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The building blocks of a Java program

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• Summary and review: organization of a Java program
  • Review:

    A Java program consists of one or more classes A class contains one or more methods A method contains one or more statements A statement is the smallest unit of execution in a Java program

    We will explore these concept further in this webpage.

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• Overview: building blocks of a Java program
  • The building blocks discussed in this webpage:

    Statements                                     Methods Keywords and Identifiers Classes Blocks Comments

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• Statement: the smallest unit of execution in a Java program
  • Statement:

    Statement = the smallest (unit) of execution in a Java program

    (You cannot execute a portion of a statement)

    A statement is like a sentence:   it is the smallest unit that you write in a book.

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  • Examples of a statement:

    System.out.println("Hello Class");

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  • Effect of a statement:

    A statement tells the computer to do something Example: When the statement        System.out.println("Hello Class"); is executed, the computer will print the text "Hello Class" to the terminal

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• Types of statements
  • There are different kinds (types) of statements in Java

  • We will discuss the details of each statement in Java later in the course

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• Syntax of statements
  • Each type of statement follows a specific syntax

    This is similar to the English language Each type of sentence in English has its own syntax Example: An ordinary sentence follows the syntax: subject verb A question (sentence) follows syntax: verb subject

  • We will discuss the details of the syntax of each type of statement in Java later in the course

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• Method: a container for (many) statements that perform a complex task

  • Important facts:

    One statement can only perform a simple operation One statement is insufficient to solve a complex problem A complex task can be performed by many statements

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  • Method:

    For convenience, you can put a number of statement into a unit called a method The method is given a unique name to identify the group of statements.

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  • Example:

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• Defining a method
  • Terminology:

    Define a method = constructing a method

  • You must use a specify syntax to define a new method

    We will discuss a simplified syntax on how to define a method here and discuss the details on how to define methods later

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  • Simplified syntax used to define a method:

    MethodProperties METHOD_NAME( ... ) { statements contained in the method }

    Explanation:

    The MethodProperties describes the properties of the new method The METHOD_NAME is the name of the method (used for identification) The brackets ( ... ) contains parameters for the method You have seen parameters before if you have used a TI-83 calculator: The X in the formula Y1 = 4sin(X) is a parameter BTW, sin(..) is a method !!! Parameters provide information to the method Finally, the braces { ..... } encloses the statements

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  • Example:

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• Method invocation: executing a method
  • The computer can execute a method

    A method is a larger execution unit than a (single) statement

  • Computer jargon:

    Invoke a method = execute a method

  • When a method is invoked (executed), then:

    all statements contained between the braces { .... } are executed one statement at a time

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  • Example:

    Java program: When the Hello.java is run using the command: java Hello the computer will invoke the main() method The execution of the main() method will execute the statements contained in the main() method one at a time: System.out.println("Hello Class"); System.out.println(" How is... ");

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• Method: define once and run as many times as you want
  • Fact:

    You only have to define a method once (In a method definition, you are telling the computer what it needs to do when you call that method. You only have to instruct the computer once.) After you have defined a method, you can invoke the method as many times as you need/want to !!!

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• Header and body of a method
  • Computer Science jargon:

    Body of a method = the part of the method definition that is enclosed between the braces { ... } Header of a method = the part of the method definition before the body of the method

    Graphically explained:

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  • Note:

    When a method is executed, the statements in its body are executed

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• Keywords (or reserved words) in a programming language
  • Each programming language has reserved a number of words for some specific purpose

  • Keyword:

    Keyword = a reserved word in a programming language

    Examples: public, static and void

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  • Keywords have special meaning in a programming language:

    A keyword must be used for that specified purpose

    Example:

    A public method is a method that has the highest level of accessibility (Java allows you to define methods with more limited accessibility with the keyword private. You will learn about this topic much later - in another course)

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• Identifiers
  • Identifier:

    Identifier = a name chosen by the programmer to identify something defined inside a program

    There are some rules in Java that you must follow to form the identifier name

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  • Rules to form the name of an identifier in Java:

    An identifier consists of a number of characters There is no limit on the number of characters in the identifier (but do not try using identifiers that are too long because you will have to type it yourself...) The first character of an identifier must be one of the following: A letter (a, b, ..., z, A, B, ..., Z), or The underscore character The subsequent characters of an identifier must be one of the following: A letter (a, b, ..., z, A, B, ..., Z), or The underscore character, or A digit (0, 1, ..., 9) Identifiers are case-sensitive ! You cannot use a keyword as identifier (keywords are reserved for a specific purpose !)

