Common: Add an API Functions.returnToString(). (#1443)

* Common: Add an API Functions.returnToString().

* Fix comments and add tests.

2 files changed, 26 insertions, 0 deletions

diff --git a/api/src/main/java/io/opencensus/common/Functions.java b/api/src/main/java/io/opencensus/common/Functions.java
index eb5984af..ea3457ca 100644
--- a/api/src/main/java/io/opencensus/common/Functions.java
+++ b/api/src/main/java/io/opencensus/common/Functions.java
@@ -53,6 +53,14 @@ public final class Functions {
         }
       };
 
+  private static final Function<Object, /*@Nullable*/ String> RETURN_TO_STRING =
+      new Function<Object, /*@Nullable*/ String>() {
+        @Override
+        public /*@Nullable*/ String apply(Object input) {
+          return input == null ? null : input.toString();
+        }
+      };
+
   /**
    * A {@code Function} that always ignores its argument and returns {@code null}.
    *
@@ -82,6 +90,16 @@ public final class Functions {
   }
 
   /**
+   * A {@code Function} that always returns the {@link #toString()} value of the input.
+   *
+   * @return a {@code Function} that always returns the {@link #toString()} value of the input.
+   * @since 0.17
+   */
+  public static Function<Object, /*@Nullable*/ String> returnToString() {
+    return RETURN_TO_STRING;
+  }
+
+  /**
    * A {@code Function} that always ignores its argument and throws an {@link
    * IllegalArgumentException}.
    *
diff --git a/api/src/test/java/io/opencensus/common/FunctionsTest.java b/api/src/test/java/io/opencensus/common/FunctionsTest.java
index e4d37569..55d58d4d 100644
--- a/api/src/test/java/io/opencensus/common/FunctionsTest.java
+++ b/api/src/test/java/io/opencensus/common/FunctionsTest.java
@@ -40,6 +40,14 @@ public class FunctionsTest {
   }
 
   @Test
+  public void testReturnToString() {
+    assertThat(Functions.returnToString().apply("input")).isEqualTo("input");
+    assertThat(Functions.returnToString().apply(Boolean.FALSE)).isEqualTo("false");
+    assertThat(Functions.returnToString().apply(Double.valueOf(123.45))).isEqualTo("123.45");
+    assertThat(Functions.returnToString().apply(null)).isEqualTo(null);
+  }
+
+  @Test
   public void testThrowIllegalArgumentException() {
     Function<Object, Void> f = Functions.throwIllegalArgumentException();
     thrown.expect(IllegalArgumentException.class);