Generic examples

Generics.java

//src: https://github.com/eugenp/tutorials/blob/887eeaf802e0b8728ec9367ad23495303f9a1f38/core-java-modules/core-java-lang-syntax/src/main/java/com/baeldung/generics/Generics.java
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Generics {

    // definition of a generic method
    public static <T> List<T> fromArrayToList(T[] a) {
        return Arrays.stream(a).collect(Collectors.toList());
    }

    // definition of a generic method
    public static <T, G> List<G> fromArrayToList(T[] a, Function<T, G> mapperFunction) {
        return Arrays.stream(a).map(mapperFunction).collect(Collectors.toList());
    }

    // example of a generic method that has Number as an upper bound for T
    public static <T extends Number> List<T> fromArrayToListWithUpperBound(T[] a) {
        return Arrays.stream(a).collect(Collectors.toList());
    }

    // example of a generic method with a wild card, this method can be used
    // with a list of any subtype of Building
    public static void paintAllBuildings(List<? extends Building> buildings) {
        buildings.forEach(Building::paint);
    }

    public static List<Integer> createList(int a) {
        List<Integer> list = new ArrayList<>();
        list.add(a);
        return list;
    }

    public <T> List<T> genericMethod(List<T> list) {
        return list.stream().collect(Collectors.toList());
    }
    
    public List<Object> withErasure(List<Object> list) {
        return list.stream().collect(Collectors.toList());
    }
}

// src: https://github.com/eugenp/tutorials/blob/887eeaf802e0b8728ec9367ad23495303f9a1f38/core-java-modules/core-java-lang-syntax/src/test/java/com/baeldung/generics/GenericsUnitTest.java
import org.junit.Test;

import java.util.ArrayList;
import java.util.List;

import static org.hamcrest.CoreMatchers.hasItems;
import static org.hamcrest.CoreMatchers.is;
import static org.hamcrest.MatcherAssert.assertThat;
import static org.junit.Assert.fail;

public class GenericsUnitTest {

    // testing the generic method with Integer
    @Test
    public void givenArrayOfIntegers_thanListOfIntegersReturnedOK() {
        Integer[] intArray = { 1, 2, 3, 4, 5 };
        List<Integer> list = Generics.fromArrayToList(intArray);

        assertThat(list, hasItems(intArray));
    }

    // testing the generic method with Integer and String type
    @Test
    public void givenArrayOfIntegers_thanListOfStringReturnedOK() {
        Integer[] intArray = { 1, 2, 3, 4, 5 };
        List<String> stringList = Generics.fromArrayToList(intArray, Object::toString);
        assertThat(stringList, hasItems("1", "2", "3", "4", "5"));
    }

    // testing the generic method with String
    @Test
    public void givenArrayOfStrings_thanListOfStringsReturnedOK() {
        String[] stringArray = { "hello1", "hello2", "hello3", "hello4", "hello5" };
        List<String> list = Generics.fromArrayToList(stringArray);

        assertThat(list, hasItems(stringArray));
    }

    // testing the generic method with Number as upper bound with Integer
    // if we test fromArrayToListWithUpperBound with any type that doesn't
    // extend Number it will fail to compile
    @Test
    public void givenArrayOfIntegersAndNumberUpperBound_thanListOfIntegersReturnedOK() {
        Integer[] intArray = { 1, 2, 3, 4, 5 };
        List<Integer> list = Generics.fromArrayToListWithUpperBound(intArray);

        assertThat(list, hasItems(intArray));
    }

    // testing paintAllBuildings method with a subtype of Building, the method
    // will work with all subtypes of Building
    @Test
    public void givenSubTypeOfWildCardBoundedGenericType_thanPaintingOK() {
        try {
            List<Building> subBuildingsList = new ArrayList<>();
            subBuildingsList.add(new Building());
            subBuildingsList.add(new House());

            // prints
            // Painting Building
            // Painting House
            Generics.paintAllBuildings(subBuildingsList);
        } catch (Exception e) {
            fail();
        }
    }

    @Test
    public void givenAnInt_whenAddedToAGenericIntegerList_thenAListItemCanBeAssignedToAnInt() {
        int number = 7;
        List<Integer> list = Generics.createList(number);
        int otherNumber = list.get(0);
        assertThat(otherNumber, is(number));
    }

}