Section 2.3
Strings, Objects, and Subroutines
THE PREVIOUS SECTION introduced the eight primitive
data types and the type String. There is a fundamental difference
between the primitive types and the String type: Values of type
String are objects. While we will not study objects in detail
until Chapter 5, it will be useful for you
to know a little about them and about a closely related topic: classes.
This is not just because strings are useful but because objects and classes
are essential to understanding another important programming concept,
subroutines.
Recall that a subroutine is a set of program instructions that have been
chunked together and given a name. In Chapter 4,
you'll learn how to write your own subroutines, but you can get a lot done
in a program just by calling subroutines that have already been written for
you. In Java, every subroutine is contained in a class or in an object. Some
classes that are standard parts of the Java language contain predefined
subroutines that you can use. A value of type String, which is an object, contains
subroutines that can be used to manipulate that string. You can call all
these subroutines without understanding how they were written or how they
work. Indeed, that's the whole point of subroutines: A subroutine is
a "black box" which can be used without knowing what goes on inside.
Classes in Java have two very different functions. First of all, a class
can group together variables and subroutines that are contained in that class. These
variables and subroutines are called static members
of the class. You've seen one example: In a class that defines a program,
the main() routine is a static member of the class. The parts of a class definition
that define static members are marked with the reserved word "static",
just like the main() routine of a program. However, classes have a second
function. They are used to describe objects. In this role, the class of an
object specifies what subroutines and variables are contained in that object.
The class is a type -- in the technical sense of a specification of a
certain type of data value -- and the object is a value of that type.
For example, String is actually the name of a class that is included as a standard
part of the Java language. It is also a type, and actual strings such as
"Hello World" are values of type String.
So, every subroutine is contained either in a class or in an object.
Classes contain subroutines called static member subroutines.
Classes also describe objects and the subroutines that are contained
in those objects.
This dual use can be confusing, and in practice most classes are designed
to perform primarily or exclusively in only one of the two possible roles.
For example, although the String class does contain a few rarely-used
static member subroutines, it exists mainly to specify a large number of
subroutines that are contained in objects of type String. Another
standard class, named Math, exists entirely to group together a number
of static member subroutines that compute various common mathematical functions.
To begin to get a handle on all of this complexity, let's look at the
subroutine System.out.print as an example. As you have seen earlier
in this chapter, this subroutine is used to display information to the user.
For example, System.out.print("Hello World") displays the
message, Hello World.
System is one of Java's standard classes. One of the static member
variables in this class is named out. Since this variable is contained
in the class System, its full name -- which you have to use
to refer to it in your programs -- is System.out.
The variable System.out refers to an object,
and that object in turn contains a subroutine named print. The compound identifier
System.out.print refers to the subroutine print in the object
out in the class System.
(As an aside, I will note that the object referred to by System.out
is an object of the class PrintStream. PrintStream is another class that
is a standard part of Java.
Any object of type PrintStream is a destination to which information
can be printed; any object of type PrintStream has a print
subroutine that can be used to send information to that destination. The object System.out is
just one possible destination, and System.out.print is the subroutine that sends
information to that destination. Other objects of type PrintStream might send
information to other destinations such as files or across a network to other computers.
This is object-oriented programming: Many different things which have something in common --
they can all be used as destinations for information -- can all be used in the same way --
through a print subroutine. The PrintStream class expresses the commonalities
among all these objects.)
Since class names and variable names are used in similar ways, it might be hard
to tell which is which. All the built-in, predefined names in Java follow the rule that
class names begin with an upper case letter while variable names begin with
a lower case letter. While this is not a formal syntax rule, I recommend that you
follow it in your own programming. Subroutine names should also begin with lower case
letters. There is no possibility of confusing a variable with a subroutine, since
a subroutine name in a program is always followed by a left parenthesis.
Classes can contain static member subroutines, as well as static member variables.
For example, the System class contains a subroutine named exit. In a
program, of course, this subroutine must be referred to as System.exit. Calling this
subroutine will terminate the program. You could use it if you had some reason to terminate
the program before the end of the main routine. (For historical reasons,
this subroutine takes an integer as a parameter, so the subroutine call statement might
look like "System.exit(0);" or "System.exit(1);".
