Cara menggunakan generik Java untuk menghindari ClassCastExceptions

Java 5 membawa generik ke dalam bahasa Java. Pada artikel ini, saya memperkenalkan Anda pada obat generik dan membahas jenis generik, metode generik, generik dan inferensi tipe, kontroversi generik, dan generik dan polusi tumpukan.

unduh Dapatkan kodenya Unduh kode sumber untuk contoh dalam tutorial Java 101 ini. Dibuat oleh Jeff Friesen untuk JavaWorld.

Apa itu obat generik?

Generik adalah kumpulan fitur bahasa terkait yang memungkinkan tipe atau metode untuk beroperasi pada objek dari berbagai tipe sambil memberikan keamanan tipe waktu kompilasi. Fitur generik mengatasi masalah java.lang.ClassCastExceptionyang dilemparkan saat runtime, yang merupakan hasil dari kode yang bukan tipe aman (yaitu, mentransmisikan objek dari tipe saat ini ke tipe yang tidak kompatibel).

Generik dan Java Collections Framework

Generik banyak digunakan dalam Java Collections Framework (secara resmi diperkenalkan di artikel Java 101 mendatang ), tetapi tidak eksklusif untuk itu. Generik juga digunakan di bagian lain dari perpustakaan kelas standar Java termasuk java.lang.Class, java.lang.Comparable, java.lang.ThreadLocal, dan java.lang.ref.WeakReference.

Pertimbangkan fragmen kode berikut, yang menunjukkan kurangnya keamanan tipe (dalam konteks kelas Java Collections Framework java.util.LinkedList) yang umum dalam kode Java sebelum generik diperkenalkan:

Daftar doubleList = new LinkedList (); doubleList.add (baru Double (3.5)); Double d = (Double) doubleList.iterator (). Next ();

Meskipun tujuan dari program di atas adalah untuk menyimpan hanya java.lang.Doubleobjek dalam daftar, tidak ada yang mencegah jenis objek lain untuk disimpan. Misalnya, Anda dapat menentukan doubleList.add("Hello");untuk menambahkan java.lang.Stringobjek. Namun, saat menyimpan jenis objek lain, (Double)operator cor baris terakhir menyebabkan ClassCastExceptionterlempar saat dihadapkan dengan non- Doubleobjek.

Karena kurangnya keamanan jenis ini tidak terdeteksi hingga runtime, pengembang mungkin tidak menyadari masalah tersebut, menyerahkannya kepada klien (bukan kompilator) untuk menemukannya. Generik membantu kompilator memperingatkan pengembang untuk masalah penyimpanan objek dengan non- Doubletipe dalam daftar dengan mengizinkan pengembang untuk menandai daftar sebagai hanya berisi Doubleobjek. Bantuan ini ditunjukkan di bawah ini:

Daftar doubleList = new LinkedList (); doubleList.add (baru Double (3.5)); Double d = doubleList.iterator (). Next ();

Listsekarang menjadi " Listdari Double." Listadalah antarmuka generik, yang dinyatakan sebagai List, yang mengambil Doubleargumen tipe, yang juga ditentukan saat membuat objek sebenarnya. Kompilator sekarang dapat menerapkan ketepatan tipe saat menambahkan objek ke daftar - misalnya, daftar hanya dapat menyimpan Double nilai. Penegakan hukum ini menghilangkan kebutuhan akan (Double)pemeran.

Menemukan tipe generik

Sebuah jenis generik adalah kelas atau interface yang memperkenalkan satu set jenis parameter melalui daftar jenis parameter formal , yang merupakan daftar dipisahkan koma nama jenis parameter antara sepasang kurung sudut. Jenis generik mengikuti sintaks berikut:

pengenal kelas < formalTypeParameterList > {// class body} pengenal antarmuka < formalTypeParameterList > {// interface body}

Java Collections Framework menawarkan banyak contoh tipe generik dan daftar parameternya (dan saya merujuknya di seluruh artikel ini). Misalnya, java.util.Setadalah tipe generik,   adalah daftar parameter tipe formalnya, dan E merupakan parameter tipe soliter daftar. Contoh lainnya adalah  java.util.Map.

Konvensi penamaan parameter tipe Java

Konvensi pemrograman Java menentukan bahwa nama parameter tipe menjadi huruf besar tunggal, seperti Euntuk elemen, Kuntuk kunci, Vuntuk nilai, dan Tuntuk tipe. Jika memungkinkan, hindari menggunakan nama yang tidak berarti seperti P- java.util.Listberarti daftar elemen, tetapi apa yang mungkin Anda maksud denganList

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: