Fix minor sonar issues (#478)

Author: acoburn

api/src/main/java/com/inrupt/client/spi/NoopCache.java

@@ -33,23 +33,26 @@
  */
 class NoopCache<T, U> implements ClientCache<T, U> {
 
+    private static final String KEY_NOT_NULL = "cache key may not be null!";
+    private static final String VALUE_NOT_NULL = "cache value may not be null!";
+
     @Override
     public U get(final T key) {
-        Objects.requireNonNull(key, "cache key may not be null!");
+        Objects.requireNonNull(key, KEY_NOT_NULL);
         return null;
     }
 
     @Override
     public void put(final T key, final U value) {
         /* no-op */
-        Objects.requireNonNull(key, "cache key may not be null!");
-        Objects.requireNonNull(value, "cache value may not be null!");
+        Objects.requireNonNull(key, KEY_NOT_NULL);
+        Objects.requireNonNull(value, VALUE_NOT_NULL);
     }
 
     @Override
     public void invalidate(final T key) {
         /* no-op */
-        Objects.requireNonNull(key, "cache key may not be null!");
+        Objects.requireNonNull(key, KEY_NOT_NULL);
     }
 
     @Override
@@ -60,7 +63,7 @@ public void invalidateAll() {
     public static class NoopCacheBuilder implements CacheBuilderService {
         @Override
         public <T, U> ClientCache<T, U> build(final int size, final Duration duration) {
-            return new NoopCache<T, U>();
+            return new NoopCache<>();
         }
     }
 }

caffeine/src/main/java/com/inrupt/client/caffeine/CaffeineCacheBuilder.java

@@ -49,7 +49,7 @@ public <T, U> ClientCache<T, U> build(final int maximumSize, final Duration dura
      * @return a cache suitable for use in the Inrupt Client libraries
      */
     public static <T, U> ClientCache<T, U> ofCache(final Cache<T, U> cache) {
-        return new CaffeineCache<T, U>(cache);
+        return new CaffeineCache<>(cache);
     }
 }
 

guava/src/main/java/com/inrupt/client/guava/GuavaCache.java

@@ -33,6 +33,9 @@
  */
 public class GuavaCache<T, U> implements ClientCache<T, U> {
 
+    private static final String KEY_NOT_NULL = "cache key may not be null!";
+    private static final String VALUE_NOT_NULL = "cache value may not be null!";
+
     private final Cache<T, U> cache;
 
     /**
@@ -41,25 +44,25 @@ public class GuavaCache<T, U> implements ClientCache<T, U> {
      * @param cache the guava cache
      */
     public GuavaCache(final Cache<T, U> cache) {
-        this.cache = Objects.requireNonNull(cache, "cache may not be null!");
+        this.cache = Objects.requireNonNull(cache, KEY_NOT_NULL);
     }
 
     @Override
     public U get(final T key) {
-        Objects.requireNonNull(key, "cache key may not be null!");
+        Objects.requireNonNull(key, KEY_NOT_NULL);
         return cache.getIfPresent(key);
     }
 
     @Override
     public void put(final T key, final U value) {
-        Objects.requireNonNull(key, "cache key may not be null!");
-        Objects.requireNonNull(value, "cache value may not be null!");
+        Objects.requireNonNull(key, KEY_NOT_NULL);
+        Objects.requireNonNull(value, VALUE_NOT_NULL);
         cache.put(key, value);
     }
 
     @Override
     public void invalidate(final T key) {
-        Objects.requireNonNull(key, "cache key may not be null!");
+        Objects.requireNonNull(key, KEY_NOT_NULL);
         cache.invalidate(key);
     }
 

guava/src/main/java/com/inrupt/client/guava/GuavaCacheBuilder.java

@@ -50,7 +50,7 @@ public <T, U> ClientCache<T, U> build(final int maximumSize, final Duration dura
      * @return a cache suitable for use in the Inrupt Client libraries
      */
     public static <T, U> ClientCache<T, U> ofCache(final Cache<T, U> cache) {
-        return new GuavaCache<T, U>(cache);
+        return new GuavaCache<>(cache);
     }
 }