Generics in Java

Generics are a facility of generic programming that was added to the Java programming language in 2004 as part of J2SE 5.0. They allow "a type or method to operate on objects of various types while providing compile-time type safety." A common use of this feature is when using a Java Collection that can hold objects of any type, to specify the specific type of object stored in it.

Hierarchy and classification

As per Java Language Specification:

• A type variable is an unqualified identifier. Type variables are introduced by generic class declarations, generic interface declarations, generic method declarations, and by generic constructor declarations.
• A class is generic if it declares one or more type variables. These type variables are known as the type parameters of the class. It defines one or more type variables that act as parameters. A generic class declaration defines a set of parameterized types, one for each possible invocation of the type parameter section. All of these parameterized types share the same class at runtime.
• An interface is generic if it declares one or more type variables. These type variables are known as the type parameters of the interface. It defines one or more type variables that act as parameters. A generic interface declaration defines a set of types, one for each possible invocation of the type parameter section. All parameterized types share the same interface at runtime.
• A method is generic if it declares one or more type variables. These type variables are known as the formal type parameters of the method. The form of the formal type parameter list is identical to a type parameter list of a class or interface.
• A constructor can be declared as generic, independently of whether the class that the constructor is declared in is itself generic. A constructor is generic if it declares one or more type variables. These type variables are known as the formal type parameters of the constructor. The form of the formal type parameter list is identical to a type parameter list of a generic class or interface.

Motivation for generics

The following block of Java code illustrates a problem that exists when not using generics. First, it declares an ArrayList of type Object. Then, it adds a String to the ArrayList. Finally, it attempts to retrieve the added String and cast it to an Integer.

  List v = new ArrayList();
  v.add("test");
  Integer i = (Integer)v.get(0);        // Run time error

Although the code compiles without error, it throws a runtime exception (java.lang.ClassCastException) when executing the third line of code. This type of problem can be avoided by using generics and is the primary motivation for using generics.

Using generics, the above code fragment can be rewritten as follows:

  List<String> v = new ArrayList<String>();
  v.add("test");
  Integer i = v.get(0); // (type error)  Compile time error

The type parameter String within the angle brackets declares the ArrayList to be constituted of String (a descendant of the ArrayList's generic Object constituents). With generics, it is no longer necessary to cast the third line to any particular type, because the result of v.get(0) is defined as String by the code generated by the compiler.

Compiling the third line of this fragment with J2SE 5.0 (or later) will yield a compile-time error because the compiler will detect that v.get(0) returns String instead of Integer. For a more elaborate example, see reference.

Here is a small excerpt from the definitions of the interfaces List and Iterator in package java.util:

  public interface List<E> { 
    void add(E x);
    Iterator<E> iterator();
  }
  public interface Iterator<E> { 
    E next();
    boolean hasNext();
  }

Wildcards

Main article: Wildcard (Java)

Generic type parameters in Java are not limited to specific classes. Java allows the use of wildcards to specify bounds on the type of parameters a given generic object may have. Wildcards are type parameters of the form "?", possibly annotated with a bound. Given that the exact element type of an object with a wildcard is unknown, restrictions are placed on the type of methods that may be called on the object.

As an example of an unbounded wildcard, List<?> indicates a list which has an unknown object type. Methods which take such a list as an argument can take any type of list, regardless of parameter type. Reading from the list will return objects of type Object, and writing non-null elements to the list is not allowed, since the parameter type is not known.

To specify the upper bound of a generic element, the extends keyword is used, which indicates that the generic type is a subtype of the bounding class. Thus it must either extend the class, or implement the interface of the bounding class. So List<? extends Number> means that the given list contains objects of some unknown type which extends the Number class. For example, the list could be List<Float> or List<Number>. Reading an element from the list will return a Number, while writing non-null elements is once again not allowed.

The use of wildcards above is necessary since objects of one type parameter cannot be converted to objects of another parameter. Neither List<Number> nor List<Integer> is a subtype of the other, even though Integer is a subtype of Number. So, code that deals with List<Number> does not work with List<Integer>. If it did, it would be possible to insert a Number that is not an Integer into it, which violates type safety. Here is sample code that explains the contradiction it brings if List<Integer> were a subtype of List<Number>:

List<Integer> ints = new ArrayList<Integer>();
ints.add(2);
List<Number> nums = ints;  //valid if List<Integer> were a subtype of List<Number> according to substitution rule. 
nums.add(3.14);  
Integer x=ints.get(1); // now 3.14 is assigned to an Integer variable!

