Cara menggunakan generik Java untuk mengelakkan ClassCastExceptions

Java 5 membawa generik ke bahasa Java. Dalam artikel ini, saya memperkenalkan anda kepada generik dan membincangkan jenis generik, kaedah generik, generik dan inferens jenis, kontroversi generik, dan pencemaran generik dan timbunan.

muat turun Dapatkan kod Muat turun kod sumber untuk contoh dalam tutorial Java 101 ini. Dicipta oleh Jeff Friesen untuk JavaWorld.

Apa itu generik?

Generik adalah sekumpulan ciri bahasa yang berkaitan yang membolehkan jenis atau kaedah beroperasi pada objek dari pelbagai jenis sambil memberikan keselamatan jenis waktu kompilasi. Ciri-ciri generik mengatasi masalah java.lang.ClassCastExceptiondilemparkan semasa menjalankan, yang merupakan hasil kod yang tidak selamat jenisnya (iaitu, membuang objek dari jenisnya sekarang ke jenis yang tidak sesuai).

Generik dan Kerangka Koleksi Java

Generik banyak digunakan dalam Java Collections Framework (diperkenalkan secara formal dalam artikel Java 101 yang akan datang ), tetapi tidak eksklusif untuk itu. Generik juga digunakan di bahagian lain di perpustakaan kelas standard Java termasuk java.lang.Class, java.lang.Comparable, java.lang.ThreadLocal, dan java.lang.ref.WeakReference.

Pertimbangkan fragmen kod berikut, yang menunjukkan kurangnya keselamatan jenis (dalam konteks kelas Java Collections Framework java.util.LinkedList) yang biasa terjadi dalam kod Java sebelum generik diperkenalkan:

Senaraikan doubleList = LinkedList baru (); doubleList.add (Double baru (3.5)); Double d = (Double) doubleList.iterator (). Seterusnya ();

Walaupun tujuan program di atas adalah hanya menyimpan java.lang.Doubleobjek dalam senarai, tidak ada yang menghalang objek lain daripada disimpan. Sebagai contoh, anda boleh menentukan doubleList.add("Hello");untuk menambahkan java.lang.Stringobjek. Namun, ketika menyimpan jenis objek lain, (Double)pengendali pemain barisan akhir ClassCastExceptionakan dilemparkan ketika berhadapan dengan Doubleobjek bukan .

Oleh kerana kekurangan jenis keselamatan ini tidak dapat dikesan sehingga waktu operasi, pembangun mungkin tidak menyedari masalahnya, menyerahkannya kepada klien (bukannya penyusun) untuk mengetahui. Generik membantu pengkompil memberi amaran kepada pengembang mengenai masalah menyimpan objek dengan Doublejenis bukan dalam senarai dengan membenarkan pembangun menandakan senarai sebagai mengandungi Doubleobjek sahaja . Bantuan ini ditunjukkan di bawah:

Senaraikan doubleList = LinkedList baru (); doubleList.add (Double baru (3.5)); Double d = doubleList.iterator (). Seterusnya ();

Listsekarang berbunyi " Listdaripada Double." Listadalah antara muka generik, dinyatakan sebagai List, yang mengambil Doubleargumen jenis, yang juga ditentukan ketika membuat objek sebenarnya. Penyusun kini dapat menegakkan ketepatan jenis ketika menambahkan objek ke dalam senarai - misalnya, senarai itu Double hanya dapat menyimpan nilai. Penguatkuasaan ini menghilangkan keperluan (Double)pemeran.

Mencari jenis generik

A jenis generik adalah kelas atau antara muka yang memperkenalkan satu set jenis parameterized melalui senarai jenis parameter formal , yang merupakan senarai-dipisah-koma jenis nama parameter antara sepasang kurungan sudut. Jenis generik mematuhi sintaks berikut:

pengecam kelas < formalTypeParameterList > {// class body} pengecam antara muka < formalTypeParameterList > {// antara muka badan}

Java Collections Framework menawarkan banyak contoh jenis generik dan senarai parameternya (dan saya merujuknya sepanjang artikel ini). Sebagai contoh, java.util.Setadalah jenis generik,   adalah senarai parameter jenis formal, dan E merupakan parameter jenis tunggal. Contoh lain ialah  java.util.Map.

