code: public static String generateColumnCheckSet(TypeSpec.Builder builder, SQLiteModelMethod method, Set<String> columnNames) {
  		String columnNameSet = method.contentProviderMethodName + "ColumnSet";
  		StringBuilder initBuilder = new StringBuilder();
  		String temp = "";
  
  		for (String item : columnNames) {
  			initBuilder.append(temp + "\"" + item + "\"");
  			temp = ", ";
  		}
  
  		FieldSpec.Builder fieldBuilder = FieldSpec.builder(ParameterizedTypeName.get(Set.class, String.class), columnNameSet, Modifier.STATIC, Modifier.PRIVATE, Modifier.FINAL);
  		fieldBuilder.initializer("$T.asSet($T.class, $L)", CollectionUtils.class, String.class, initBuilder.toString());
  
  		builder.addField(fieldBuilder.build());
  
  		return columnNameSet;
  	}
docs: check used columns.
  
  @param builder
  the builder
  @param method
  the method
  @param columnNames
  the column names
  @return name of column name set