Generics in Java


Generics are a facility of generic programming that were added to the Java programming language in 2004 within version J2SE 5.0. They were designed to extend Java's type system to allow "a type or method to operate on objects of various types while providing compile-time type safety". The aspect compile-time type safety was not fully achieved, since it was shown in 2016 that it is not guaranteed in all cases.
The Java collections framework supports generics to specify the type of objects stored in a collection instance.
In 1998, Gilad Bracha, Martin Odersky, David Stoutamire and Philip Wadler created Generic Java, an extension to the Java language to support generic types. Generic Java was incorporated in Java with the addition of wildcards.

Hierarchy and classification

According to Java Language Specification:
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—an error in logic, as it is not generally possible to cast an arbitrary string to an integer.

List v = new ArrayList;
v.add; // A String that cannot be cast to an Integer
Integer i = v.get; // Run time error

Although the code is compiled without error, it throws a runtime exception when executing the third line of code. This type of logic error can be detected during compile time by using generics and is the primary motivation for using them.
The above code fragment can be rewritten using generics as follows:

List v = new ArrayList;
v.add;
Integer i = v.get; // compilation-time error

The type parameter String within the angle brackets declares the ArrayList to be constituted of String. With generics, it is no longer necessary to cast the third line to any particular type, because the result of v.get is defined as String by the code generated by the compiler.
The logical flaw in the third line of this fragment will be detected as a compile-time error because the compiler will detect that v.get returns String instead of Integer. For a more elaborate example, see reference.
Here is a small excerpt from the definition of the interfaces List and Iterator in package :

public interface List
public interface Iterator

Type wildcards

A type argument for a parameterized type is not limited to a concrete class or interface. Java allows the use of type wildcards to serve as type arguments for parameterized types. Wildcards are type arguments in the form ""; optionally with an upper or lower bound. Given that the exact type represented by a wildcard is unknown, restrictions are placed on the type of methods that may be called on an object that uses parameterized types.
Here is an example where the element type of a Collection is parameterized by a wildcard:

Collection c = new ArrayList;
c.add); // compile-time error
c.add; // allowed
Since we don’t know what the element type of c stands for, we cannot add objects to it. The add method takes arguments of type E, the element type of the Collection generic interface. When the actual type argument is ?, it stands for some unknown type. Any method argument value we pass to the add method would have to be a subtype of this unknown type. Since we don't know what type that is, we cannot pass anything in. The sole exception is null; which is a member of every type.
To specify the upper bound of a type wildcard, the extends keyword is used to indicate that the type argument is a subtype 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. Adding null elements is, again, also allowed.
The use of wildcards above adds flexibility since there is not any inheritance relationship between any two parameterized types with concrete type as type argument. Neither List nor List is a subtype of the other; even though Integer is a subtype of Number. So, any method that takes List as a parameter does not accept an argument of List. If it did, it would be possible to insert a Number that is not an Integer into it; which violates type safety. Here is an example that demonstrates how type safety would be violated if List were a subtype of List:

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

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

List nums = ints; // OK
nums.add; // compile-time error
nums.add; // allowed

To specify the lower bounding class of a type wildcard, the super keyword is used. This keyword indicates that the type argument is a supertype of the 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. Adding to such a list requires either elements of type Number, any subtype of Number or null.
The mnemonic PECS from the book Effective Java by Joshua Bloch gives an easy way to remember when to use wildcards in Java.

Generic class definitions

Here is an example of a generic Java class, which can be used to represent individual entries in a map:

public class Entry

This generic class could be used in the following ways, for example:

Entry grade = new Entry;
Entry mark = new Entry;
System.out.println;
System.out.println;
Entry prime = new Entry;
if ) System.out.println;
else System.out.println;

It outputs:

grade: 
mark: 
13 is prime.

Diamond operator

Thanks to type inference, Java SE 7 and above allow the programmer to substitute an empty pair of angle brackets for a pair of angle brackets containing the one or more type parameters that a sufficiently-close context implies. Thus, the above code example using Entry can be rewritten as:

Entry grade = new Entry<>;
Entry mark = new Entry<>;
System.out.println;
System.out.println;
Entry prime = new Entry<>;
if ) System.out.println;
else System.out.println;

Generic method definitions

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

public static Entry twice

Note: If we remove the first in the above method, we will get compilation error since it represents the declaration of the symbol.
In many cases the user of the method need not indicate the type parameters, as they can be inferred:

Entry pair = Entry.twice;

The parameters can be explicitly added if needed:

Entry pair = Entry.twice;

The use of primitive types is not allowed, and boxed versions must be used instead:

Entry pair; // Fails compilation. Use Integer instead.

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

public Type toArray

In such cases you can't use primitive types either, e.g.:

Integer array = toArray;

Generics in throws clause

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

public void throwMeConditional throws T

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.
Because 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 may have been parameterized with Integer.
Demonstrating this point, the following code outputs "Equal":

ArrayList li = new ArrayList;
ArrayList lf = new ArrayList;
if lf.getClass)

Another effect of type erasure is that a generic class cannot extend the Throwable class in any way, directly or indirectly:

public class GenericException extends Exception

The reason why this is not supported is due to type erasure:

try
catch
catch

Due to type erasure, the runtime will not know which catch block to execute, so this is prohibited by the compiler.
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 cannot be compiled:

T instantiateElementType

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. Consequently, the type parameter cannot be used in the declaration of static variables or in static methods.

Project on generics

is an experimental project to incubate improved Java generics and language features, for future versions potentially from Java 10 onwards. Potential enhancements include: