Create Env utility class (#48)

* Add Env utility class

Fixes #47

src/main/java/info/freelibrary/util/Env.java

@@ -0,0 +1,41 @@
+
+package info.freelibrary.util;
+
+import java.util.Objects;
+
+/**
+ * An extension to the Java environment properties interface.
+ */
+public final class Env {
+
+    /**
+     * Creates a new environment object.
+     */
+    private Env() {
+        // This is intentionally left empty.
+    }
+
+    /**
+     * Gets an environmental property value, optionally falling back to the supplied alternative.
+     *
+     * @param aPropertyName An environmental property name
+     * @param aFallbackValue A fallback value used if the required property isn't set
+     * @return The environmental property value or its supplied fallback value
+     */
+    public static String get(final String aPropertyName, final String aFallbackValue) {
+        final String envProperty = StringUtils.trimToNull(System.getenv(aPropertyName));
+        return envProperty == null ? aFallbackValue : envProperty;
+    }
+
+    /**
+     * Gets an environmental property value or throws a NullPointerException if that property isn't found.
+     *
+     * @param aPropertyName An environmental property name
+     * @param aMessage An exception message used if property is not found
+     * @return The environmental property value
+     * @throws NullPointerException If the requested property name isn't found in the environment
+     */
+    public static String getOrFail(final String aPropertyName, final String aMessage) {
+        return Objects.requireNonNull(StringUtils.trimToNull(System.getenv(aPropertyName)), aMessage);
+    }
+}

src/main/java/info/freelibrary/util/warnings/PMD.java

@@ -18,6 +18,11 @@ public final class PMD {
     @SuppressWarnings("PMD.LongVariable")
     public static final String ABSTRACT_CLASS_WITHOUT_ABSTRACT_METHOD = "PMD.AbstractClassWithoutAbstractMethod";
 
+    /**
+     * Cf. https://pmd.github.io/latest/pmd_rules_java_design.html#avoidthrowingnullpointerexception
+     */
+    public static final String AVOID_THROWING_NULLPOINTEREXCEPTION = "AvoidThrowingNullPointerException";
+
     /**
      * Cf. https://pmd.github.io/latest/pmd_rules_java_design.html#avoiddeeplynestedifstmts
      */