Added PR files for BAEL-2031 (#4844)

* Added source files for BAEL-2031

* Added test files for BAEL-2031

core-java-8/src/main/java/com/baeldung/nullsafecollectionstreams/NullSafeCollectionStreamsUsingCommonsEmptyIfNull.java

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+package com.baeldung.nullsafecollectionstreams;
+
+import java.util.Collection;
+import java.util.stream.Stream;
+import static org.apache.commons.collections4.CollectionUtils.emptyIfNull;
+
+public class NullSafeCollectionStreamsUsingCommonsEmptyIfNull {
+
+    /**
+    * This method shows how to make a null safe stream from a collection through the use of 
+    * emptyIfNull() method from Apache Commons CollectionUtils library
+    * 
+     * @param collection The collection that is to be converted into a stream
+     * @return The stream that has been created from the collection or an empty stream if the collection is null
+    */
+    public Stream<String> collectionAsStream(Collection<String> collection) {
+        return emptyIfNull(collection).stream();
+    }
+}

core-java-8/src/main/java/com/baeldung/nullsafecollectionstreams/NullSafeCollectionStreamsUsingJava8OptionalContainer.java

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+package com.baeldung.nullsafecollectionstreams;
+
+import java.util.Collection;
+import java.util.Optional;
+import java.util.stream.Stream;
+
+public class NullSafeCollectionStreamsUsingJava8OptionalContainer {
+
+    /**
+    * This method shows how to make a null safe stream from a collection  through the use of 
+    * Java SE 8’s Optional Container
+    * 
+     * @param collection The collection that is to be converted into a stream
+     * @return The stream that has been created from the collection or an empty stream if the collection is null
+    */
+    public Stream<String> collectionAsStream(Collection<String> collection) {
+        return Optional.ofNullable(collection)
+            .map(Collection::stream)
+            .orElseGet(Stream::empty);
+    }
+}

core-java-8/src/main/java/com/baeldung/nullsafecollectionstreams/NullSafeCollectionStreamsUsingNullDereferenceCheck.java

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+package com.baeldung.nullsafecollectionstreams;
+
+import java.util.Collection;
+import java.util.stream.Stream;
+
+public class NullSafeCollectionStreamsUsingNullDereferenceCheck {
+    
+    /**
+    * This method shows how to make a null safe stream from a collection through the use of a check 
+    * to prevent null dereferences
+    * 
+    * @param collection The collection that is to be converted into a stream
+    * @return The stream that has been created from the collection or an empty stream if the collection is null
+    */
+    public Stream<String> collectionAsStream(Collection<String> collection) {
+        return collection == null ? Stream.empty() : collection.stream();
+    }    
+
+
+}

core-java-8/src/test/java/com/baeldung/nullsafecollectionstreams/NullSafeCollectionStreamsUsingCommonsEmptyIfNullUnitTest.java

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+package com.baeldung.nullsafecollectionstreams;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.stream.Stream;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+
+public class NullSafeCollectionStreamsUsingCommonsEmptyIfNullUnitTest {
+
+    private final NullSafeCollectionStreamsUsingCommonsEmptyIfNull instance = 
+            new NullSafeCollectionStreamsUsingCommonsEmptyIfNull();
+
+    @Test
+    public void whenCollectionIsNull_thenExpectAnEmptyStream() {
+        Collection<String> collection = null;
+        Stream<String> expResult = Stream.empty();
+        Stream<String> result = instance.collectionAsStream(collection);
+        assertStreamEquals(expResult, result);
+
+    }
+
+    @Test
+    public void whenCollectionHasElements_thenExpectAStreamOfExactlyTheSameElements() {
+
+        Collection<String> collection = Arrays.asList("a", "b", "c");
+        Stream<String> expResult = Arrays.stream(new String[] { "a", "b", "c" });
+        Stream<String> result = instance.collectionAsStream(collection);
+        assertStreamEquals(expResult, result);
+    }
+
+    private static void assertStreamEquals(Stream<?> s1, Stream<?> s2) {
+        Iterator<?> iter1 = s1.iterator(), iter2 = s2.iterator();
+        while (iter1.hasNext() && iter2.hasNext())
+            assertEquals(iter1.next(), iter2.next());
+        assert !iter1.hasNext() && !iter2.hasNext();
+    }
+    
+}

core-java-8/src/test/java/com/baeldung/nullsafecollectionstreams/NullSafeCollectionStreamsUsingJava8OptionalContainerUnitTest.java

