PR for BAEL-1340 - Java StringJoiner class (#3324)

* unit tests for stringjoiner

* formatting changes

* slit test cases and formatting fixes

* minor formatting fix

core-java-8/src/test/java/com/baeldung/stringjoiner/StringJoinerUnitTest.java

@@ -0,0 +1,101 @@
+package com.baeldung.stringjoiner;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.StringJoiner;
+import java.util.stream.Collectors;
+
+import org.junit.Test;
+
+public class StringJoinerUnitTest {
+    private final String DELIMITER_COMMA = ",";
+    private final String DELIMITER_HYPHEN = "-";
+    private final String PREFIX = "[";
+    private final String SUFFIX = "]";
+    private final String EMPTY_JOINER = "empty";
+
+    @Test
+    public void whenJoinerWithoutPrefixSuffixWithoutEmptyValue_thenReturnDefault() {
+        StringJoiner commaSeparatedJoiner = new StringJoiner(DELIMITER_COMMA);
+        assertEquals(0, commaSeparatedJoiner.toString().length());
+    }
+
+    @Test
+    public void whenJoinerWithPrefixSuffixWithoutEmptyValue_thenReturnDefault() {
+        StringJoiner commaSeparatedPrefixSuffixJoiner = new StringJoiner(DELIMITER_COMMA, PREFIX, SUFFIX);
+        assertEquals(commaSeparatedPrefixSuffixJoiner.toString(), PREFIX + SUFFIX);
+    }
+
+    @Test
+    public void whenJoinerWithoutPrefixSuffixWithEmptyValue_thenReturnDefault() {
+        StringJoiner commaSeparatedJoiner = new StringJoiner(DELIMITER_COMMA);
+        commaSeparatedJoiner.setEmptyValue(EMPTY_JOINER);
+
+        assertEquals(commaSeparatedJoiner.toString(), EMPTY_JOINER);
+    }
+
+    @Test
+    public void whenJoinerWithPrefixSuffixWithEmptyValue_thenReturnDefault() {
+        StringJoiner commaSeparatedPrefixSuffixJoiner = new StringJoiner(DELIMITER_COMMA, PREFIX, SUFFIX);
+        commaSeparatedPrefixSuffixJoiner.setEmptyValue(EMPTY_JOINER);
+
+        assertEquals(commaSeparatedPrefixSuffixJoiner.toString(), EMPTY_JOINER);
+    }
+
+    @Test
+    public void whenAddElements_thenJoinElements() {
+        StringJoiner rgbJoiner = new StringJoiner(DELIMITER_COMMA, PREFIX, SUFFIX);
+        rgbJoiner.add("Red")
+          .add("Green")
+          .add("Blue");
+
+        assertEquals(rgbJoiner.toString(), "[Red,Green,Blue]");
+    }
+
+    @Test
+    public void whenAddListElements_thenJoinListElements() {
+        List<String> rgbList = new ArrayList<String>();
+        rgbList.add("Red");
+        rgbList.add("Green");
+        rgbList.add("Blue");
+
+        StringJoiner rgbJoiner = new StringJoiner(DELIMITER_COMMA, PREFIX, SUFFIX);
+
+        for (String color : rgbList) {
+            rgbJoiner.add(color);
+        }
+
+        assertEquals(rgbJoiner.toString(), "[Red,Green,Blue]");
+    }
+
+    @Test
+    public void whenMergeJoiners_thenReturnMerged() {
+        StringJoiner rgbJoiner = new StringJoiner(DELIMITER_COMMA, PREFIX, SUFFIX);
+        StringJoiner cmybJoiner = new StringJoiner(DELIMITER_HYPHEN, PREFIX, SUFFIX);
+
+        rgbJoiner.add("Red")
+          .add("Green")
+          .add("Blue");
+        cmybJoiner.add("Cyan")
+          .add("Magenta")
+          .add("Yellow")
+          .add("Black");
+
+        rgbJoiner.merge(cmybJoiner);
+
+        assertEquals(rgbJoiner.toString(), "[Red,Green,Blue,Cyan-Magenta-Yellow-Black]");
+    }
+
+    @Test
+    public void whenUsedWithinCollectors_thenJoin() {
+        List<String> rgbList = Arrays.asList("Red", "Green", "Blue");
+        String commaSeparatedRGB = rgbList.stream()
+          .map(color -> color.toString())
+          .collect(Collectors.joining(","));
+
+        assertEquals(commaSeparatedRGB, "Red,Green,Blue");
+    }
+}