[java] Add test cases for InefficientStringBuffering

Originally from #1932

Author: jborgers <jborgers@jpinpoint.com>

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/performance/xml/InefficientStringBuffering.xml

@@ -340,6 +340,84 @@ public class Foo {
         String str2 = "b";
         StringBuffer sb = new StringBuffer(str1.length() + str2.length());
     }
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>Violation: Avoid contact in append method invocations</description>
+        <expected-problems>3</expected-problems>
+        <expected-linenumbers>9,18,24</expected-linenumbers>
+        <code><![CDATA[
+public class Foo {
+
+    private static final String STATIC_FINAL_F = "static_final_field";
+    private static String static_f = "static_field";
+    private final String FINAL_F = "final_field";
+
+    private void concatIsBad(String arg) {
+        StringBuilder sb = new StringBuilder();
+        sb.append("arg='" + arg + "'"); // bad
+    }
+
+    private void concatNumericIsBad() {
+        StringBuilder sb = new StringBuilder();
+        sb.append(1 + 2); // bad
+    }
+
+    private String testStaticBad() {
+        StringBuilder sb = new StringBuilder().append("fld:" + static_f); // bad
+        return sb.toString();
+    }
+
+    private String testFinalLocalBad() {
+        String local = "local"; // non-final, jdk9+ optimized with indified Strings, assumed still worse
+        return new StringBuilder().append("fld:" + local).toString(); // assumed bad
+    }
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>No violation: Avoid contact in append method invocations</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public class Foo {
+
+    private static final String STATIC_FINAL_F = "static_final_field";
+    private final String FINAL_F = "final_field";
+
+    private void good(String arg) {
+        StringBuilder sb = new StringBuilder();
+        sb.append("arg='").append(arg).append("'");
+    }
+
+    public void testAddInSecondOrThirdArgAppendGood() {
+        StringBuilder wrappedLine = new StringBuilder();
+        String str = "bar";
+        int offset = 1;
+        int wrapLength = 2;
+        wrappedLine.append(str, offset + 1, wrapLength + offset + 1); // + but no string concat
+    }
+
+    private String testLiteralsGood(String arg) {
+        StringBuilder sb = new StringBuilder();
+        sb.append("lit:" + "etc");
+        return sb.toString();
+    }
+
+    private String testFinalFieldGood() { // final assumed a constant expression
+        return new StringBuilder().append("fld:" + FINAL_F).toString(); //good
+    }
+
+    private String testStaticFinalFieldGood() {
+        return new StringBuilder().append("fld:" + STATIC_FINAL_F).toString(); // good
+    }
+
+    private String testFinalLocalGood() {
+        final String local = "local"; // final assumed a constant expression
+        return new StringBuilder().append("fld:" + local).toString(); // good
+    }
 }
         ]]></code>
     </test-code>