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+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package com.baeldung.nullsafecollectionstreams;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.stream.Stream;
+import org.junit.After;
+import org.junit.AfterClass;
+import org.junit.Before;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ *
+ * @author Kwaje Anthony <kwajeanthony@gmail.com>
+ */
+public class NullSafeCollectionStreamsUsingJava8OptionalContainerUnitTest {
+    
+    private final NullSafeCollectionStreamsUsingJava8OptionalContainer instance = 
+            new NullSafeCollectionStreamsUsingJava8OptionalContainer();
+
+    @Test
+    public void whenCollectionIsNull_thenExpectAnEmptyStream() {
+        Collection<String> collection = null;
+        Stream<String> expResult = Stream.empty();
+        Stream<String> result = instance.collectionAsStream(collection);
+        assertStreamEquals(expResult, result);
+
+    }
+
+    @Test
+    public void whenCollectionHasElements_thenExpectAStreamOfExactlyTheSameElements() {
+
+        Collection<String> collection = Arrays.asList("a", "b", "c");
+        Stream<String> expResult = Arrays.stream(new String[] { "a", "b", "c" });
+        Stream<String> result = instance.collectionAsStream(collection);
+        assertStreamEquals(expResult, result);
+    }
+
+    private static void assertStreamEquals(Stream<?> s1, Stream<?> s2) {
+        Iterator<?> iter1 = s1.iterator(), iter2 = s2.iterator();
+        while (iter1.hasNext() && iter2.hasNext())
+            assertEquals(iter1.next(), iter2.next());
+        assert !iter1.hasNext() && !iter2.hasNext();
+    }
+    
+}

core-java-8/src/test/java/com/baeldung/nullsafecollectionstreams/NullSafeCollectionStreamsUsingNullDereferenceCheckUnitTest.java

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+
+package com.baeldung.nullsafecollectionstreams;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.stream.Stream;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ *
+ * @author Kwaje Anthony <kwajeanthony@gmail.com>
+ */
+public class NullSafeCollectionStreamsUsingNullDereferenceCheckUnitTest {
+
+    private final NullSafeCollectionStreamsUsingNullDereferenceCheck instance = 
+            new NullSafeCollectionStreamsUsingNullDereferenceCheck();
+
+    @Test
+    public void whenCollectionIsNull_thenExpectAnEmptyStream() {
+        Collection<String> collection = null;
+        Stream<String> expResult = Stream.empty();
+        Stream<String> result = instance.collectionAsStream(collection);
+        assertStreamEquals(expResult, result);
+
+    }
+
+    @Test
+    public void whenCollectionHasElements_thenExpectAStreamOfExactlyTheSameElements() {
+
+        Collection<String> collection = Arrays.asList("a", "b", "c");
+        Stream<String> expResult = Arrays.stream(new String[] { "a", "b", "c" });
+        Stream<String> result = instance.collectionAsStream(collection);
+        assertStreamEquals(expResult, result);
+    }
+
+    private static void assertStreamEquals(Stream<?> s1, Stream<?> s2) {
+        Iterator<?> iter1 = s1.iterator(), iter2 = s2.iterator();
+        while (iter1.hasNext() && iter2.hasNext())
+            assertEquals(iter1.next(), iter2.next());
+        assert !iter1.hasNext() && !iter2.hasNext();
+    }
+
+}