Generic Java Classes Implementing your own generic classes

Generic Java Classes Implementing your own generic classes CS-2851 Dr. Mark L. Hornick 1

Array. List vs. Array. List<E> ( or Linked. List vs. Linked. List<E>) JDK 1. 4 (deprecated): To declare an Array. List that can contain any datatype: Array. List some. List; You’ll get a warning if you do this (not an error) – don’t use this approach! JDK 1. 5 and later: To declare an Array. List that can contain only Double’s: Array. List<Double> salary. List; CS-2851 Dr. Mark L. Hornick 2

Generics – new to J 2 SE 5. 0/JDK 1. 5 Eclipse must be configured to use JDK 1. 5 or 1. 6 l you should have 1. 6 on your PC CS-2851 Dr. Mark L. Hornick 3

Generic Array. List<E> Array. List<Double> salary. List = new Array. List<Double>(); E specifies the specific type of data that the Array. List can manage l You must substitute a class name for E This creates an instance, salary. List, of the Array. List collection class. The elements in salary. List must be (references to) Doubles. Q: Why do we have to use Array. List<Double>? Can’t we just use Array. List<double>? A: No. The generic type E must represent a class, not a primitive. Double is a wrapper class that just represents a double primitive. CS-2851 Dr. Mark L. Hornick 4

Wrapper-to-primitive type conversion is done automatically through boxing and unboxing Example: You want to insert a double value into an Array. List<Double> at index 30: salary. List. add(30, 40000. 00); // works!! This is called boxing: The automatic conversion of a primitive double value (40000. 00) to the appropriate Double wrapper object To retrieve the value, no explicit cast is needed: double pay = /*(double)*/ salary. List. get(30); This is because although the get() method returns a Double, a Double can be automatically converted to a double primitive type. This is called unboxing. CS-2851 Dr. Mark L. Hornick 5

Datatype parameter placeholders appear in the definition of classes and interfaces public class Array. List<E> {…} l E (for “Element”) is a type parameter E is not a keyword, just a placeholder l The letter T is also used as a placeholder l l l But you can use any (non-keyword) placeholder name E and T are commonly-used symbols Generics allow you to create a class template that can use different types of objects when instantiated CS-2851 Dr. Mark L. Hornick 6

Writing your own generic class Consider a class that deals with ints public class Maxinator { public int get. Max. Value(int a, int b) { int max=0; int result = a-b; if( result > 0 ) max=a; else if( result < 0 ) max=b; else // a and b are the same max=a; return max; } } CS-2851 Dr. Mark L. Hornick 7

Next, replace int with a generic placeholder Typically, we use the letter E: public class Maxinator<E> { public E print. Max. Value(E a, E b) { E max=0; E result = a-b; // any issues here? ? if( result > 0 ) max=a; else if( result < 0 ) max=b; else // a and b are the same max=a; return max; } } CS-2851 Dr. Mark L. Hornick 8

The compiler doesn’t “know” what datatype E represents However, we can specify that E must be a subclass of Number (the superclass of Double, Integer, Float, etc). public class Maxinator<E extends Number> { public print. Max. Value(E a, E b) { int max=0; int result = a. int. Value() – b. int. Value(); // all Numbers support the int. Value() method if( result > 0 ) max=a; else if( result < 0 ) max=b; else // a and b are the same max=a; return max; CS-2851 } Dr. Mark L. Hornick } 9

So far, we have limited our Maxinator class to handle subtypes of Number What if we wanted to allow it to compare other datatypes, like Strings and Students? ? l We need another way to compare the objects, regardless of the datatype represented by E The Comparable<E> interface is the key to doing this for any datatype represented by E. CS-2851 Dr. Mark L. Hornick 10

The Comparable<E> interface in the java. lang package This interface defines a single method: int compare. To(E o) // returns -1, 0, or 1 This method compares this object (the one invoking the compare. To() method) to another object of the same datatype (the object represented by o) Many datatypes already implement Comparable: String msg = “Hello”; int result = msg. compare. To(“hello”); // returns 1 Double x = 3. 0; result = x. compare. To(4. 0); // returns -1 result = x. compare. To(3. 0); // returns 0 CS-2851 Dr. Mark L. Hornick 11

We can specify that the class represented by E must implement the Comparable<E> interface Any class E that implements Comparable contains the compare. To() method!! public class Maxinator<E extends Comparable<E>> { public print. Max. Value(E a, E b) { int max=0; int result = a. compare. To(b); // returns -1, 0, or 1 if( result > 0 ) max=a; else if( result < 0 ) max=b; else // a and b are the same max=a; return max; } CS-2851 12 Dr. Mark L. Hornick }

Writing Generic Classes Write the basic class using a simple type 1. l e. g. int Test it using a test program 2. l Make sure it works Replace the simple type identifier with E 3. l Or other generic type name Add the generic suffix 4. l Fall 2004 public class My. Generic. Class<E> CS-183 Dr. Mark L. Hornick 13

Using Generic Classes Declare the specific class using a specific type 1. l l l My. Generic. Class<Integer> My. Generic. Class<String> My. Generic. Class<Student> Test it using a test program 2. l Make sure it still works All specific types (Integer, String, Student) must support the same methods, (e. g. compare. To()) Fall 2004 CS-183 Dr. Mark L. Hornick 14