Add Provider interface and implementation, cover with unit test

src/main/java/io/beanmapper/providers/DefaultValuesProvider.java

@@ -0,0 +1,33 @@
+package io.beanmapper.providers;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static java.util.Map.entry;
+import static java.util.Objects.isNull;
+
+public class DefaultValuesProvider implements Provider {
+    private static final Map<Class<?>, Object> DEFAULT_VALUES = Map.ofEntries(
+            entry(boolean.class, false),
+            entry(byte.class, (byte) 0),
+            entry(short.class, (short) 0),
+            entry(int.class, 0),
+            entry(long.class, 0L),
+            entry(char.class, '\0'),
+            entry(float.class, 0.0F),
+            entry(double.class, 0.0),
+            entry(Optional.class, Optional.empty()),
+            entry(List.class, Collections.emptyList()),
+            entry(Set.class, Collections.emptySet()),
+            entry(Map.class, Collections.emptyMap())
+    );
+
+    @Override
+    public <T extends S, S> T get(Class<S> source) {
+        if (isNull(source)) return null;
+        return (T) DEFAULT_VALUES.get(source);
+    }
+}

src/main/java/io/beanmapper/providers/Provider.java

@@ -0,0 +1,12 @@
+package io.beanmapper.providers;
+
+public interface Provider {
+
+    /**
+     * Get the default value given some S type.
+     *
+     * @param source {@link Class} to get the default value of.
+     * @return the default value given the type.
+     */
+    <T extends S, S> T get(Class<S> source);
+}

src/test/java/io/beanmapper/providers/DefaultValuesProviderTest.java

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+package io.beanmapper.providers;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static java.util.Collections.emptyList;
+import static java.util.Collections.emptyMap;
+import static java.util.Collections.emptySet;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+class DefaultValuesProviderTest {
+    private final DefaultValuesProvider defaultValuesProvider = new DefaultValuesProvider();
+
+    @Test
+    void shouldGetNull() {
+        Object actualValue = defaultValuesProvider.get(null);
+        UndefinedClassInProvider actualValue2 = defaultValuesProvider.get(UndefinedClassInProvider.class);
+
+        assertNull(actualValue);
+        assertNull(actualValue2);
+    }
+
+    @Test
+    void shouldGetDefault() {
+        assertEquals(false, defaultValuesProvider.get(boolean.class));
+        byte actualDefaultByte = defaultValuesProvider.get(byte.class);
+        assertEquals((byte) 0, actualDefaultByte);
+        short actualDefaultShort = defaultValuesProvider.get(short.class);
+        assertEquals((short) 0, actualDefaultShort);
+        int actualDefaultInt = defaultValuesProvider.get(int.class);
+        assertEquals(0, actualDefaultInt);
+        long actualDefaultLong = defaultValuesProvider.get(long.class);
+        assertEquals(0L, actualDefaultLong);
+        char actualDefaultChar = defaultValuesProvider.get(char.class);
+        assertEquals('\0', actualDefaultChar);
+        float actualDefaultFloat = defaultValuesProvider.get(float.class);
+        assertEquals(0.0F, actualDefaultFloat);
+        double actualDefaultDouble = defaultValuesProvider.get(double.class);
+        assertEquals(0.0, actualDefaultDouble);
+        assertEquals(Optional.empty(), defaultValuesProvider.get(Optional.class));
+        assertEquals(emptyList(), defaultValuesProvider.get(List.class));
+        assertEquals(emptySet(), defaultValuesProvider.get(Set.class));
+        assertEquals(emptyMap(), defaultValuesProvider.get(Map.class));
+    }
+
+    private static final class UndefinedClassInProvider {
+    }
+}