Add basic helper functional interface

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-core/src/main/java/com/alibaba/csp/sentinel/util/function/Function.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 1999-2018 Alibaba Group Holding Ltd.
+ *
+ * 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 com.alibaba.csp.sentinel.util.function;
+
+/**
+ * Function functional interface from JDK 8.
+ */
+public interface Function<T, R> {
+
+    /**
+     * Applies this function to the given argument.
+     *
+     * @param t the function argument
+     * @return the function result
+     */
+    R apply(T t);
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/util/function/Predicate.java

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+/*
+ * Copyright 1999-2018 Alibaba Group Holding Ltd.
+ *
+ * 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 com.alibaba.csp.sentinel.util.function;
+
+/**
+ * Predicate functional interface from JDK 8.
+ */
+public interface Predicate<T> {
+
+    /**
+     * Evaluates this predicate on the given argument.
+     *
+     * @param t the input argument
+     * @return {@code true} if the input argument matches the predicate,
+     * otherwise {@code false}
+     */
+    boolean test(T t);
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/util/function/Tuple2.java

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+package com.alibaba.csp.sentinel.util.function;
+
+/**
+ * A tuple of 2 elements.
+ */
+public class Tuple2<R1, R2> {
+
+    public final R1 r1;
+    public final R2 r2;
+
+    public Tuple2(R1 r1, R2 r2) {
+        this.r1 = r1;
+        this.r2 = r2;
+    }
+
+    /**
+     * Factory method for creating a Tuple.
+     *
+     * @return new Tuple
+     */
+    public static <C1, C2> Tuple2<C1, C2> of(C1 c1, C2 c2) {
+        return new Tuple2<C1, C2>(c1, c2);
+    }
+
+    /**
+     * Swaps the element of this Tuple.
+     *
+     * @return a new Tuple where the first element is the second element of this Tuple and the second element is the first element of this Tuple.
+     */
+    public Tuple2<R2, R1> swap() {
+        return new Tuple2<R2, R1>(this.r2, this.r1);
+    }
+
+    @Override
+    public String toString() {
+        return "Tuple2{" +
+            "r1=" + r1 +
+            ", r2=" + r2 +
+            '}';
+    }
+}