BAEL-5583 String concatenation: concat() vs + operator (#12719)

* BAEL-5583 String concatenation: concat() vs + operator (#10)

* BAEL-5583 String concatenation: concat() vs + operator

core-java-modules/core-java-string-operations-5/src/test/java/com/baeldung/concatenation/StringConcatenationUnitTest.java

@@ -0,0 +1,110 @@
+package com.baeldung.concatenation;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.StringJoiner;
+import java.util.stream.Collectors;
+
+import org.junit.jupiter.api.Test;
+
+public class StringConcatenationUnitTest {
+
+    @Test
+    void whenUsingStringBuilder_thenAssertEquals() {
+        StringBuilder builderOne = new StringBuilder("Hello");
+        StringBuilder builderTwo = new StringBuilder(" World");
+        StringBuilder builder = builderOne.append(builderTwo);
+        assertEquals("Hello World", builder.toString());
+
+    }
+
+    @Test
+    void whenUsingStringBuffer_thenAssertEquals() {
+        StringBuffer buffer = new StringBuffer("Hello");
+        buffer.append(" World");
+        assertEquals("Hello World", buffer.toString());
+    }
+
+    @Test
+    void whenUsingStringFormat_thenAssertEquals() {
+        String stringOne = "Hello";
+        String stringTwo = " World";
+        assertEquals("Hello World", String.format("%s%s", stringOne, stringTwo));
+    }
+
+    @Test
+    void whenUsingStringJoin_thenAssertEquals() {
+        String stringOne = "Hello";
+        String stringTwo = " World";
+        assertEquals("Hello World", String.join("", stringOne, stringTwo));
+    }
+
+    @Test
+    void whenUsingStringJoiner_thenAssertEquals() {
+        StringJoiner joiner = new StringJoiner(", ");
+        joiner.add("Hello");
+        joiner.add("World");
+        assertEquals("Hello, World", joiner.toString());
+    }
+
+    @Test
+    void whenUsingCollectors_thenAssertEquals() {
+        List<String> words = Arrays.asList("Hello", "World");
+        String collect = words.stream()
+            .collect(Collectors.joining(", "));
+        assertEquals("Hello, World", collect);
+    }
+
+    @Test
+    void whenUsingConcat_thenAssertEquals() {
+        String stringOne = "Hello";
+        String stringTwo = " World";
+        assertEquals("Hello World", stringOne.concat(stringTwo));
+    }
+
+    @Test
+    void whenUsingConcatWithOutAssignment_thenAssertNotEquals() {
+        String stringOne = "Hello";
+        String stringTwo = " World";
+        stringOne.concat(stringTwo);
+        assertNotEquals("Hello World", stringOne);
+    }
+
+    @Test
+    void whenUsingConcatWithAssignment_thenAssertEquals() {
+        String stringOne = "Hello";
+        String stringTwo = " World";
+        stringOne = stringOne.concat(stringTwo);
+        assertEquals("Hello World", stringOne);
+    }
+
+    @Test
+    void whenUsingConcatToMultipleStringConcatenation_thenAssertEquals() {
+        String stringOne = "Hello";
+        String stringTwo = "World";
+        String stringThree = ", in Jav";
+        stringOne = stringOne.concat(" ")
+            .concat(stringTwo)
+            .concat(stringThree)
+            .concat("@");
+        assertEquals("Hello World, in Jav@", stringOne);
+    }
+
+    @Test
+    void whenUsingConcatAppendANull_thenAssertEquals() {
+        String stringOne = "Hello";
+        String stringTwo = null;
+        assertThrows(NullPointerException.class, () -> stringOne.concat(stringTwo));
+    }
+
+    @Test
+    void whenUsingPlusOperatorANull_thenAssertEquals() {
+        String stringOne = "Hello ";
+        String stringTwo = null;
+        assertEquals("Hello null", stringOne + stringTwo);
+    }
+}