#11 Added supporting class and exception

src/main/java/de/rwu/easydrop/exception/InvalidCatalogueException.java

@@ -0,0 +1,27 @@
+package de.rwu.easydrop.exception;
+
+/**
+ * Exception that signifies the data of a Product are invalid.
+ *
+ * @since 0.3.0
+ */
+public class InvalidCatalogueException extends RuntimeException {
+    /**
+     * Throws an exception that signifies the data of a Product are invalid.
+     *
+     * @param message
+     */
+    public InvalidCatalogueException(final String message) {
+        super(message);
+    }
+
+    /**
+     * Throws an exception that signifies the data of a Product are invalid.
+     *
+     * @param message
+     * @param cause
+     */
+    public InvalidCatalogueException(final String message, final Throwable cause) {
+        super(message, cause);
+    }
+}

src/main/java/de/rwu/easydrop/model/ProductPair.java

@@ -0,0 +1,31 @@
+package de.rwu.easydrop.model;
+
+import lombok.Data;
+
+/**
+ * Associates two related Products to one another.
+ *
+ * @since 0.3.0
+ */
+@Data
+public class ProductPair {
+    /**
+     * The first product.
+     */
+    private final Product product1;
+    /**
+     * The second product.
+     */
+    private final Product product2;
+
+    /**
+     * Constructs a Product Pair.
+     *
+     * @param a First product
+     * @param b Second product
+     */
+    public ProductPair(final Product a, final Product b) {
+        this.product1 = a;
+        this.product2 = b;
+    }
+}