Static Class Members Wrapper Classes Autoboxing Unboxing

1. Static Class Members Wrapper Classes Autoboxing Unboxing

2. Static Class Members • Recall that a static method is one that can be invoked through its class name • For example, the methods of the Math class are static: result = Math.sqrt(25) • Variables can be static as well.

3. Static Class Members • Determining if a method or variable should be static is an important design decision

4. The static Modifier • We declare static methods and variables using the static modifier • It associates the method or variable with the class rather than with an object of that class • Static methods are sometimes called class methods and static variables are sometimes called class variables.

5. Static Variables • Normally, each object has its own data space, but if a variable is declared as static, only one copy of the variable exists private static float price; • Memory space for a static variable is created when the class is first referenced.

6. Static Variables • All objects instantiated from the class share its static variables • Changing the value of a static variable in one object changes it for all others

7. Static Methods class Helper { public static int cube (int num) { return num * num * num; } } Because it is declared as static, the method can be invoked as value = Helper.cube(5);

8. Static Class Members • The order of the modifiers can be interchanged, but by convention visibility modifiers come first • Recall that the main method is static – it is invoked by the Java interpreter without creating an object

9. Static Class Members • Static methods cannot reference instance variables because instance variables don't exist until an object exists • However, a static method can reference static variables or local variables

10. Wrapper Classes • The java.lang package contains a wrapper class that corresponds to each primitive type:

11. Wrapper Classes • The following declaration creates an Integer object which is a reference to an object with the integer value 40 Integer age = new Integer(40); • An object of a wrapper class is used in situations where a primitive value will not suffice • For example, some objects serve as containers of other objects • Primitive values could not be stored in such containers, but wrapper objects could be

12. Wrapper Classes • Wrapper classes may contain static methods that help manage the associated type • For example, the Integer class contains a method to convert an integer stored in a String to an int value: num = Integer.parseInt(str); • Wrapper classes often contain useful constants • For example, the Integer class contains MIN_VALUE and MAX_VALUE for the smallest and largest int values

13. Autoboxing • Autoboxing is the automatic conversion of a primitive value to a corresponding wrapper object: Integer obj; int num = 42; obj = int; • The assignment creates the appropriate Integer object

14. Unboxing • The reverse conversion (called unboxing) also occurs automatically as needed