add simpleentry test (#2654)

libraries/src/test/java/com/baeldung/pairs/CoreJavaSimpleEntryUnitTest.java

@@ -0,0 +1,39 @@
+package com.baeldung.pairs;
+
+import static org.junit.Assert.*;
+
+import java.util.AbstractMap;
+
+import org.junit.Test;
+
+public class CoreJavaSimpleEntryUnitTest {
+
+    @Test
+    public void givenSimpleEntry_whenGetValue_thenOk() {
+        AbstractMap.SimpleEntry<Integer, String> entry = new AbstractMap.SimpleEntry<Integer, String>(1, "one");
+        Integer key = entry.getKey();
+        String value = entry.getValue();
+
+        assertEquals(key.intValue(), 1);
+        assertEquals(value, "one");
+    }
+    
+    @Test(expected = UnsupportedOperationException.class)
+    public void givenSimpleImmutableEntry_whenSetValue_thenException() {
+        AbstractMap.SimpleImmutableEntry<Integer, String> entry = new AbstractMap.SimpleImmutableEntry<Integer, String>(1, "one");
+       
+        entry.setValue("two");
+        
+    }
+    
+    @Test
+    public void givenSimpleImmutableEntry_whenGetValue_thenOk() {
+        AbstractMap.SimpleImmutableEntry<Integer, String> entry = new AbstractMap.SimpleImmutableEntry<Integer, String>(1, "one");
+        Integer key = entry.getKey();
+        String value = entry.getValue();
+
+        assertEquals(key.intValue(), 1);
+        assertEquals(value, "one");
+    }
+
+}