Command line arguments

1
Command line arguments

• Argument to main
• array of strings
• args that appear after the class name

2
Program exit value

• main function returns void
• to return a value from main use System.exit(int)
• exit stops the program even if other threads are
  running

3
environment

• java cannot read environment variables
• java can read system properties
• String xSystem.getProperty(user.home)
• the system sets some properties
• you may set properties using the -D option
• java -Dmyapp.debugtrue myapp

4
name space

• no global variables or functions
• every variable or function is a member of a class
• every class is a member of a package
• the fully qualified name of a member includes the
  package name, class name and member name

5
import statement

• makes classes available under an abbreviated name
• import package.class
• import package.

6
access

• all classes and interfaces in a package are
  accessible to all other classes and interfaces in
  the same package
• a class declared public is accessible in other
  packages
• members of a class are accessible from other
  class in the same package if not declared
  private.

7
access

• a member of class A is accessible from a class B
  in a different package if A is public and the
  member is public, or if A is public, the member
  is protected and B is a subclass of A

8
Exceptions

• a signal that indicates an exceptional condition,
  such as an error, has occurred
• to throw an exception is to signal an exceptional
  condition
• to catch an exception is to handle it.

9
exceptions

• if an exception is not caught by the block of
  code that throws it, it propagates up to the next
  level
• if never caught it causes java to print an error
  message and terminate

10
exception object

• an exception is an object
• subclass of java.lang.Throwable
• different exception objects have different data
  describing the exception
• Throwable includes an error message that is
  inherited by all exceptions

11
try/catch/finally

• try establishes a block of code to have its
  exceptions handled
• followed by zero or more catch clauses that
  handle specified types of exception
• followed by a finally clause that always gets
  executed

12
exceptions
trysomething catch(Exception e)do something
about an exception finallydo this even if no
exception occurred
13
declaring exceptions

• any method that can cause an exception must
  either catch the exception or specify the type of
  exception with a throws clause
• public void open_file() throws IOException

14
Generating exceptions

• throw new MyException(my exception occurred)
• class MyException extends Exception
• public MyException(String s)super(s)