Add java interface single implementation examples (#13972)

core-java-modules/core-java-lang-oop-patterns/src/main/java/com/baeldung/interfacesingleimpl/Animal.java

@@ -0,0 +1,5 @@
+package com.baeldung.interfacesingleimpl;
+
+public interface Animal {
+    String makeSound();
+}

core-java-modules/core-java-lang-oop-patterns/src/main/java/com/baeldung/interfacesingleimpl/AnimalCare.java

@@ -0,0 +1,13 @@
+package com.baeldung.interfacesingleimpl;
+
+public class AnimalCare {
+    private Animal animal;
+
+    public AnimalCare(Animal animal) {
+        this.animal = animal;
+    }
+
+    public String animalSound() {
+        return animal.makeSound();
+    }
+}

core-java-modules/core-java-lang-oop-patterns/src/main/java/com/baeldung/interfacesingleimpl/Cat.java

@@ -0,0 +1,21 @@
+package com.baeldung.interfacesingleimpl;
+
+public class Cat {
+    private String name;
+
+    public Cat(String name) {
+        this.name = name;
+    }
+
+    public String makeSound() {
+        return "Meow! My name is " + name;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+}

core-java-modules/core-java-lang-oop-patterns/src/main/java/com/baeldung/interfacesingleimpl/Dog.java

@@ -0,0 +1,22 @@
+package com.baeldung.interfacesingleimpl;
+
+public class Dog implements Animal {
+    private String name;
+
+    public Dog(String name) {
+        this.name = name;
+    }
+
+    @Override
+    public String makeSound() {
+        return "Woof! My name is " + name;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+}

core-java-modules/core-java-lang-oop-patterns/src/test/java/com/baeldung/interfacesingleimpl/InterfaceSingleImplUnitTest.java

@@ -0,0 +1,51 @@
+package com.baeldung.interfacesingleimpl;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class InterfaceSingleImplUnitTest {
+    @Test
+    public void whenUsingMockAnimal_thenAnimalSoundIsCorrect() {
+        MockAnimal mockAnimal = new MockAnimal();
+        String expected = "Mock animal sound!";
+        AnimalCare animalCare = new AnimalCare(mockAnimal);
+        assertThat(animalCare.animalSound()).isEqualTo(expected);
+    }
+
+    @Test
+    public void whenCreatingDog_thenDogMakesWoofSound() {
+        Dog dog = new Dog("Buddy");
+        String expected = "Woof! My name is Buddy";
+        assertThat(dog.makeSound()).isEqualTo(expected);
+    }
+
+    @Test
+    public void whenCreatingCat_thenCatMakesMeowSound() {
+        Cat cat = new Cat("FuzzBall");
+        String expected = "Meow! My name is FuzzBall";
+        assertThat(cat.makeSound()).isEqualTo(expected);
+    }
+
+    @Test
+    public void whenCreatingAnimalCareWithDog_thenDogMakesWoofSound() {
+        Animal dog = new Dog("Ham");
+        AnimalCare animalCare = new AnimalCare(dog);
+        String expected = "Woof! My name is Ham";
+        assertThat(animalCare.animalSound()).isEqualTo(expected);
+    }
+
+    @Test
+    public void whenCreatingCatCareWithCat_thenCatMakesMeowSound() {
+        Cat cat = new Cat("Grumpy");
+        String expected = "Meow! My name is Grumpy";
+        assertThat(cat.makeSound()).isEqualTo(expected);
+    }
+
+    @Test
+    public void whenCreatingMockAnimal_thenMockAnimalMakesMockAnimalSound() {
+        MockAnimal mockAnimal = new MockAnimal();
+        String expected = "Mock animal sound!";
+        assertThat(mockAnimal.makeSound()).isEqualTo(expected);
+    }
+}

core-java-modules/core-java-lang-oop-patterns/src/test/java/com/baeldung/interfacesingleimpl/MockAnimal.java

@@ -0,0 +1,8 @@
+package com.baeldung.interfacesingleimpl;
+
+public class MockAnimal implements Animal {
+    @Override
+    public String makeSound() {
+        return "Mock animal sound!";
+    }
+}