1 files changed, 130 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
new file mode 100644
index 00000000..ea3457ca
--- /dev/null
+++ b/api/src/main/java/io/opencensus/common/Functions.java
@@ -0,0 +1,130 @@
+/*
+ * Copyright 2017, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.opencensus.common;
+
+/*>>>
+import org.checkerframework.checker.nullness.qual.Nullable;
+*/
+
+/**
+ * Commonly used {@link Function} instances.
+ *
+ * @since 0.5
+ */
+public final class Functions {
+  private Functions() {}
+
+  private static final Function<Object, /*@Nullable*/ Void> RETURN_NULL =
+      new Function<Object, /*@Nullable*/ Void>() {
+        @Override
+        @javax.annotation.Nullable
+        public Void apply(Object ignored) {
+          return null;
+        }
+      };
+
+  private static final Function<Object, Void> THROW_ILLEGAL_ARGUMENT_EXCEPTION =
+      new Function<Object, Void>() {
+        @Override
+        public Void apply(Object ignored) {
+          throw new IllegalArgumentException();
+        }
+      };
+
+  private static final Function<Object, Void> THROW_ASSERTION_ERROR =
+      new Function<Object, Void>() {
+        @Override
+        public Void apply(Object ignored) {
+          throw new AssertionError();
+        }
+      };
+
+  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}.
+   *
+   * @return a {@code Function} that always ignores its argument and returns {@code null}.
+   * @since 0.5
+   */
+  public static <T> Function<Object, /*@Nullable*/ T> returnNull() {
+    // It is safe to cast a producer of Void to anything, because Void is always null.
+    @SuppressWarnings("unchecked")
+    Function<Object, /*@Nullable*/ T> function = (Function<Object, /*@Nullable*/ T>) RETURN_NULL;
+    return function;
+  }
+
+  /**
+   * A {@code Function} that always ignores its argument and returns a constant value.
+   *
+   * @return a {@code Function} that always ignores its argument and returns a constant value.
+   * @since 0.5
+   */
+  public static <T> Function<Object, T> returnConstant(final T constant) {
+    return new Function<Object, T>() {
+      @Override
+      public T apply(Object ignored) {
+        return constant;
+      }
+    };
+  }
+
+  /**
+   * 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}.
+   *
+   * @return a {@code Function} that always ignores its argument and throws an {@link
+   *     IllegalArgumentException}.
+   * @since 0.5
+   */
+  public static <T> Function<Object, T> throwIllegalArgumentException() {
+    // It is safe to cast this function to have any return type, since it never returns a result.
+    @SuppressWarnings("unchecked")
+    Function<Object, T> function = (Function<Object, T>) THROW_ILLEGAL_ARGUMENT_EXCEPTION;
+    return function;
+  }
+
+  /**
+   * A {@code Function} that always ignores its argument and throws an {@link AssertionError}.
+   *
+   * @return a {@code Function} that always ignores its argument and throws an {@code
+   *     AssertionError}.
+   * @since 0.6
+   */
+  public static <T> Function<Object, T> throwAssertionError() {
+    // It is safe to cast this function to have any return type, since it never returns a result.
+    @SuppressWarnings("unchecked")
+    Function<Object, T> function = (Function<Object, T>) THROW_ASSERTION_ERROR;
+    return function;
+  }
+}