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| Figure 4 - A compiler error. |
|---|
Generics04.java:34: error: no suitable method found for add(String)
var1.add("abcd");^
method ArrayList.add(int,Date) is not applicable(actual and formal argument lists differ in length)
method ArrayList.add(Date) is not applicable(actual argument String cannot be converted to Date by method invocation
conversion)1 error |
Without the use of generics, a reference to an object of any type could be added to the collection. This could result in a runtime error laterif the programmer expected an object of type Date when in fact the object is of type String . If you are going to write programs containing errors, compiler errors are almost always preferable to runtimeerrors.
Iterators
Listing 7 and Listing 8 showed the syntax that you must use to cause a collection object to be treated as a generic collection. You must also usea special syntax when working with generic iterators and the Java Collections Framework.
The enhanced for loop
Another new feature of Java version 1.5, referred to by Oracle as an enhanced for loop, can be used in certain situations to provide most of the benefits of an iterator with a somewhat simpler syntax.
(The enhanced for loop is also sometimes referred to as a for-each loop.)
The program named Generics05 , which begins in Listing 1 , illustrates both of these concepts. A complete listing of the program is provided in Listing 9 .
| Listing 1 - Beginning of the program named Generics05. |
|---|
import java.util.*;
public class Generics05{//Create an ArrayList object suitable for
// storing references to Date objects.ArrayList<Date>var1 = new ArrayList<Date>(); |
The main method
As in the previous programs, the main method instantiates an object of the Generics05 class and calls the runIt method on that object. You can view the main method in Listing 9 near the end of the module.
Instantiate a generic ArrayList object
This program is a little longer than the previous programs, so I will break it down and explain it in fragments.
Listing 1 shows the beginning of the Generics05 class. The code in Listing 1 instantiates a new ArrayList object capable of storing references to objects of type Date only. The code in Listing 1 also saves that object's reference in a generic instance variable named var1 . This is the same syntax that you have seen in previous listings.
Populate the collection
Listing 2 shows the beginning of the runIt method. This code populates the ArrayList object with references to three Date objects. The first object encapsulates the current date and time.The second object encapsulates the date and time one day later than the first. The third object encapsulates the date and time two days later than the firstobject.
| Listing 2 - Beginning of the runIt method. |
|---|
void runIt(){
//Get current date and time in milliseconds.long now = new Date().getTime();
//Get length of one day in millisecondslong oneDay = 24 * 60 * 60 * 1000;
//Populate the ArrayList objectvar1.add(new Date(now));
var1.add(new Date(now + oneDay));var1.add(new Date(now + 2 * oneDay)); |
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