. F Create Once like in Golang

Author: ScottBob

approvaltests-util-tests/src/test/java/org/lambda/utils/OnceTest.java

@@ -0,0 +1,39 @@
+package org.lambda.utils;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class OnceTest {
+    public static int count = 0;
+
+    @Test
+    public void testOnce() {
+        increment();
+        increment();
+        increment();
+        increment();
+        increment();
+        assertEquals(1, count);
+    }
+
+    private void increment() {
+        Once.run(() -> OnceTest.count++);
+    }
+
+    @Test
+    public void testFunctionCallOnce() {
+        int count = 0;
+        count = increment(count);
+        count = increment(count);
+        count = increment(count);
+        count = increment(count);
+        count = increment(count);
+        assertEquals(1, count);
+    }
+
+    private int increment(int count) {
+        return Once.run(() -> count + 1);
+    }
+
+}

approvaltests-util/src/main/java/org/lambda/utils/Once.java

@@ -0,0 +1,20 @@
+package org.lambda.utils;
+
+import org.lambda.actions.Action0;
+import org.lambda.functions.Function0;
+
+import java.util.*;
+
+public class Once {
+    private static final Set<Action0> actions = Collections.synchronizedSet(new HashSet<>());
+    private static final Map<Class, Object> functions = Collections.synchronizedMap(new HashMap<>());
+    public static void run(Action0 runnable) {
+        if (!actions.contains(runnable)) {
+            actions.add(runnable);
+            runnable.call();
+        }
+    }
+    public static <T> T run(Function0<T> runnable) {
+        return (T) functions.computeIfAbsent(runnable.getClass(), k -> runnable.call());
+    }
+}