ValuableAmountAdapter.java: added factory method wrap to better handle null

    made constructor private
    updated ConfigurableStorage

src/main/java/de/zmt/storage/ConfigurableStorage.java

@@ -204,15 +204,15 @@ public class ConfigurableStorage<Q extends Quantity> extends BaseStorage<Q> impl
 
     public class MyPropertiesProxy {
 	public Valuable getAmount() {
-	    return new ValuableAmountAdapter(ConfigurableStorage.this.getAmount());
+	    return ValuableAmountAdapter.wrap(ConfigurableStorage.this.getAmount());
 	}
 
 	public Valuable getLowerLimit() {
-	    return new ValuableAmountAdapter(ConfigurableStorage.this.getLowerLimit());
+	    return ValuableAmountAdapter.wrap(ConfigurableStorage.this.getLowerLimit());
 	}
 
 	public Valuable getUpperLimit() {
-	    return new ValuableAmountAdapter(ConfigurableStorage.this.getUpperLimit());
+	    return ValuableAmountAdapter.wrap(ConfigurableStorage.this.getUpperLimit());
 	}
 
 	public double getFactorIn() {

src/main/java/de/zmt/util/ValuableAmountAdapter.java

@@ -13,7 +13,7 @@ import sim.util.Valuable;
 public class ValuableAmountAdapter implements Valuable {
     private final Amount<?> amount;
 
-    public ValuableAmountAdapter(Amount<?> amount) {
+    private ValuableAmountAdapter(Amount<?> amount) {
 	super();
 	this.amount = amount;
     }
@@ -27,4 +27,18 @@ public class ValuableAmountAdapter implements Valuable {
     public String toString() {
 	return amount.toString();
     }
+
+    /**
+     * Returns {@code amount} wrapped into a {@link ValuableAmountAdapter} or
+     * <code>null</code> if {@code amount} is null.
+     * 
+     * @param amount
+     * @return wrapped {@code amount}
+     */
+    public static ValuableAmountAdapter wrap(Amount<?> amount) {
+	if (amount != null) {
+	    return new ValuableAmountAdapter(amount);
+	}
+	return null;
+    }
 }