Konvensyen penamaan parameter jenis Java

Konvensyen pengaturcaraan Java menetapkan bahawa nama parameter jenis menjadi huruf besar tunggal, seperti Euntuk elemen, Kuntuk kunci, Vuntuk nilai, dan Tuntuk jenis. Sekiranya boleh, elakkan menggunakan nama yang tidak bermakna seperti P- java.util.Listbermaksud senarai elemen, tetapi apa yang mungkin anda maksudkanList

A parameterized type is a generic type instance where the generic type’s type parameters are replaced with actual type arguments (type names). For example, Set is a parameterized type where String is the actual type argument replacing type parameter E.

The Java language supports the following kinds of actual type arguments:

  • Concrete type: A class or other reference type name is passed to the type parameter. For example, in List, Animal is passed to E.
  • Concrete parameterized type: A parameterized type name is passed to the type parameter. For example, in Set , List is passed to E.
  • Array type: An array is passed to the type parameter. For example, in Map, String is passed to K and String[] is passed to V.
  • Type parameter: A type parameter is passed to the type parameter. For example, in class Container { Set elements; }, E is passed to E.
  • Wildcard: The question mark (?) is passed to the type parameter. For example, in Class, ? is passed to T.

Each generic type implies the existence of a raw type, which is a generic type without a formal type parameter list. For example, Class is the raw type for Class. Unlike generic types, raw types can be used with any kind of object.

Declaring and using generic types in Java

Declaring a generic type involves specifying a formal type parameter list and accessing these type parameters throughout its implementation. Using the generic type involves passing actual type arguments to its type parameters when instantiating the generic type. See Listing 1.

Listing 1:GenDemo.java (version 1)

class Container { private E[] elements; private int index; Container(int size) { elements = (E[]) new Object[size]; index = 0; } void add(E element) { elements[index++] = element; } E get(int index) { return elements[index]; } int size() { return index; } } public class GenDemo { public static void main(String[] args) { Container con = new Container(5); con.add("North"); con.add("South"); con.add("East"); con.add("West"); for (int i = 0; i < con.size(); i++) System.out.println(con.get(i)); } }

Listing 1 demonstrates generic type declaration and usage in the context of a simple container type that stores objects of the appropriate argument type. To keep the code simple, I’ve omitted error checking.

The Container class declares itself to be a generic type by specifying the formal type parameter list. Type parameter E is used to identify the type of stored elements, the element to be added to the internal array, and the return type when retrieving an element.

The Container(int size) constructor creates the array via elements = (E[]) new Object[size];. If you’re wondering why I didn’t specify elements = new E[size];, the reason is that it isn’t possible. Doing so could lead to a ClassCastException.

Compile Listing 1 (javac GenDemo.java). The (E[]) cast causes the compiler to output a warning about the cast being unchecked. It flags the possibility that downcasting from Object[] to E[] might violate type safety because Object[] can store any type of object.

Note, however, that there is no way to violate type safety in this example. It’s simply not possible to store a non-E object in the internal array. Prefixing the Container(int size) constructor with @SuppressWarnings("unchecked") would suppress this warning message.

Execute java GenDemo to run this application. You should observe the following output:

North South East West

Bounding type parameters in Java

The E in Set is an example of an unbounded type parameter because you can pass any actual type argument to E. For example, you can specify Set, Set, or Set.

Sometimes you’ll want to restrict the types of actual type arguments that can be passed to a type parameter. For example, perhaps you want to restrict a type parameter to accept only Employee and its subclasses.

You can limit a type parameter by specifying an upper bound, which is a type that serves as the upper limit on the types that can be passed as actual type arguments. Specify the upper bound by using the reserved word extends followed by the upper bound’s type name.

