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Week10 packages using objects in objects
- 1. Packages • Package: a set of related classes • Some packages in the Java library: Package Purpose Sample Class java.lang Language support Math java.util Utilities Random java.io Input and output PrintStream java.awt Abstract Windowing Toolkit Color java.net Networking Socket java.sql Database Access ResultSet javax.swing Swing user interface JButton
- 2. Working with Packages • We can create our own packages – Helps organize classes by function • To put a class in a package, you must create the package in Eclipse:
- 3. Working with Packages • After package is created, drag and drop your class into the package or create your class within the package. • Eclipse will automatically add a package statement to the top of the class
- 4. Working with Packages • Default package has no name, no package statement
- 5. Package Names • Recommendation: start with reversed domain name: edu.dmacc • edu.dmacc.cis171.smith: for Sammy Smith’s classes
- 6. How Classes Are Located in Your workspace
- 7. Static Variables • A static variable belongs to the class, not to any object of the class. • To assign bank account numbers sequentially • Have a single value of lastAssignedNumber that is a property of the class, not any object of the class. • Declare it using the static reserved word
- 8. Static Variables • Every BankAccount object has its own balance andaccountNumber instance variables • All objects share a single copy of the lastAssignedNumber variable • That variable is stored in a separate location, outside any BankAccount objects • Static variables should always be declared as private • This ensures that methods of other classes do not change their values • static constants may be either private or public
- 10. Static Methods • Sometimes a class defines methods that are not invoked on an object • Called a static method • Example: sqrt method of Math class • if x is a number, then the call x.sqrt() is not legal • Math class provides a static method: invoked as Math.sqrt(x) • No object of the Math class is constructed. • The Math qualifier simply tells the compiler where to find the sqrt • method.
- 11. Defining Your Own Static Methods public class Financial{ /** Computes a percentage of an amount. @param percentage the percentage to apply @param amount the amount to which the percentage is applied @return the requested percentage of the amount */ public static double percentOf(double percentage, double amount){ return (percentage / 100) * amount; } } When calling such a method, supply the name of the class containing it: double tax = Financial.percentOf(taxRate, total);
- 12. Main Method • The main method is always static. • When the program starts, there aren’t any objects. • Therefore, the first method of a program must be a static method. Name two static variables of the System class.
- 13. Using Objects in Objects • Create the object that you want to use inside the other object first. • Insert it into the object by using the name/reference of it. • Employee billSmith = new Employee(“Bill”, “Smith”); • Manager sales = new Manager(“Sales”, billSmith); • Error!!! Trying to use the object before it’s created! • Manager sales = new Manager(“Sales”, billSmith); • Employee billSmith = new Employee(“Bill”, “Smith”);
- 14. Using Objects in Objects • Event – String eventName – LocalDate dateOfEvent – Location location Location String address; String city; String state; LocalDate
Editor's Notes
- #13: Answer: System.in and System.out.