Lecture 10 Static Methods and Variables Last time 1 Project 2 Hints 2 Aliasing 3 Constructors Accessors Mutators 4 Equality 5 Printing an object 6 for loops Today 1 for loops from the previous set of slides 2 Static variables and methods CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr Static Variables and Methods We have seen how to declare Instance variables in classes public int tokenLevel 3 Methods in classes public void setName String nameDesired Objects created from a class receive their own copies of instance variables and methods Java also has static variables and methods which are shared by all objects in a class CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 1 Why Have Static Variables Methods Sometimes info needs to be shared data among all objects of a specific class type e g How many objects in a class have been created A constant that needs to be the same for all objects of that type Sometimes it is useful to have methods that are in a class that can be invoked without first creating objects of that type Static components help for these types of things CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 2 1 Declaring Static Methods and variables and constants Static methods public static void main public static void drawLetter DrawingGrid grid char letter Color color How do we call static methods LetterMaker drawLetter grid choice charAt 0 color Can have static variables and constants too public static int numStudents 0 public static final int MAX ENROLLMENT 0 How do we use static variables and constants see next example StudentRoster numStudents StudentRoster MAX ENROLLMENT CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 3 Example Object Counting public class StudentRoster public static int numStudents 0 public static final int MAX ENROLLMENT 50 public static int getNumStudents return numStudents public class Student code from previous Student class Student String newName name newName StudentRoster numStudents CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 4 What Is Printed Student s1 new Student John Doe Student s2 new Student Mary Roe System out println StudentRoster getNumStudents 2 Student s3 new Student Eduardo Duhalde System out println StudentRoster getNumStudents 3 System out println StudentRoster MAX ENROLLMENT 50 CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 5 2 Class Access to Static Variables and Methods If Ct is a class sv is a static variable and sm is a static method Then sv sm can be accessed via Ct sv Ct sm I e no object of type Ct needs to be created at all but if they do exist that is OK too CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 6 When To Use Static Variables Class wide constants static final int MAX ENROLLMENT 50 Class wide aggregate data static int numStudents 0 CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 7 When To Use Static Methods When a method should be invocable without object creation When a method should not change instance variables A static method can only change static variables Instance variables can only be changed by nonstatic methods CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 8 3 Default Values Static and instance variables initialized most types to 0 char to value 0 non printable character strings are assigned to null Local variables do not have a default value and you get a error from eclipse CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 9 Calling one method from another static and non static non static methods when running they are associated with a specific instance do have a current object are called with objectName methodName static methods when running they are NOT associated with a specific instance do NOT have a current object are called with ClassName methodName CMSC 131 Spring 2007 Jan Plane adapted from Bonnie Dorr 10 4
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