The Java Lesson 2: Anatomy of a simple Java program, page 2

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Statement blocks

• Are modules of code consisting of one or more statements

• Begin with an opening brace "{" and end with a closing brace "}"

• May be entirely nested within another statement block. For example, the block for the main method is nested within the block for the App class in the sample program.

The main method

• A required method (program module) in all Java programs except applets

• The first method executed by the JVM when processing begins

• In a small program, it defines application processing. In larger programs, it is the highest-level module and calls other methods to perform detailed processing.

• Should have a method header coded as follows:

public static void main(String[] args)

This states that the method has public access, is a class (static) method not to be associated with any objects of the class, does not return a value to the caller (void), has the name main, and receives an array of string references (String[]) named args as parameters. It is not important that you understand all this now, just code the statement exactly as shown. The only flexibility you have in coding the header is the name of the array. The sample program will run just fine if you re-code the method header as

public static void main(String[] x)

Writing to the system output device (the console)

• The system output device is already open and may be referenced by the identifier System.out where out is a public reference to a PrintStream defined within the System class. Again, it is not important that you understand all this now.

• println is a public method of PrintStream objects that accepts a string parameter and copies it to the output device. In the sample program, the string is coded as a string literal enclosed within quotation marks.

Compiler requirements

• All Java statements are delimited by a semicolon ";"

• The compiler ignores "white space" characters (spaces, tabs, and the use of the return key). A good programmer, however, will use "white space" liberally to make their code easier to write, read, and maintain.

DO NOT write code like this

public class App {public static void main(String[] args){
System.out.println("Hello World!");}}

even though it compiles and runs just fine. Try it and see!

Review questions

1. Which of the following are invalid headers for the main method (choose three)

1. public static void Main(String[] args)

2. public static void main(String[] parms)

3. public static void main(String args)

4. public static void main(string[] args)

5. public static void main(String[] args)

2. What will result from attempting to compile and execute the following?

public class App
{
public static void main(String[] parms)
{
// System.out.println("Java is fun");
}
}

1. it will not compile

2. it will compile but an error will occur at run time

3. it will compile and run but nothing will display

4. it will compile and run to display the message "Java is fun" on the console

3. What will result from attempting to compile and execute the following?

public class App {
public static void main(String[] parms) {
System.out.println("Java is fun");
}
}

1. it will not compile

2. it will compile but an error will occur at run time

3. it will compile and run but nothing will display

4. it will compile and run to display the message "Java is fun" on the console

4. What will result from attempting to compile and execute the following?

public class App
{
public static void main(String[] args)
{
System.out.println("Java is fun")
}
}

1. it will not compile

2. it will compile but an error will occur at run time

3. it will compile and run but nothing will display

4. it will compile and run to display the message "Java is fun" on the console

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