@Override
 public void visitAssignmentExpression(AssignmentExpressionTree tree) {
   Type wrapper = ((JavaType) tree.expression().symbolType()).primitiveWrapperType();
   if (tree.is(Kind.PLUS_ASSIGNMENT)
       && tree.variable().symbolType().is("java.lang.String")
       && wrapper != null) {
     createIssue(tree, wrapper.name());
   }
   super.visitAssignmentExpression(tree);
 }
 private void checkConcatenation(
     Tree tree, ExpressionTree leftOperand, ExpressionTree rightOperand) {
   Type wrapper = null;
   if (isEmptyString(leftOperand)) {
     wrapper = ((JavaType) rightOperand.symbolType()).primitiveWrapperType();
   } else if (isEmptyString(rightOperand)) {
     wrapper = ((JavaType) leftOperand.symbolType()).primitiveWrapperType();
   }
   if (wrapper != null) {
     createIssue(tree, wrapper.name());
   }
 }