java - NullPointerException:在不期望的地方抛出

Question

public void setSpecifications(ObservableList<Specifications> data) {
    String sql = "select * from specifications where ( bid=" + bid[0] + " or bid=" + bid[1] + " or bid=" + bid[2] + " or bid=" + bid[3] + " ) and ( mid=" + mid[0] + " or mid=" + mid[1] + " or mid=" + mid[2] + " or mid=" + mid[3] + " ) and ( vid=" + vid[0] + " or vid=" + vid[1] + " or vid=" + vid[2] + " or vid=" + vid[3] + " );";
    int colCount = 0;
    String  specVal[], paramVal = null;
    specVal = new String[]{" ", " ", " ", " "};
    try {
        stmt = conn.createStatement();
        rs = stmt.executeQuery(sql);
        rsmd = rs.getMetaData();
        colCount = rsmd.getColumnCount();
        for (int colIndex = 0; colIndex < colCount;  colIndex++) {
            rs.beforeFirst();
            paramVal=rsmd.getColumnName(colIndex+1);
            for (int rowIndex = 0; rs.next(); rowIndex++) {
                specVal[rowIndex] = rs.getString(paramVal);
            }
            data.add(new Specifications(paramVal, specVal[0], specVal[1], specVal[2], specVal[3]));
            //**NullPointerException**
        }
    } catch (SQLException ex) {
        Logger.getLogger(Compare.class.getName()).log(Level.SEVERE, null, ex);
    }



}

堆栈跟踪:

java.lang.NullPointerException
file:<path>.jar!/carshowroomsystem/details/compare/Compare.fxml
  at carshowroomsystem.dataInterface.Compare.setSpecifications(Compare.java:138)

  at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
  at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
  at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
 ...  rest of the trace

Caused by: java.lang.NullPointerException
    at carshowroomsystem.dataInterface.Compare.setSpecifications(Compare.java:138)
    at carshowroomsystem.details.compare.CompareController.addCar(CompareController.java:74)
    at carshowroomsystem.details.compare.CompareController.initialize(CompareController.java:65)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2152)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2028)
    at carshowroomsystem.WelcomeController.handleCompareAction(WelcomeController.java:106)
    ... 44 more

它在 data.add(...) 行抛出 NullPointerException，我已经检查了所有参数是否具有一定的值!不知道该怎么办。为什么跟踪显示 Compare.fxml 文件？它应该是 .java

Answer 1

您有String specVal[], paramVal = null;它初始化 paramVal为空。表达式paramVal=rsmd.getColumnName(colIndex+1)位于 for 的第三部分表达式，因此它是在迭代之后执行的，因此第一次迭代使用 null paramVal 的值。您需要重写代码，因为它没有意义，并了解如何for语句在 Java 中有效。