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Chapter 4 Generic Vector Class

Chapter 4 Generic Vector Class. Agenda. A systemic problem with Vector of Object Several approaches at a solution Generic structures Converting classes to generic classes Association class Vector class. Reading the args array in main. public static void main(String[] args ) {

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Chapter 4 Generic Vector Class

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  1. Chapter 4 Generic Vector Class

  2. Agenda • A systemic problem with Vector of Object • Several approaches at a solution • Generic structures • Converting classes to generic classes • Association class • Vector class

  3. Reading the args array in main public static void main(String[] args) { Vector longWords = new Vector(); int i; for (i = 0; i < args.length; i++) { if (args[i].length() > 4) { longWords.add(args[i]); // line 12 suppose you forget [i] ? } } ... for (i = 0; i < longWords.size(); i++) { String word = (String)longWords.get(i); // line 31 System.out.println(word+", length "+word.length()); } }

  4. Generics • A language feature for generalizing the type of data a method or class will process • The data types are specified by the code • That invokes of the method • That declares an object in the class • Method parameter types or class data types can be generic types

  5. Generic Methods • A generic placeholder (e.g., <T>) is coded in the method heading and used in the parameter list • Invoker’s argument(s) type is substituted for the generic place holder at run time. • E.g., generic code to output an array of any type of primitive (int, float, double, char, etc.) public static<T>void outputNumericArray( T[] array) { for(int i=0; i< array.length; i++) System.out.println(array[i]); }

  6. Generic Classes • A generic placeholder (e.g., <T>) is coded in the class’ heading and used in the class’ code • Types used in an object declaration is substituted for the generic place holder at run time. • Assuming there are two generic types in the class PersonGeneric, an instance declaration would be: PersonGeneric <Integer, Double> bill = new PersonGeneric <Integer, Double> (10, 102.56);

  7. Generic Class Code public class PersonGeneric <T, E> { // definition of the data members privateT age; privateE weight; // definition of member functions public PersonGeneric(T a, Ew ) // the constructor { age = a; weight = w; } public String toString( ) { return( "this person’s age is: " + age + "\n and their weight is: " + weight); } // end of toString method } // end of Person class For the declaration on the previous slide, Integer and Double will be substituted for placeholders T and E respectively

  8. Bailey's Structure--Generic • Bailey has two versions of his package: • structure for non generic versions • structure5 for generic versions • import structure.Vector; non-generic version • can only say Vector wordList; • import structure5.Vector; generic version • can be used either way • Vector<String> wordList; OR Vector wordList (unsafe ops warning)

  9. BlueJ Warning using Generics • This happens when you use Java 1.4-style collections (non-generic) with Java 5. The Java 5 compiler produces this warning. In BlueJ, you can switch off this warning in the preferences: Open the 'Miscellaneous' tab in the preferences, and uncheck the option "Show compiler warnings when unsafe collections are used". • Alternatively, import structure.Vector NOT structure5.Vector

  10. Errors using Generics • Type mismatch – good, want to know about these • Sometimes overspecifing types will trigger warning • Not all casts are strictly necessary • For now be open to modifying your expressions

  11. Database concepts • Load a file of students into a Vector<Student> • Find the student with certain ID • Update a student record, replace back in Vector

  12. What it looks like • Vector<Student> database = new Vector<Student>(); • // .. read student100 file into database database elementCountfirstName elementDatalastName ID GPA firstName lastName ID

  13. Another Approach • Each student record is a Vector of Associations • such as

  14. More like what we are doing …

  15. Java Vector vsArrayList classes • http://www.javaworld.com/javaqa/2001-06/03-qa-0622-vector.html • Both share the same interface (same methods to access, insert) • java.util.Vector • “Thread Safe” meaning it can be used in multithreaded apps • When extending array size, it doubles the capacity • java.util.ArrayList • “Not Thread Safe” should not be used in multithreaded apps • When extending array size it increases capacity by 50% • Overhead from resizing can dampen performance • In general try to estimate the actual size you need in program.

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