The BASE for JAVA LEARNING
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SECTION II - A Stand-alone JAVA Program
Step 3: Classes for Encapsulation
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SECTION II - A Stand-alone JAVA Program
Step 3: Classes for Encapsulation
Let's first have a class.
A class is used as the ENCAPSULATION block in JAVA. It begins with the opening curly brace { and ends with the closing curly brace }. That is the equivalent of BEGIN and END in other languages.
Other blocks in JAVA also use the curly braces as delimiters. For instance, a method is defined also within curly braces, as well as constructs, such as 'if' statements and 'for' loops.
See a JAVA class below. The class name is Exercise.
class Exercise
{
//
// in here we can have a method defined or attributes
//
}
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SECTION II - A Stand-alone JAVA Program
Step 4: The main Method
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SECTION II - A Stand-alone JAVA Program
Well, now that we have a class, let's then have the main method in the class. The main method uses some modifiers which will be discussed later. The name of this main method is 'main'. The modifiers are: public, static, and void.
Any JAVA method will have a set of parenthesis used for parameter passing. In the case of the main method the parameter passed comes from the command line, and it's an array of the String class.
Here below see the class Exercise with the main method defined in it.
class Exercise
{
public static void main(String[] args)
{
// here the main method is defined. (code, attributes, etc.)
}
}
The BASE for JAVA LEARNING
Step 5: Printing-out to the Standard Console
In order to have an output on our video console, we can invoke a JAVA method that is used to do exactly that.
It's the "println()" method referenced by the "out" field
of the System class. The out field is an outstream of
the "PrintStream" class.
// where x can be a primitive data type or String.
Notice that the String that we want to be outputted to the video terminal is within the set of parenthesis. That String is the parameter passed to that method.
For example to display "Hello" to the console we write: System.out.println("Hello");
To continue on our example, let's have this output method invoked from withint the main method. When this code is compiled, and executed, it will display "Hello" on your console.
class Exercise
{
public static void main(String[] args)
{
System.out.println("Hello");
}
}
A pair of curly braces is used to encapsulate the class, and another pair is used to delimit the main method.
The 'ln' in 'println' stands for line. It places a carriage return/line feed by the end of the string displayed.
If we write 'print' without the 'ln' at the end, that's another output method which also can be used. That method
print() does not display a carriage return/line feed by the end of the string.
Another important observation is the fact that after we write a line of code, we end it with a semi-colon ";" .
In above example the line of code is "System.out.println("Hello");"
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SECTION II - A Stand-alone JAVA Program
Step 6: Methods for Keyboard Input
JAVA has I/O methods for input as well as output. The I/O methods are contained in classes which are part of the IO package of JAVA.
We already have seen the method utilized for console output. That is, System.out.println(x); where x can be a String, or any primitive type.
Many methods contained in classes of the IO package raise Exceptions. Invoking methods that raise Exceptions may be handled in different ways. We will see later in Section XII of this Mini-Course, Exception Handling and we will have a better idea of the way Exceptions can be handled in JAVA.
For Keyboard Input we will use some methods contained in the Stream classes which are part of the IO package and will need to handle IOException. We will see the Stream classes in later Section.
However, for the time being we can utilize a class that we developed that handles those raised IOExceptions, and can be used for Keyboard Input. That class will let you enter all data primitives and their corresponding arrays (except array of chars), as well as any String. More about String and arrays in a later Section.
In the Table given below you find the various methods that can be used from your JAVA program, when you are using our Input.class.
An example is given for each method. Notice that any method used is preceded by the class name Input and a dot. This is because all methods of this class are class methods. ('static') We'll get into class methods in a later Section of this Mini-course.
For instance, to invoke the method that lets you enter an 'int' from the keyboard, you can write:
Class Method | Entry type | Example | Comments |
enterInt() | int | int year = Input.enterInt(); | An int is entered. |
enterChar() | char | char letter = Input.enterChar(); | A char is entered. |
enterShort() | short | short month = Input.enterShort(); | A short is entered. |
enterByte() | byte | byte number = Input.enterByte(); | A byte is entered. |
enterLong() | long | long atoms = Input.enterLong(); | A long is entered. |
enterFloat() | float | float value = Input.enterFloat(); | A float is entered. |
enterDouble() | double | double amount = Input.enterDouble(); | A double is entered. |
enterBoolean() | boolean | boolean flag = Input.enterBoolean(); | A boolean is entered. |
enterString() | String | String name = Input.enterString(); | A String is entered. |
In a later Section we'll provide the methods (also in this Input.class) that can be utilized to enter arrays.