Building Java Applications for the iSeries Server with VisualAge for Java 3.5

Chapter 7. Java for RPG programmers 317

These four integer variables are all different entities in Java. The RPG compiler folds these

names to uppercase and treats them as a single entity.

Comments may begin with either the C style of a slash followed by an asterisk and end with

an asterisk followed by a slash, or the C++ style of a double slash:

/* The first line of a C style comment line

** The second line of a C style comment line

** The final line follows ....

*/

// A C++ style comment line

// Another C++ style comment line

Java is also a strongly typed language. This means that all named entities in the program

must have a specific type (for example, char, int, Object, and so on). The type is always

specified when declaring a variable in Java.

7.1.3 Object creation

Every class in Java has at least one constructor method. This is a method with the same

name as the class. It may or may not accept arguments. An object is created by creating a

new instance of a class:

Thing myThing = new Thing();

This statement performs both the definition and the declaration of a variable. The definition is

the part to the left of the equal sign and says define a variable called myThing that is a type of

Thing. The declaration is the part to the right of the equal sign and says create a new Thing.

The equal sign is an assignment statement. The statement creates a new Thing and stores

the reference to that new object in the variable myThing.

Thing() is the default constructor for the Thing class and does not accept any arguments. It is

possible for a class to have multiple constructors each accepting different arguments. A

constructor is used to initialize a new object.

7.1.4 Class variables

Class variables are those with a static modifier. These variables are associated with the class

and, therefore, are accessible from every instance of the class. They are used where the

variable represents something that is either independent of each object or is dependent on all

objects. For example, a counter of the number of objects (instances of a particular class) can

be implemented as a class variable and increased by the constructor for that class giving all

objects knowledge of how many of them exist:

static int howMany = 0;

7.1.5 Class methods

Class methods also use the static modifier. You can start these methods without an instance

of the class having been created. This may be necessary where the methods operate on one

or more of the various primitive data types. For example, the Math class (in java.lang)

declares all its methods as static, because no object is required to use the Math functions.

They all accept numeric primitive data types. They also do not reference any object instance

data.

double aRoot = Math.sqrt(someValue);