Download APCS Lecture Notes – Wrapper Classes

APCS Lecture Notes – Wrapper Classes -- Integer and Double
Wrapper classes are used to make objects from primitive data types – int, double, boolean,
etc.
java.lang package provides “wrapper” classes for primitive data types to convert them to
objects.
Question: Why would we want to convert primitive data types to objects?
Answer: To store them in our many Object–only data structures.
Like String, wrapper classes are immutable -- they have NO modifier/mutator methods and,
once created, can NEVER BE CHANGED internally. They can only be overwritten.
Integer class constructor takes an int parameter
Integer obj = new Integer(123);
int num = obj.intValue(); //puts 123 in primitive int num
Wrapper classes inherit several important methods from Object:
1. Use .equals() method when comparing two Integers or two Doubles
2. toString() shows the integer value in the object
Important:
== and != compare addresses, not values – Check for aliasing!
Comparable interface
class java.lang.Integer implements java.lang.Comparable
Wrapper classes implement the Comparable interface
compareTo method – obj1.compareTo(obj2)
returns positive number if obj1 is greater than obj2 (like subtraction)
returns negative number if obj1 is less than obj2 (like subtraction)
returns 0 if obj1 is equal to obj2 (like subtraction)
Introduction to Casting – The compiler complains
ArrayList<Object> aL = new ArrayList<Object>();
aL.add(new Fraction(1,3));
System.out.println(aL.get(0));
//What if I just want to get the Fraction's numerator?
System.out.println(aL.get(0).getNumerator());
//Fix:
System.out.println(((Fraction)aL.get(0)).getNumerator());
The compiler must be able to find the definition of all methods before it will successfully
compile. Sometimes you must direct it where to look.
D:\582677150.doc