feat: basic Constant value provider

Author: nergal-perm

plan.md

@@ -18,8 +18,8 @@ in the commit message.
 
 ## Fetching data from a constant value provider
 
-- Creating an empty `ConstantValueProvider` should not be possible.
-- Test creating a `ConstantValueProvider` with a single String value and getting that value back.
+- [x] Creating an empty `ConstantValueProvider` should not be possible.
+- [x] Test creating a `ConstantValueProvider` with a single String value and getting that value back.
 
 ## Fetching data from a complex object
 

src/main/java/ru/ewc/decita/core/value/ConstantValueProvider.java

@@ -24,6 +24,7 @@
 
 package ru.ewc.decita.core.value;
 
+import java.util.Objects;
 import ru.ewc.decisions.api.ComputationContext;
 
 /**
@@ -44,7 +45,7 @@ public final class ConstantValueProvider<T> implements ValueProvider<T> {
      * @param value The constant value to be provided.
      */
     public ConstantValueProvider(final T value) {
-        this.value = value;
+        this.value = Objects.requireNonNull(value, "Constant value cannot be null");
     }
 
     @Override

src/test/java/ru/ewc/decita/core/value/ConstantValueProviderTest.java

@@ -33,12 +33,15 @@
  * @since 0.10.0
  */
 final class ConstantValueProviderTest {
-    /**
-     * A simple test to check that the provider returns the value it was created with.
-     */
     @Test
     void returnsTheValueItWasCreatedWith() {
         final var provider = new ConstantValueProvider<>("Hello, World!");
         Assertions.assertThat(provider.valueFrom(null)).isEqualTo("Hello, World!");
     }
+
+    @Test
+    void doesNotAllowEmptyValue() {
+        Assertions.assertThatThrownBy(() -> new ConstantValueProvider<>(null))
+            .isInstanceOf(NullPointerException.class);
+    }
 }