For example, class Employees restricts the types that can be passed to Employees to Employee or a subclass (e.g., Accountant). Specifying new Employees would be legal, whereas new Employees would be illegal.

You can assign more than one upper bound to a type parameter. However, the first bound must always be a class, and the additional bounds must always be interfaces. Each bound is separated from its predecessor by an ampersand (&). Check out Listing 2.

Listing 2: GenDemo.java (version 2)

import java.math.BigDecimal; import java.util.Arrays; abstract class Employee { private BigDecimal hourlySalary; private String name; Employee(String name, BigDecimal hourlySalary) { this.name = name; this.hourlySalary = hourlySalary; } public BigDecimal getHourlySalary() { return hourlySalary; } public String getName() { return name; } public String toString() { return name + ": " + hourlySalary.toString(); } } class Accountant extends Employee implements Comparable { Accountant(String name, BigDecimal hourlySalary) { super(name, hourlySalary); } public int compareTo(Accountant acct) { return getHourlySalary().compareTo(acct.getHourlySalary()); } } class SortedEmployees
    
      { private E[] employees; private int index; @SuppressWarnings("unchecked") SortedEmployees(int size) { employees = (E[]) new Employee[size]; int index = 0; } void add(E emp) { employees[index++] = emp; Arrays.sort(employees, 0, index); } E get(int index) { return employees[index]; } int size() { return index; } } public class GenDemo { public static void main(String[] args) { SortedEmployees se = new SortedEmployees(10); se.add(new Accountant("John Doe", new BigDecimal("35.40"))); se.add(new Accountant("George Smith", new BigDecimal("15.20"))); se.add(new Accountant("Jane Jones", new BigDecimal("25.60"))); for (int i = 0; i < se.size(); i++) System.out.println(se.get(i)); } }
    

Listing 2’s Employee class abstracts the concept of an employee that receives an hourly wage. This class is subclassed by Accountant, which also implements Comparable to indicate that Accountants can be compared according to their natural order, which happens to be hourly wage in this example.

The java.lang.Comparable interface is declared as a generic type with a single type parameter named T. This interface provides an int compareTo(T o) method that compares the current object with the argument (of type T), returning a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

The SortedEmployees class lets you store Employee subclass instances that implement Comparable in an internal array. This array is sorted (via the java.util.Arrays class’s void sort(Object[] a, int fromIndex, int toIndex) class method) in ascending order of the hourly wage after an Employee subclass instance is added.

Compile Listing 2 (javac GenDemo.java) and run the application (java GenDemo). You should observe the following output:

George Smith: 15.20 Jane Jones: 25.60 John Doe: 35.40

Lower bounds and generic type parameters

You cannot specify a lower bound for a generic type parameter. To understand why I recommend reading Angelika Langer’s Java Generics FAQs on the topic of lower bounds, which she says “would be confusing and not particularly helpful.”

Considering wildcards

Let’s say you want to print out a list of objects, regardless of whether these objects are strings, employees, shapes, or some other type. Your first attempt might look like what’s shown in Listing 3.

Listing 3: GenDemo.java (version 3)

import java.util.ArrayList; import java.util.Iterator; import java.util.List; public class GenDemo { public static void main(String[] args) { List directions = new ArrayList(); directions.add("north"); directions.add("south"); directions.add("east"); directions.add("west"); printList(directions); List grades = new ArrayList(); grades.add(new Integer(98)); grades.add(new Integer(63)); grades.add(new Integer(87)); printList(grades); } static void printList(List list) { Iterator iter = list.iterator(); while (iter.hasNext()) System.out.println(iter.next()); } }

It seems logical that a list of strings or a list of integers is a subtype of a list of objects, yet the compiler complains when you attempt to compile this listing. Specifically, it tells you that a list-of-string cannot be converted to a list-of-object, and similarly for a list-of-integer.

The error message you've received is related to the fundamental rule of generics: