch <= 'z')
                       || (ch >= 'A' && ch <= 'Z') ) {
                // ch is a letter, so append it to the string
                //      and read the next character.
                str += ch;   // (note: same as  str = str + ch;)
                ch = console.getChar();
             } // end of while loop
             
End of Chapter 2

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Section 2.8 Details of Strings

Section 2.8
Details of Strings