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  • Examples of correct identifiers:

    age Age (is different from age) greaterCommonDivisor R2D2 radius_of_the_circle

  • Examples of illegal identifiers:

    3cpo (cannot start with a digit) radius-of-the-circle (cannot have minus sign in an identifier) public (cannot use a keyword !)

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• Class: container for methods
  • Important facts:

    One method is used to perform one complex task In order to solve one problem, you may need to perform multiple complex tasks For each complex task, you will have to write one method to perform the task. The different methods have a common purpose It makes sense to collect methods that serve similar purpose together.

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  • Class:

    For organizational purpose, you can put a number of methods into a unit called a class The class is given a unique name

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• Defining (constructing) a class
  • You must use a specify syntax to define a new class

  • Syntax to define a class:

    ClassProperties class CLASSNAME { methodDefinitions }

    Explanation:

    The ClassProperties describes the properties of the new class A word class is a keyword The use of this keyword tells Java that you want to define a new class The CLASSNAME is an identified used as the name of the new class The CLASSNAME must be unique (chosen by the programmer) The name of the class must be the same as the name of the file that contain the class

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  • Example:

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• Block: grouping unit in Java
  • Block:

    Block = a pair of "{" and "}" braces that groups components in a Java program together

  • Example:

    Because this block is part of a class definition, it is called a class block

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  • Blocks can be nested: one block can be placed inside another block

    Example:

    The outer block is a class block

    The inner block is the body of a method and it is called a method block

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  • Notes:

    An opening brace "{" must be matched with a closing brace "}" Programming tip: Whenever you type an opening brace "{", immediately type a closing brace "}" on the next line Go back and insert the rest of the program.

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• Java is case-sensitive
  • Every letter in a Java program is case-sensitive

    Meaning:

    A lower-case letter and its corresponding upper-case letter are counted as different Example: public and Public are not the same. (You can use Public as an identifier, but not public !)

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• Comments
  • Comment:

    Comment = text inside a Java program that is ignored by the Java compiler Comments are used to annotate the program to help humans understand the operation of the Java program

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  • Syntax for a comment in Java:

    Single line comment syntax: // ... single line comment The text on the line following the symbol // will be ignored Multiple lines comment syntax: /* comment line 1 comment line 2 ... ... */ All text between the comment brackets /* ..... */ will be ignored

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  • Example:

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  • Example Program: (Demo above code)                                                
    • The Hello prog file with comments: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Hello2.java To run:          java Hello2

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• Java programs are format-free
  • The Java compiler (translator) consider white space characters (i.e., SPACE, TAB and New line) as insignificant

    All that the Java compiler cares about is syntactical correctness

  • Consequence:

    You can make a Java program look very ugly and it may still be syntactically correct to the Java compiler

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  • Example:

    public class Hello2 { public static void main(String[] args) { System.out.println("Hello Class"); System.out.println(" How is everyone doing so far ?"); } }

    Same program with many insignificant white spaces:

    public class Hello3 { public static void main (String[] args) { System.out.println ( "Hello Class"); System.out.println( " How is everyone doing so far ?"); }}

    (But... I had to call it Hello3 because the name Hello2 is already used)

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  • Example Program: (Try it out ! It will run and print the same thing)                        
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Hello3.java To run:          java Hello3

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  • Moral of this lesson:

    Do yourself a favor Don't abuse the format-free feature of Java Indent your programs properly so the structure of the algorithm is easily visible It will help you understand what the program is doing and find errors

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• Summary
  • What a Java program look like abstractly:

  • What a Java program look like more concretely:

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  • Keyword = a (English) word that is reserved for a special purpose

    Each keyword has a special meaning in Java

  • Identifier = an (artificial) name made up by the programmer

    The (artificial) name is used to identify things defined in a java program

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  • Statement = the smallest unit of execution in Java

  • Method = contains multiple statements and identified by a method name which is an identifier

    When a method is invoked, all statements contained in the method are executed

  • Class = contains methods that serve a similar purpose

    Each class is identified by a class name which is an identifier

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  • Syntax to define a public class:

    public class CLASSNAME { (method definitions) }

  • Syntax to define the main method in Java:

    public static void main(String[] args) { (statements) }