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Exam Objectives Covered

• Describe the components of a basic Java program


• Identify the conventions to be followed in a Java program


• Use Java reserved words


• Use single-line and multi-line comments in Java programs

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Java Source Files

• In Java, almost everything is in classes, i.e., inside:

class {
  ...
}         
          

• There should generally be only one class per source file
• And the names should match - class "Student" goes in file "Student.java"


• JAVA IS CASE SENSITIVE Java Is Case Sensitive
• The sooner you accept this reality, the better
• There are a few things that can be expressed both in upper and lower case, but not many

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Java Language Elements

• Java source files contain:
• Classes
• Variables
• Methods
• Comments
• Reserved words
• Modifiers
• Statements
• Blocks


• All will be introduced at the right time

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Classes

• A class consists of two fundamental kinds of things:
• Variables
• Methods


• A class describes its objects
• The data (variables) that each object of the class will have
• The operations (methods) defined on those objects


• A class is a way to model an OO concept
• Much more on this later


• Let's learn more about the language first

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Packages

• Java classes are organized into packages
• Packages are directories on the file system (more later)
• Think "name space" or "surname"


• A class without a package statement is in the default package
• A package with no name


• Class names must be unique within a package


• The Java Core API library is divided up into packages
• java.lang, java.util, java.io, etc.

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Identifiers

• Names used for classes, variables, and methods


• Can be made up of any length of:
• Letters
• Digits
• Underscore (_)
• Dollar ($)
• Cannot begin with a digit
• Cannot be a Java keyword (we'll see these soon)


• Java is case sensitive
• "User" and "user" are completely different identifiers

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Identifiers

• Valid identifiers:
• MyClass
• myVariable
• MY_VARIABLE
• x
• _myvariable
• $myvariable


• Invalid identifiers:
• My Variable
• contains a space
• 9pins
• begins with a digit
• a+c
• "+" is not an alphanumeric character
• testing1-2-3
• "-" is not an alphanumeric character

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Java Keywords

• * not used

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Java Naming Conventions

• While not strictly required (by the compiler), Java naming conventions are pretty much law
• Class names begin with a capital letter
• Variable and method names begin with a lowercase letter
• In both cases, subsequent "words" in the name are capitalized


• Class examples:
• String, System, Person, HelloWorld, SecurityManager


• Variable examples:
• name, age, initialCount, numWords, bufferSize


• Method examples:
• getAge(), setAge(), println(), flushBuffer(), maxVal()

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Comments

• Multi-line or "block"
• Begin with /* and end with */
• Cannot nest these (why not?)


• Single line
• Begin with //
• Continues to end of line


• Javadoc documentation comments
• Begin with /** and end with */
• Read by javadoc tool to generate HTML-based API doc (later)

// single-line comment
/*
 this comment spans
 several lines
*/

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Lab 3.1 - Declaring Comments

• Declare a class with a valid identifier that follows java naming convention.


• Use comments to list 3 vaild identifiers for classes, methods, and variables.


• Use comments to list 3 invaild identifiers for classes, methods, and variables.

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Whitespace

• Spaces, blank lines, and tabs
• Used to separate words and symbols in a program


• Extra white space is ignored by the compiler
• Add white space to make your code more readable


• Code should be formatted to enhance readability, using consistent indentation

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Whitespace Example

• Which of the two code blocks below is easier to read?

class WhitespaceExample {
public static void main(String[] args) {
  System.out.println("try to be neat");                 /*
but somehow can't */
System.out.
  println("good luck trying to maintain this ");  System.
out.
println("code");
}}

class WhitespaceExample {
  public static void main(String[] args) {
    System.out.println("try to be neat");
    /* but somehow can't */
    System.out.println("good luck trying to maintain this ");
    System.out.println("code");
  }
}