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

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

* Fix comments and add tests.

6 files changed, 33 insertions, 19 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 364d5775..39cc492e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 - Add `AttributeValueDouble` to `AttributeValue`.
 - Add `createWithSender` to `JaegerTraceExporter` to allow use of `HttpSender`
   with extra configurations
+- Add an API `Functions.returnToString()`.
 
 ## 0.16.1 - 2018-09-18
 - Fix ClassCastException in Log4j log correlation
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);
diff --git a/contrib/zpages/src/main/java/io/opencensus/contrib/zpages/TracezZPageHandler.java b/contrib/zpages/src/main/java/io/opencensus/contrib/zpages/TracezZPageHandler.java
index 0da29d73..f6a36996 100644
--- a/contrib/zpages/src/main/java/io/opencensus/contrib/zpages/TracezZPageHandler.java
+++ b/contrib/zpages/src/main/java/io/opencensus/contrib/zpages/TracezZPageHandler.java
@@ -654,12 +654,7 @@ final class TracezZPageHandler extends ZPageHandler {
   // The return type needs to be nullable when this function is used as an argument to 'match' in
   // attributeValueToString, because 'match' doesn't allow covariant return types.
   private static final Function<Object, /*@Nullable*/ String> returnToString =
-      new Function<Object, /*@Nullable*/ String>() {
-        @Override
-        public String apply(Object input) {
-          return input.toString();
-        }
-      };
+      Functions.returnToString();
 
   @javax.annotation.Nullable
   private static String attributeValueToString(AttributeValue attributeValue) {
diff --git a/exporters/trace/instana/src/main/java/io/opencensus/exporter/trace/instana/InstanaExporterHandler.java b/exporters/trace/instana/src/main/java/io/opencensus/exporter/trace/instana/InstanaExporterHandler.java
index 1bbb665a..649a026f 100644
--- a/exporters/trace/instana/src/main/java/io/opencensus/exporter/trace/instana/InstanaExporterHandler.java
+++ b/exporters/trace/instana/src/main/java/io/opencensus/exporter/trace/instana/InstanaExporterHandler.java
@@ -114,12 +114,7 @@ final class InstanaExporterHandler extends SpanExporter.Handler {
   // The return type needs to be nullable when this function is used as an argument to 'match' in
   // attributeValueToString, because 'match' doesn't allow covariant return types.
   private static final Function<Object, /*@Nullable*/ String> returnToString =
-      new Function<Object, /*@Nullable*/ String>() {
-        @Override
-        public String apply(Object input) {
-          return input.toString();
-        }
-      };
+      Functions.returnToString();
 
   @javax.annotation.Nullable
   private static String attributeValueToString(AttributeValue attributeValue) {
diff --git a/exporters/trace/zipkin/src/main/java/io/opencensus/exporter/trace/zipkin/ZipkinExporterHandler.java b/exporters/trace/zipkin/src/main/java/io/opencensus/exporter/trace/zipkin/ZipkinExporterHandler.java
index 711dcaa9..70bc725c 100644
--- a/exporters/trace/zipkin/src/main/java/io/opencensus/exporter/trace/zipkin/ZipkinExporterHandler.java
+++ b/exporters/trace/zipkin/src/main/java/io/opencensus/exporter/trace/zipkin/ZipkinExporterHandler.java
@@ -172,14 +172,11 @@ final class ZipkinExporterHandler extends SpanExporter.Handler {
 
   // The return type needs to be nullable when this function is used as an argument to 'match' in
   // attributeValueToString, because 'match' doesn't allow covariant return types.
-  private static final Function<Object, String> returnToString =
-      new Function<Object, String>() {
-        @Override
-        public String apply(Object input) {
-          return input.toString();
-        }
-      };
+  private static final Function<Object, /*@Nullable*/ String> returnToString =
+      Functions.returnToString();
 
+  // TODO: Fix the Checker Framework warning.
+  @SuppressWarnings("nullness")
   private static String attributeValueToString(AttributeValue attributeValue) {
     return attributeValue.match(
         returnToString,