Add stream supplier test (#2631)

core-java-8/src/test/java/com/baeldung/stream/SupplierStreamTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.stream;
+
+import static org.junit.Assert.fail;
+
+import java.util.Optional;
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+
+import org.junit.Test;
+
+public class SupplierStreamTest {
+
+    @Test(expected = IllegalStateException.class)
+    public void givenStream_whenStreamUsedTwice_thenThrowException() {
+        Stream<String> stringStream = Stream.of("A", "B", "C", "D");
+        Optional<String> result1 = stringStream.findAny();
+        System.out.println(result1.get());
+        Optional<String> result2 = stringStream.findFirst();
+        System.out.println(result2.get());
+    }
+
+    @Test
+    public void givenStream_whenUsingSupplier_thenNoExceptionIsThrown() {
+        try {
+            Supplier<Stream<String>> streamSupplier = () -> Stream.of("A", "B", "C", "D");
+            Optional<String> result1 = streamSupplier.get().findAny();
+            System.out.println(result1.get());
+            Optional<String> result2 = streamSupplier.get().findFirst();
+            System.out.println(result2.get());
+        } catch (IllegalStateException e) {
+            fail();
+        }
+    }
+
+}