1 /*
2  * Copyright 2017, OpenCensus Authors
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *     http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package io.opencensus.common;
18 
19 /*>>>
20 import org.checkerframework.checker.nullness.qual.Nullable;
21 */
22 
23 /**
24  * Commonly used {@link Function} instances.
25  *
26  * @since 0.5
27  */
28 public final class Functions {
Functions()29   private Functions() {}
30 
31   private static final Function<Object, /*@Nullable*/ Void> RETURN_NULL =
32       new Function<Object, /*@Nullable*/ Void>() {
33         @Override
34         @javax.annotation.Nullable
35         public Void apply(Object ignored) {
36           return null;
37         }
38       };
39 
40   private static final Function<Object, Void> THROW_ILLEGAL_ARGUMENT_EXCEPTION =
41       new Function<Object, Void>() {
42         @Override
43         public Void apply(Object ignored) {
44           throw new IllegalArgumentException();
45         }
46       };
47 
48   private static final Function<Object, Void> THROW_ASSERTION_ERROR =
49       new Function<Object, Void>() {
50         @Override
51         public Void apply(Object ignored) {
52           throw new AssertionError();
53         }
54       };
55 
56   private static final Function<Object, /*@Nullable*/ String> RETURN_TO_STRING =
57       new Function<Object, /*@Nullable*/ String>() {
58         @Override
59         public /*@Nullable*/ String apply(Object input) {
60           return input == null ? null : input.toString();
61         }
62       };
63 
64   /**
65    * A {@code Function} that always ignores its argument and returns {@code null}.
66    *
67    * @return a {@code Function} that always ignores its argument and returns {@code null}.
68    * @since 0.5
69    */
returnNull()70   public static <T> Function<Object, /*@Nullable*/ T> returnNull() {
71     // It is safe to cast a producer of Void to anything, because Void is always null.
72     @SuppressWarnings("unchecked")
73     Function<Object, /*@Nullable*/ T> function = (Function<Object, /*@Nullable*/ T>) RETURN_NULL;
74     return function;
75   }
76 
77   /**
78    * A {@code Function} that always ignores its argument and returns a constant value.
79    *
80    * @return a {@code Function} that always ignores its argument and returns a constant value.
81    * @since 0.5
82    */
returnConstant(final T constant)83   public static <T> Function<Object, T> returnConstant(final T constant) {
84     return new Function<Object, T>() {
85       @Override
86       public T apply(Object ignored) {
87         return constant;
88       }
89     };
90   }
91 
92   /**
93    * A {@code Function} that always returns the {@link #toString()} value of the input.
94    *
95    * @return a {@code Function} that always returns the {@link #toString()} value of the input.
96    * @since 0.17
97    */
98   public static Function<Object, /*@Nullable*/ String> returnToString() {
99     return RETURN_TO_STRING;
100   }
101 
102   /**
103    * A {@code Function} that always ignores its argument and throws an {@link
104    * IllegalArgumentException}.
105    *
106    * @return a {@code Function} that always ignores its argument and throws an {@link
107    *     IllegalArgumentException}.
108    * @since 0.5
109    */
110   public static <T> Function<Object, T> throwIllegalArgumentException() {
111     // It is safe to cast this function to have any return type, since it never returns a result.
112     @SuppressWarnings("unchecked")
113     Function<Object, T> function = (Function<Object, T>) THROW_ILLEGAL_ARGUMENT_EXCEPTION;
114     return function;
115   }
116 
117   /**
118    * A {@code Function} that always ignores its argument and throws an {@link AssertionError}.
119    *
120    * @return a {@code Function} that always ignores its argument and throws an {@code
121    *     AssertionError}.
122    * @since 0.6
123    */
124   public static <T> Function<Object, T> throwAssertionError() {
125     // It is safe to cast this function to have any return type, since it never returns a result.
126     @SuppressWarnings("unchecked")
127     Function<Object, T> function = (Function<Object, T>) THROW_ASSERTION_ERROR;
128     return function;
129   }
130 }
131