![]() ![]() The Javacompiler will then enforce this by allowing only String objects to be added. This new definitiondeclares that all elements of this ArrayList will be of type String. Thus, we can read the newconstruct " ArrayList" as"ArrayList of Strings". We can read the angle brackets roughly as "of". We have to make a corresponding change where the ArrayList object is created inthe constructor. We can now declare it as private ArrayList notes Java 5 introduces a new piece of syntax to allow us to do this.Instead of declaring notes as follows: private ArrayList notes ![]() Java 5 encourages programmers to use typed collections - collectionsthat store objects of a known type - rather than untyped collections - collections that can store objects of mixedtypes.In this particular case, we are encouraged to indicate to the compiler that notes will store objects of type String. This warning is indicating that the compiler cannot check whether or not it isappropriate to add an object of type String to the notes collection. The message suggests recompiling with an additional compiler option: -Xlint.When this option is added (e.g., via the fs configuration file)the more detailed explanation is: Notebook.java:30: warning: unchecked call to add(E) as a member of the raw type notes.add(note) ^ \projects\chapter04\notebook1\Notebook.java uses unchecked or unsafe operations.Note: Recompile with -Xlint:unchecked for details. If the notebook1 project of Chapter 4 is compiled with a Java 5 compiler,the following warning message is produced: Note. In Java 5, the new Generics feature offers a way around this 'type loss'problem and casting requirement through its support of typed collections. This is understandable, because the compiler has no way of knowing what type ofobjects have been stored into the collection.Indeed, a particular collection might have objects of several different typesstored within it at any one time. One drawback, however, is that storing an object into a collection effectively'loses' details of an object's type, so that retrieval requires the use of a cast: Lot selectedLot = (Lot) lots.get(lotNumber - 1) In this supplementary material we discuss some of the new language featuresavailable in Java 5.We do this in the context of a sample of projects to be found in the secondedition of Objects First with Java - A Practical Introduction using BlueJ,Pearson Education, 2005 (in the USA: Prentice Hall, 2005),ISBN 3-9.Ĭollection classes such as ArrayList, LinkedList, and HashMap permit objects of any class to be stored withinthem.This feature makes them extremely useful for storing arbitrary numbers ofobjects in many different application contexts. Supplementary Material for Java 5 David J. Objects First with Java: Supplementary Material Objects First with Java
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