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Generic examples
//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));
}
}
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