Github user peteryhwong commented on a diff in the pull request:
https://github.com/apache/commons-lang/pull/80#discussion_r29721658
--- Diff: src/main/java/org/apache/commons/lang3/concurrent/Memoizer.java
---
@@ -0,0 +1,133 @@
+/*
+ * 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.lang3.concurrent;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.CancellationException;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Future;
+import java.util.concurrent.FutureTask;
+
+/**
+ * <p>Definition of an interface for a wrapper around a calculation that
takes a single parameter and returns a result.
+ * The results for the calculation will be cached for future requests.</p>
+ * <p/>
+ * <p>This is not a fully functional cache, there is no way of limiting or
removing results once they have been generated.
+ * However, it is possible to get the implementation to regenerate the
result for a given parameter, if an error was
+ * thrown during the previous calculation, by setting the option during
the construction of the class. If this is not
+ * set the class will return the cached exception.</p>
+ * <p/>
+ * <p>Thanks should go to Brian Goetz, Tim Peierls and the members of JCP
JSR-166 Expert Group for coming up with the
+ * original implementation of the class. It was also published within Java
Concurreny in Practice as a sample.</p>
+ *
+ * @param <A> the type of the input to the calculation
+ * @param <V> the type of the output of the calculation
+ */
+public class Memoizer<A, V> implements Computable<A, V> {
+ private final ConcurrentMap<A, Future<V>> cache
+ = new ConcurrentHashMap<A, Future<V>>();
+ private final Computable<A, V> c;
+ private final boolean recalculate;
+
+ /**
+ * <p>Constructs a Memoizer for the provided Computable calculation.</p>
+ * <p/>
+ * <p>If a calculation is thrown an exception for any reason, this
exception will be cached and returned for
+ * all future calls with the provided parameter.</p>
+ *
+ * @param c the computation whose results should be memorized
+ */
+ public Memoizer(Computable<A, V> c) {
+ this(c, false);
+ }
+
+ /**
+ * <p>Constructs a Memoizer for the provided Computable calculation,
with the option of whether a Computation
+ * that experiences an error should recalculate on subsequent calls or
return the same cached exception.</p>
+ *
+ * @param c the computation whose results should be memorized
+ * @param recalculate determines whether the computation should be
recalculated on subsequent calls if the previous
+ * call failed
+ */
+ public Memoizer(Computable<A, V> c, boolean recalculate) {
+ this.c = c;
+ this.recalculate = recalculate;
+ }
+
+ /**
+ * <p>This method will return the result of the calculation and cache
it, if it has not previously been calculated.</p>
+ * <p/>
+ * <p>This cache will also cache exceptions that occur during the
computation if the {@code recalculate} parameter is
+ * the constructor was set to {@code false}, or not set. Otherwise, if
an exception happened on the previous
+ * calculation, the method will attempt again to generate a value.</p>
+ *
+ * @param arg the argument for the calculation
+ * @return the result of the calculation
+ * @throws InterruptedException thrown if the calculation is
interrupted
+ * @throws IllegalStateException a wrapper around any checked exception
that occurs during the computation of the result
+ */
+ public V compute(final A arg) throws InterruptedException,
IllegalStateException {
--- End diff --
Please use Overrides annotation. Moreover, IlegalStateException is a
runtime (unchecked) exception and so need not be declared at the method
signature.
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