functionaljava · gliptak · Apr 23, 2019 · Apr 6, 2019
diff --git a/core/src/main/java/fj/function/Longs.java b/core/src/main/java/fj/function/Longs.java
@@ -1,11 +1,16 @@
 package fj.function;
 
 import fj.F;
+import fj.Monoid;
+import fj.data.List;
+import fj.data.Option;
 
 import static fj.Function.curry;
 import static fj.Semigroup.longAdditionSemigroup;
 import static fj.Semigroup.longMultiplicationSemigroup;
 
+import static fj.data.Option.none;
+import static fj.data.Option.some;
 import static java.lang.Math.abs;
 
 /**
@@ -48,4 +53,38 @@ private Longs() {
    */
   public static final F<Long, F<Long, Long>> remainder = curry((a, b) -> a % b);
 
+  /**
+   * Sums a list of longs.
+   *
+   * @param longs A list of longs to sum.
+   * @return The sum of the longs in the list.
+   */
+  public static long sum(final List<Long> longs) {
+    return Monoid.longAdditionMonoid.sumLeft(longs);
+  }
+
+  /**
+   * Returns the product of a list of integers.
+   *
+   * @param longs A list of longs to multiply together.
+   * @return The product of the longs in the list.
+   */
+  public static long product(final List<Long> longs) {
+    return Monoid.longMultiplicationMonoid.sumLeft(longs);
+  }
+
+  /**
+   * A function that converts strings to integers.
+   *
+   * @return A function that converts strings to integers.
+   */
+  public static F<String, Option<Long>> fromString() {
+    return s -> {
+      try { return some(Long.valueOf(s)); }
+      catch (final NumberFormatException ignored) {
+        return none();
+      }
+    };
+  }
+
 }
diff --git a/core/src/test/java/fj/function/LongsTest.java b/core/src/test/java/fj/function/LongsTest.java
@@ -0,0 +1,62 @@
+package fj.function;
+
+import org.junit.Test;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+
+import static fj.data.List.list;
+import static fj.data.Option.none;
+import static org.hamcrest.core.Is.is;
+import static org.junit.Assert.*;
+
+public class LongsTest {
+
+    @Test
+    public void testSum() {
+        assertThat(Longs.sum(list(3L, 4L, 5L)), is(12L));
+    }
+
+    @Test
+    public void testProduct() {
+        assertThat(Longs.product(list(3L, 4L, 5L)), is(60L));
+    }
+
+    @Test
+    public void testAdd() {
+        assertThat(Longs.add.f(10L).f(20L), is(30L));
+    }
+
+    @Test
+    public void testMultiply() {
+        assertThat(Longs.multiply.f(3L).f(5L), is(15L));
+    }
+
+    @Test
+    public void testAbs() {
+        assertThat(Longs.abs.f(-5L), is(5L));
+    }
+
+    @Test
+    public void testFromString() {
+        assertThat(Longs.fromString().f("-123").some(), is(-123L));
+    }
+
+    @Test
+    public void testFromStringFail() {
+        assertThat(Longs.fromString().f("w"), is(none()));
+    }
+
+    @Test
+    public void testCannotInstantiate() throws NoSuchMethodException, IllegalAccessException, InstantiationException {
+        Constructor<Longs> constructor = Longs.class.getDeclaredConstructor();
+        constructor.setAccessible(true);
+        try {
+            constructor.newInstance();
+            fail("expected InvocationTargetException");
+        } catch (InvocationTargetException ite) {
+            assertTrue(ite.getCause() instanceof UnsupportedOperationException);
+        }
+    }
+
+}