The solution with wildcards works because it disallows operations that would violate type safety.

To specify the lower bounding class of a generic element, the super keyword is used. This keyword indicates that the aforementioned generic type is a super-type of said bounding class. So, List<? super Number> could represent List<Number> or List<Object>. Reading from a list defined as List<? super Number> returns elements of type Object. Writing to such a list requires elements of type Number.

Generic class definitions

Here is an example of a generic class:

public class Entry<K, V> {
  
  private final K key;
  private final V value;

  public Entry(K k,V v) {  
    key = k;
    value = v;   
  }

  public K getKey() {
    return key;
  }

  public V getValue() {
    return value;
  }

  public String toString() { 
    return "(" + key + ", " + value + ")";  
  }
}

Note that the above class is for illustration of Java generics only.

A better implementation of Entry/Pair can be found in this stackoverflow.com discussion.

This generic class can be used in the following way:

Entry<String, String> grade440 = new Entry<String, String>("mike", "A");
Entry<String, Integer> marks440 = new Entry<String, Integer>("mike", 100);
System.out.println("grade: " + grade440);
System.out.println("marks: " + marks440);

Generic method definitions

Here is an example of a generic method using the generic class above:

public static <T> Entry<T,T> twice(T value) {
   return new SimpleImmutableEntry<T,T>(value, value);
}

In many cases the user of the method need not indicate the type parameters, as they can be inferred:

Entry<String, String> pair = twice("Hello");

The parameters can be explicitly added if needed:

Entry<String, String> pair = this.<String>twice("Hello");

Note that you cannot use native types, ex:

Entry<int, int> pair; // this fails. You have to use Integer instead.

There is also the possibility to create generic methods based on given parameters.

public <T> T[] toArray(T... elements) {
   return elements;
}

In such cases you can't use native types either, ex:

   Integer[] array = toArray(1,2,3,4,5);

Generics in throws clause

Although exceptions themselves cannot be generic, generic parameters can appear in a throws clause:

public <T extends Throwable> void throwMeConditional
      (boolean conditional, T exception) throws T {
   if(conditional)
     throw exception;
}

Problems with type erasure

Generics are checked at compile-time for type-correctness. The generic type information is then removed in a process called type erasure. For example, List<Integer> will be converted to the non-generic type List, which ordinarily contains arbitrary objects. The compile-time check guarantees that the resulting code is type-correct.

As a result of type erasure, type parameters cannot be determined at run-time. For example, when an ArrayList is examined at runtime, there is no general way to determine whether, before type erasure, it was an ArrayList<Integer> or an ArrayList<Float>. Many people are dissatisfied with this restriction. There are partial approaches. For example, individual elements may be examined to determine the type they belong to; for example, if an ArrayList contains an Integer, that ArrayList was presumably parameterized with Integer. Reflection can also determine the type parameter. However, no approach works in all cases.

Demonstrating this point, the following code outputs "Equal":

ArrayList<Integer> li = new ArrayList<Integer>();
ArrayList<Float> lf = new ArrayList<Float>();
if (li.getClass() == lf.getClass()) // evaluates to true
  System.out.println("Equal");

Java generics differ from C++ templates. Java generics generate only one compiled version of a generic class or function regardless of the number of parameterizing types used. Furthermore, the Java run-time environment does not need to know which parameterized type is used because the type information is validated at compile-time and is not included in the compiled code. Consequently, instantiating a Java class of a parameterized type is impossible because instantiation requires a call to a constructor, which is unavailable if the type is unknown.

For example, the following code will not compile:

T instantiateElementType(List<T> arg) {
  return new T(); //causes a compile error
}

Because there is only one copy per generic class at runtime, static variables are shared among all the instances of the class, regardless of their type parameter. As a result, the type parameter cannot be used in the declaration of static variables or in static methods. Static variables and static methods are "outside" of the scope of the class's parameterized types.

See also

• Generic programming
• Wildcard (Java)
• Java
• Comparison of C# and Java
• Comparison of Java and C++

External links

• Lesson: Generics at The Java Tutorials
• Generics by Sun Microsystems
• Wild FJ by Mads Torgersen, Erik Ernst, and Christian Plesner Hansen
• Java Generics FAQ by Angelika Langer
• JavaWorld Taming the Tiger Excellent overview.
• Java Generics Tutorial by Jakob Jenkov – jenkov.com
• Type Wildcard in Java Generics - A Tutorial by Ted Gao