关于为2D对象列表赋值的NullPointerException

Question

我试图将字符串分配给2D对象列表，但每次都遇到NullPointerException。2D对象列表是自定义表模型类的数据源，该类将其提供给JTable。源是一个2D字符串列表，它只由数据对组成，但是这两部分都是需要打印的字符串，所以映射不适合。我遍历源列表并设置表数据列表的值。

public class CustomTableModel extends AbstractTableModel {

    private List<List<Object>> tableData = new ArrayList<>();

    public void setValues(List<List<String>> values) {

        //Sanity Check
        if (values == null) {
            throw new IllegalArgumentException("Values cannot be null");
        } else {

            System.out.println("Got values in table model");

            //Iterate through and set table data
            int i = 0;
            int j = 0;
            for (List<String> l : values) {
                for (String s : l) {
                    setValueAt(s, i, j);
                    j++;
                }
                i++;
                //Reset to 0 as only 2 colums needed
                j = 0;
            }
        }
    }

再往下是setValueAt方法

   @Override
    public void setValueAt(Object value, int rowIndex, int columnIndex) {
        System.out.println("Setting" + value + " at " + rowIndex + "," + columnIndex);
        //This gives a NullPointerException
        tableData.get(rowIndex).set(columnIndex, value);
        //
        fireTableCellUpdated(rowIndex, columnIndex);
    }

中的所有值

tableData.get(rowIndex).set(columnIndex, value);

并且是非空的。

private ArrayList<ArrayList<Object>> tableData = new ArrayList<ArrayList<Object>>();

                @Override
    public void setValueAt(Object value, int rowIndex, int columnIndex) {
        System.out.println("Setting" + value + " at " + rowIndex + "," + columnIndex);
        //This causes IndexOutOfBoundsException
        if(tableData.get(rowIndex) == null){
            tableData.add(rowIndex, new ArrayList<>());
        }else{
            tableData.get(rowIndex).add(columnIndex, value);
            //Called after both columns in a row are populated
            fireTableCellUpdated(rowIndex, columnIndex);
        }
        
    }

Answer 1

那么，如果rowindex、columnindex和value都清楚地设置了，那么tabledata要么为null，要么从get获得的内部list为null。

您可以通过将语句分成几行来确定哪一个语句：

List<Object> row = tableData.get(rowIndex); //NPE means tableData is null
row.set(columnIndex, value); //NPE means inner List is null