在上面的示例代码中，FunctionExtractor 类是如何工作的？

在上面的示例代码中，FunctionExtractor 类是如何工作的？

java
import org.apache.calcite.sql.SqlCall;  
import org.apache.calcite.sql.SqlFunction;  
import org.apache.calcite.sql.SqlNode;  
import org.apache.calcite.sql.parser.SqlParseException;  
import org.apache.calcite.sql.parser.SqlParser;  
import org.apache.calcite.sql.util.SqlBasicVisitor;  

import java.util.ArrayList;  
import java.util.List;  

public class CalciteTest {  

    public static void main(String[] args) throws SqlParseException {  
        String sql = "select concat('test-', upper(name)) from test limit 3";  
        SqlParser parser = SqlParser.create(sql);  
        SqlNode stmt = parser.parseStmt();  
        FunctionExtractor functionExtractor = new FunctionExtractor();  
        stmt.accept(functionExtractor);  
        System.out.println(functionExtractor.getFunctions()); // 输出 [CONCAT, UPPER]  
    }  

    private static class FunctionExtractor extends SqlBasicVisitor<Void> {  

        private final List<String> functions = new ArrayList<>();  

        @Override  
        public Void visit(SqlCall call) {  
            if (call.getOperator() instanceof SqlFunction) {  
                functions.add(call.getOperator().getName());  
            }  
            return super.visit(call);  
        }  

        public List<String> getFunctions() {  
            return functions;  
        }  
    }  
}