Further functional interfaces (apache#110)

* Fix a warning over generics

* Extract IOFunction from test case to be usable elsewhere. Complements IOConsumer

* Add IOSupplier to complement IOFunction and IOConsumer

src/main/java/org/apache/commons/io/function/IOFunction.java

@@ -0,0 +1,207 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.commons.io.function;
+
+import java.io.IOException;
+import java.util.Objects;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+/**
+ * Like {@link Function} but throws {@link IOException}.
+ *
+ * @param <T> the type of the input to the operations.
+ * @param <R> the return type of the operations.
+ * @since 2.7
+ */
+@FunctionalInterface
+public interface IOFunction<T, R> {
+
+    /**
+     * Applies this function to the given argument.
+     *
+     * @param t the function argument
+     * @return the function result
+     *
+     * @throws IOException if the function throws an IOException
+     */
+    R apply(final T t) throws IOException;
+
+    /**
+     * Returns a composed {@link IOFunction} that first applies the {@code before}
+     * function to its input, and then applies this function to the result.
+     * If evaluation of either function throws an exception, it is relayed to
+     * the caller of the composed function.
+     *
+     * @param <V> the type of input to the {@code before} function, and to the
+     *           composed function
+     * @param before the function to apply before this function is applied
+     * @return a composed function that first applies the {@code before}
+     * function and then applies this function
+     * @throws NullPointerException if before is null
+     *
+     * @see #andThen(IOFunction)
+     */
+    default <V> IOFunction<V, R> compose(final IOFunction<? super V, ? extends T> before) {
+        Objects.requireNonNull(before);
+        return (V v) -> apply(before.apply(v));
+    }
+
+    /**
+     * Returns a composed {@link IOFunction} that first applies the {@code before}
+     * function to its input, and then applies this function to the result.
+     * If evaluation of either function throws an exception, it is relayed to
+     * the caller of the composed function.
+     *
+     * @param <V> the type of input to the {@code before} function, and to the
+     *           composed function
+     * @param before the function to apply before this function is applied
+     * @return a composed function that first applies the {@code before}
+     * function and then applies this function
+     * @throws NullPointerException if before is null
+     *
+     * @see #andThen(IOFunction)
+     */
+    default <V> IOFunction<V, R> compose(final Function<? super V, ? extends T> before) {
+        Objects.requireNonNull(before);
+        return (V v) -> apply(before.apply(v));
+    }
+
+    /**
+     * Returns a composed {@link IOFunction} that first applies the {@code before}
+     * function to its input, and then applies this function to the result.
+     * If evaluation of either function throws an exception, it is relayed to
+     * the caller of the composed function.
+     *
+     * @param before the supplier which feeds the application of this function
+     * @return a composed function that first applies the {@code before}
+     * function and then applies this function
+     * @throws NullPointerException if before is null
+     *
+     * @see #andThen(IOFunction)
+     */
+    default IOSupplier<R> compose(final IOSupplier<? extends T> before) {
+        Objects.requireNonNull(before);
+        return () -> apply(before.get());
+    }
+
+    /**
+     * Returns a composed {@link IOFunction} that first applies the {@code before}
+     * function to its input, and then applies this function to the result.
+     * If evaluation of either function throws an exception, it is relayed to
+     * the caller of the composed function.
+     *
+     * @param before the supplier which feeds the application of this function
+     * @return a composed function that first applies the {@code before}
+     * function and then applies this function
+     * @throws NullPointerException if before is null
+     *
+     * @see #andThen(IOFunction)
+     */
+    default IOSupplier<R> compose(final Supplier<? extends T> before) {
+        Objects.requireNonNull(before);
+        return () -> apply(before.get());
+    }
+
+    /**
+     * Returns a composed {@link IOFunction} that first applies this function to
+     * its input, and then applies the {@code after} function to the result.
+     * If evaluation of either function throws an exception, it is relayed to
+     * the caller of the composed function.
+     *
+     * @param <V> the type of output of the {@code after} function, and of the
+     *           composed function
+     * @param after the function to apply after this function is applied
+     * @return a composed function that first applies this function and then
+     * applies the {@code after} function
+     * @throws NullPointerException if after is null
+     *
+     * @see #compose(IOFunction)
+     */
+    default <V> IOFunction<T, V> andThen(IOFunction<? super R, ? extends V> after) {
+        Objects.requireNonNull(after);
+        return (T t) -> after.apply(apply(t));
+    }
+
+    /**
+     * Returns a composed {@link IOFunction} that first applies this function to
+     * its input, and then applies the {@code after} function to the result.
+     * If evaluation of either function throws an exception, it is relayed to
+     * the caller of the composed function.
+     *
+     * @param <V> the type of output of the {@code after} function, and of the
+     *           composed function
+     * @param after the function to apply after this function is applied
+     * @return a composed function that first applies this function and then
+     * applies the {@code after} function
+     * @throws NullPointerException if after is null
+     *
+     * @see #compose(IOFunction)
+     */
+    default <V> IOFunction<T, V> andThen(Function<? super R, ? extends V> after) {
+        Objects.requireNonNull(after);
+        return (T t) -> after.apply(apply(t));
+    }
+
+    /**
+     * Returns a composed {@link IOFunction} that first applies this function to
+     * its input, and then applies the {@code after} consumer to the result.
+     * If evaluation of either function throws an exception, it is relayed to
+     * the caller of the composed function.
+     *
+     * @param after the consumer to apply after this function is applied
+     * @return a composed function that first applies this function and then
+     * applies the {@code after} consumer
+     * @throws NullPointerException if after is null
+     *
+     * @see #compose(IOFunction)
+     */
+    default IOConsumer<T> andThen(IOConsumer<? super R> after) {
+        Objects.requireNonNull(after);
+        return (T t) -> after.accept(apply(t));
+    }
+
+    /**
+     * Returns a composed {@link IOFunction} that first applies this function to
+     * its input, and then applies the {@code after} consumer to the result.
+     * If evaluation of either function throws an exception, it is relayed to
+     * the caller of the composed function.
+     *
+     * @param after the consumer to apply after this function is applied
+     * @return a composed function that first applies this function and then
+     * applies the {@code after} consumer
+     * @throws NullPointerException if after is null
+     *
+     * @see #compose(IOFunction)
+     */
+    default IOConsumer<T> andThen(Consumer<? super R> after) {
+        Objects.requireNonNull(after);
+        return (T t) -> after.accept(apply(t));
+    }
+
+    /**
+     * Returns a {@link IOFunction} that always returns its input argument.
+     *
+     * @param <T> the type of the input and output objects to the function
+     * @return a function that always returns its input argument
+     */
+    static <T> IOFunction<T, T> identity() {
+        return t -> t;
+    }
+}

src/main/java/org/apache/commons/io/function/IOSupplier.java

@@ -0,0 +1,40 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.commons.io.function;
+
+import java.io.IOException;
+import java.util.function.Supplier;
+
+/**
+ * Like {@link Supplier} but throws {@link IOException}.
+ *
+ * @param <T> the return type of the operations.
+ * @since 2.7
+ */
+@FunctionalInterface
+public interface IOSupplier<T> {
+
+    /**
+     * Gets a result.
+     *
+     * @return a result
+     *
+     * @throws IOException if an IO error occurs whilst supplying the value.
+     */
+    T get() throws IOException;
+}