The parameter tells the computer why the program is being terminated. A parameter value
of 0 indicates that the program is ending normally. Any other value indicates that the
program is being terminated because an error has been detected.)
Every subroutine performs some specific task. For some subroutines, that task is to compute
or retrieve some data value. Subroutines of this type are called functions.
We say that a function returns a value. The returned value must
then be used somehow in the program.
You are familiar with the mathematical function that computes the square root of a number.
Java has a corresponding function called Math.sqrt. This function is a static member
subroutine of the class named Math. If x is any numerical value,
then Math.sqrt(x) computes and returns the square root of that value. Since Math.sqrt(x)
represents a value, it doesn't make sense to put it on a line by itself in a subroutine
call statement such as
Math.sqrt(x); // This doesn't make sense!
What, after all, would the computer do with the value computed by the function in this case?
You have to tell the computer to do something with the value. You might tell the computer to
display it:
System.out.print( Math.sqrt(x) ); // Display the square root of x.
or you might use an assignment statement to tell the computer to store that value in a variable:
lengthOfSide = Math.sqrt(x);
The function call Math.sqrt(x) represents a value of type double,
and it can be used anyplace where a numerical value of type double could be used.
The Math class contains many static member functions.
Here is a list of some of the more important of them:
- Math.abs(x), which computes the absolute value of x.
- The usual trigonometric functions, Math.sin(x),
Math.cos(x), and Math.tan(x). (For all the
trigonometric functions, angles are measured in radians, not degrees.)
- The inverse trigonometric functions arcsin, arccos, and
arctan, which are written as: Math.asin(x),
Math.acos(x), and Math.atan(x).
- The exponential function Math.exp(x) for
computing the number e raised to the power x,
and the natural logarithm function Math.log(x)
for computing the logarithm of x in the base e.
- Math.pow(x,y) for computing x raised to the
power y.
- Math.floor(x), which rounds x down to the
nearest integer value that is less than or equal to x.
(For example, Math.floor(3.76) is 3.0.)
- Math.random(), which returns a randomly chosen double
in the range 0.0 <= Math.random() < 1.0.
(The computer actually calculates so-called "pseudorandom" numbers,
which are not truly random but are random enough for most purposes.)
For these functions, the type of the parameter -- the value
inside parentheses -- can be of any numeric type. For most
of the functions, the value returned by the function is of
type double no matter what the type of the parameter.
However, for Math.abs(x), the value returned will be
the same type as x. If x is of type int,
then so is Math.abs(x). (So, for example, while Math.sqrt(9)
is the double value 3.0, Math.abs(9) is the int
value 9.)
Note that Math.random() does not have any parameter.
You still need the parentheses, even though there's nothing between them. The
parentheses let the computer know that this is a subroutine rather
than a variable. Another example of a subroutine that has no
parameters is the function System.currentTimeMillis(),
from the System class. When this function is executed,
it retrieves the current time, expressed as the number of milliseconds
that have passed since a standardized base time (the start of the year 1970
in Greenwich Mean Time, if you care). One millisecond is one thousandth second.
The value of System.currentTimeMillis() is of type long.
This function can be used to measure the time that it takes the computer to
perform a task. Just record the time at which the task is begun and the time
at which it is finished and take the difference.
Here is a sample program that performs a few mathematical tasks and reports
the time that it takes for the program to run. On some computers, the time
reported might be zero, because it is too small to measure in milliseconds.
Even if it's not zero, you can be sure that most of the time reported by the computer
was spent doing output or working on tasks other than the program,
since the calculations performed in this program occupy only a tiny fraction of
a second of a computer's time.
public class TimedComputation {
/* This program performs some mathematical computations and displays
the results. It then reports the number of seconds that the
computer spent on this task.
*/
public static void main(String[] args) {
long startTime; // Starting time of program, in milliseconds.
long endTime; // Time when computations are done, in milliseconds.
double time; // Time difference, in seconds.
startTime = System.currentTimeMillis();
double width, height, hypotenuse; // sides of a triangle
width = 42.0;
height = 17.0;
hypotenuse = Math.sqrt( width*width + height*height );
System.out.print("A triangle with sides 42 and 17 has hypotenuse ");
System.out.println(hypotenuse);
System.out.println("\nMathematically, sin(x)*sin(x) + "
+ "cos(x)*cos(x) - 1 should be 0.");
System.out.println("Let's check this for x = 1:");
System.out.print(" sin(1)*sin(1) + cos(1)*cos(1) - 1 is ");
System.out.println( Math.sin(1)*Math.sin(1)
+ Math.cos(1)*Math.cos(1) - 1 );
System.out.println("(There can be round-off errors when "
+ " computing with real numbers!)");
System.out.print("\nHere is a random number: ");
System.out.println( Math.random() );
endTime = System.currentTimeMillis();
time = (endTime - startTime) / 1000.0;
System.out.print("\nRun time in seconds was: ");
System.out.println(time);
} // end main()
} // end class TimedComputation
Here is a simulated version of this program. If you run it several times, you should see
a different random number in the output each time.
A value of type String is an object. That object contains data, namely the sequence
of characters that make up the string. It also contains subroutines. All of these
subroutines are in fact functions. For example, length is a subroutine that
computes the length of a string. Suppose that str is a variable that refers to
a String. For example, str might have been declared and assigned a
value as follows:
String str;
str = "Seize the day!";
Then str.length() is a function call that represents the number of characters
in the string. The value of str.length() is an int.
Note that this function has no parameter; the string whose length is being
computed is str. The length subroutine is defined by the
class String, and it can be used with any value of type String.
It can even be used with String literals, which are, after all, just
constant values of type String. For example, you could have a program
count the characters in "Hello World" for you by saying
System.out.print("The number of characters in ");
System.out.println("the string \"Hello World\" is ");
System.out.println( "Hello World".length() );
The String class defines a lot of functions.
Here are some that you might find useful. Assume that s1
and s2 refer to values of type String:
- s1.equals(s2) is a function that returns a boolean value.
It returns true if s1 consists of exactly the
same sequence of characters as s2, and returns false
otherwise.
- 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.length(), as mentioned above, is an integer-valued
function that gives the number of characters in s1.
- s1.charAt(N), where N is an integer, returns a
value of type char. It 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. The final position
is s1.length() - 1. For example, the value
of "cat".charAt(1) is 'a'. An error occurs if the
value of the parameter is less than zero or greater than
s1.length() - 1.
- s1.substring(N,M), where N and M are
integers, returns a value of type String. The returned
value consists of the characters in s1 in positions N, N+1,...,
M-1. Note that the character in position M is not included.
The returned value is called a substring of s1.
- s1.indexOf(s2) returns an integer. If s2 occurs as
a substring of s1, then the returned value is the starting
position of that substring. Otherwise, the returned value is -1.
You can also use s1.indexOf(ch) to search for a particular
character, ch, in s1. To find the first occurrence
of x at or after position N, you can use
s1.indexOf(x,N).
- 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 a number less than zero, and if s1 is greater than
s2, the value returned is some number greater than zero. (If both of the strings
consist entirely of lowercase letters, then "less than"
and "greater than" refer to alphabetical order.
Otherwise, the ordering is more complicated.)
- 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. For example, "Cat".toUpperCase()
is the string "CAT".
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();".
Here is another extremely useful fact about strings: You can use the plus 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.) Let's suppose that name
is a variable of type String and that it
already refers to the name of the person using the program.
Then, the program could greet the user by executing
the statement:
System.out.println("Hello, " + name + ". Pleased to meet you!");
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 standard output, and then it is concatenated onto the string.
For example, the expression "Number" + 42
evaluates to the string "Number42". And the statements
System.out.print("After ");
System.out.print(years);
System.out.print(" years, the value is ");
System.out.print(principal);
can be replaced by the single statement:
System.out.print("After " + years +
" years, the value is " + principal);
Obviously, this is very convenient. It would have shortened
several of the examples used earlier in this chapter.
