feat: add lab7

Lab7/cs2030s/fp/Combiner.java

@@ -0,0 +1,23 @@
+package cs2030s.fp;
+
+/**
+ * Represent a function that combines two values into one.  The two inputs
+ * and the result can be of different types.
+ * CS2030S Lab 5
+ * AY20/21 Semester 2
+ *
+ * @param <R> The type of the return value
+ * @param <S> The type of the first input value
+ * @param <T> The type of the second input value
+ */
+@FunctionalInterface
+public interface Combiner<R, S, T> {
+  /**
+   * The functional method to combines two values into one.
+   *
+   * @param s The first input value
+   * @param t The second input value
+   * @return The value after combining s and t.
+   */
+  R combine(S s, T t);
+}