static Methods, static Fields and Class Math

As you know, every class provides methods that perform common tasks on objects of the class. For example, to input data from the keyboard, you have called methods on a Scanner object that was initialized in its constructor to obtain input from the standard input stream (System.in). As you will learn in Chapter 14, Files and Streams, you can initialize a Scanner to obtain input from other sources, such as a file on disk. One program could have a Scanner object that inputs information from the standard input stream and a second Scanner that inputs information from a file. Each input method called on the standard input stream Scanner would obtain input from the keyboard, and each input method called on the file Scanner would obtain input from the specified file on disk.

Although most methods execute in response to method calls on specific objects, this is not always the case. Sometimes a method performs a task that does not depend on the contents of any object. Such a method applies to the class in which it is declared as a whole and is known as a static method or a class method. It is not uncommon for a class to contain a group of convenient static methods to perform common tasks. For example, recall that we used static method pow of class Math to raise a value to a power in Fig. 5.6.

To declare a method as static, place the keyword static before the return type in the method's declaration. You can call any static method by specifying the name of the class in which the method is declared, followed by a dot (.) and the method name, as in

      ClassName.methodName( arguments )

We use various Math class methods here to present the concept of static methods. Class Math provides a collection of methods that enable you to perform common mathematical calculations. For example, you can calculate the square root of 900.0 with the static method call

 Math.sqrt( 900.0 )

The preceding expression evaluates to 30.0. Method sqrt takes an argument of type double and returns a result of type double. To output the value of the preceding method call in the command window, you might write the statement

 System.out.println( Math.sqrt( 900.0 ) );

In this statement, the value that sqrt returns becomes the argument to method println. Note that there was no need to create a Math object before calling method sqrt. Also note that all Math class methods are statictherefore, each is called by preceding the name of the method with the class name Math and a dot (.) separator.

Software Engineering Observation 6.4

Class Math is part of the java.lang package, which is implicitly imported by the compiler, so it is not necessary to import class Math to use its methods.

Method arguments may be constants, variables or expressions. If c = 13.0, d = 3.0 and f = 4.0, then the statement

 System.out.println( Math.sqrt( c + d * f ) );

calculates and prints the square root of 13.0 + 3.0 * 4.0 = 25.0namely, 5.0. Figure 6.2 summarizes several Math class methods. In the figure, x and y are of type double.

Figure 6.2. Math class methods.

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Method	Description	Example
abs( x )	absolute value of x	abs( 23.7 ) is 23.7 abs( 0.0 ) is 0.0 abs( -23.7 ) is 23.7
ceil( x )	rounds x to the smallest integer not less than x	ceil( 9.2 ) is 10.0 ceil( -9.8 ) is -9.0
cos( x )	trigonometric cosine of x (x in radians)	cos( 0.0 ) is 1.0
exp( x )	exponential method ex	exp( 1.0 ) is 2.71828 exp( 2.0 ) is 7.38906
floor( x )	rounds x to the largest integer not greater than x	floor( 9.2 ) is 9.0 floor( -9.8 ) is -10.0
log( x )	natural logarithm of x (base e)	log( Math.E ) is 1.0 log( Math.E * Math.E ) is 2.0
max( x, y )	larger value of x and y	max( 2.3, 12.7 ) is 12.7 max( -2.3, -12.7 ) is -2.3
min( x, y )	smaller value of x and y	min( 2.3, 12.7 ) is 2.3 min( -2.3, -12.7 ) is -12.7
pow( x, y )	x raised to the power y (i.e., x y)	pow( 2.0, 7.0 ) is 128.0 pow( 9.0, 0.5 ) is 3.0
sin( x )	trigonometric sine of x (x in radians)	sin( 0.0 ) is 0.0
sqrt( x )	square root of x	sqrt( 900.0 ) is 30.0
tan( x )	trigonometric tangent of x (x in radians)	tan( 0.0 ) is 0.0

 

Math Class Constants PI and E

Class Math also declares two fields that represent commonly used mathematical constants: Math.PI and Math.E. The constant Math.PI (3.14159265358979323846) is the ratio of a circle's circumference to its diameter. The constant Math.E (2.7182818284590452354) is the base value for natural logarithms (calculated with static Math method log). These fields are declared in class Math with the modifiers public, final and static. Making them public allows other programmers to use these fields in their own classes. Any field declared with keyword final is constantits value cannot be changed after the field is initialized. Both PI and E are declared final because their values never change. Making these fields static allows them to be accessed via the class name Math and a dot (.) separator, just like class Math's methods. Recall from Section 3.5 that when each object of a class maintains its own copy of an attribute, the field that represents the attribute is also known as an instance variableeach object (instance) of the class has a separate instance of the variable in memory. There are fields for which each object of a class does not have a separate instance of the field. That is the case with static fields, which are also known as class variables. When objects of a class containing static fields are created, all the objects of that class share one copy of the class's static fields. Together the class variables and instance variables represent the fields of a class. You will learn more about static fields in Section 8.11.

Why Is Method main Declared static?

Why must main be declared static? When you execute the Java Virtual Machine (JVM) with the java command, the JVM attempts to invoke the main method of the class you specifywhen no objects of the class have been created. Declaring main as static allows the JVM to invoke main without creating an instance of the class. Method main is usually declared with the header:

 public static void main( String args[] )

When you execute your application, you specify its class name as an argument to the command java, as in

      java ClassName argument1 argument2 ...

The JVM loads the class specified by ClassName and uses that class name to invoke method main. In the preceding command, ClassName is a command-line argument to the JVM that tells it which class to execute. Following the ClassName, you can also specify a list of Strings (separated by spaces) as command-line arguments that the JVM will pass to your application. Such arguments might be used to specify options (e.g., a file name) to run the application. As you will learn in Chapter 7, Arrays, your application can access those command-line arguments and use them to customize the application.

Additional Comments about Method main

In earlier chapters, every application had one class that contained only main and possibly a second class that was used by main to create and manipulate objects. Actually, any class can contain a main method. In fact, each of our two-class examples could have been implemented as one class. For example, in the application in Fig. 5.9 and Fig. 5.10, method main (lines 616 of Fig. 5.10) could have been taken as is and placed in class GradeBook (Fig. 5.9). You would then execute the application by typing the command java GradeBook in the command windowthe application results would be identical to those of the two-class version. You can place a main method in every class you declare. The JVM invokes the main method only in the class used to execute the application. Some programmers take advantage of this to build a small test program into each class they declare.