A Guide to Generic Type Arguments in Java

Posted By Amit Kumar | 30-Nov-2018

Generics capabilities were added in Java way back in version J2SE 1.5, but in Java 8, suddenly the Javadocs are filled with method signatures like the following :


static <K extends Comparable<? super K>,V> Comparator<Map.Entry<K,V>>

static <T,U extends Comparable<? super U>> Comparator<T> comparing(
    Function<? super T,? extends U> keyExtractor)

static <T,K,D,A,M extends Map<K, D>> Collector<T,?,M> groupingBy(
    Function<? super T,? extends K> classifier, Supplier<M> mapFactory,
    Collector<? super T,A,D> downstream)

In this blog, we look to understand the above parameters being used

One of the simplest usages of Generic Type parameters we know is

List<String> strings = new ArrayList<String>();
//and since Java 7 we can also use 
List<String> strings = new ArrayList<>();

Declaring the data type of the collection accomplishes two goals:
We can’t place the wrong type inside a collection and an explicit cast is not required to retrieve the values. Example - 1


List<String> strings = new ArrayList<>();
// strings.add(new Date());    
// Integer i = strings.get(0); 

for (String s : strings) {     
    System.out.printf("%s has length %d%n", s, s.length());

//Let us look through the List interface (public interface List<E> extends Collection<E>) with E as the type parameter

boolean	add(E e)                           
boolean	addAll(Collection<? extends E> c)  
void	clear()                            
boolean	contains(Object o)                 
boolean	containsAll(Collection<?> c)       
E	get(int index) 


Some of these methods use the declared generic type, E, as either an argument or a return type. Some (specifically, clear and contains) don’t use the type at all. Others involve some kind of a wildcard, using a question mark.

One of the most common mistakes a person can make in using the above functions is - getting confused between the relationship in List<tring> and List<Object>. Consider the add method in the List<E> interface - 


//declare & initialise , List<Object>
List<Object> objects = new ArrayList<Object>();
objects.add("Hello World");
objects.add(new Date());

List<String> strings = new ArrayList<>();
String s = "Hello World";
Object o = s;                          
strings.add(o)      //does not compile


The problem is that List<String> is not a subclass of List<Object>. The only instances we can add to it are of the type String and hence we say that the parameterized type is invariant.
If we could assign a List<String> to a List<Object> we could add something that wasn’t string to the list, causing a cast exception when you try to retrieve it using the List<String> reference.

For similar situations do not arise we can use wildcards instead of invariant parameter types

A wildcard is a type of argument that uses a question mark, ? with an optional upper or lower bound.

Upper Bounded Wildcards
An upper bounded wildcard defines the super class limit. For example define a list of numbers that can belongs, doubles, and even BigDecimal instances -


private static double sumList(List<? extends Number> list) {
    return list.stream()

public static void main(String[] args) {
    List<Integer> ints = Arrays.asList(1, 2, 3, 4, 5);
    List<Double> doubles = Arrays.asList(1.0, 2.0, 3.0, 4.0, 5.0);
    List<BigDecimal> bigDecimals = Arrays.asList(
        new BigDecimal("1.0"),
        new BigDecimal("2.0"),
        new BigDecimal("3.0"),
        new BigDecimal("4.0"),
        new BigDecimal("5.0")

    System.out.printf("ints sum is         %s%n", sumList(ints));
    System.out.printf("doubles sum is      %s%n", sumList(doubles));
    System.out.printf("big decimals sum is %s%n", sumList(bigDecimals));


Lower Bounded Wildcards
A lower bounded wildcard means any parent of the class is acceptable. We use super keyword to specify a lower bound. The implication, in the case of a List<? super Number>

Consider a method called numsUpTo that takes two arguments, an integer, and a list to populate with all the numbers up to the first argument, as in Example.


public void numsUpTo(Integer num, List<? super Integer> output) {
    IntStream.rangeClosed(1, num)

ArrayList<Integer> integerList = new ArrayList<>();
ArrayList<Number>  numberList = new ArrayList<>();
ArrayList<Object>  objectList = new ArrayList<>();
numsUpTo(5, integerList);
numsUpTo(5, numberList);
numsUpTo(5, objectList);


Multiple Bounds
One final note before looking at examples from the Java 8 API. A type parameter can have multiple bounds. The bounds are separated by an ampersand when they are defined:

T extends Runnable & AutoCloseable
You can have as many interface bounds as you like, but only one can be a class. If you have a class as a bound, it must be first in the list.

Examples from the Java 8 API
With all that in mind, now it’s time to review some examples from the Java 8 docs.

In the java.util.stream.Stream interface, consider the max method:

Optional<T> max(Comparator<? super T> comparator)
Note the use of the lower bounded wildcard in the Comparator. The max method returns the maximum element of a stream by applying the supplied Comparator to it. The return type is Optional<T>, because there may not be a return value if the stream is empty. The method wraps the maximum in an Optional if there is one, and returns an empty Optional if not.

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