Java表无法实例化

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 8 年前。

我有以下类(class)

类解析

class Parsing {
  private ClassWithTable c = new ClassWithTable();

  public void doAction(){
    parseRowData();
    c.initComponents();
  }

  private void parseRowData(){
    Object[][] rowData = new Object[3][2];
    for(int i = 0; i < 3; i++){
        rowData[i][0] = "A string";
        rowData[i][1] = null;
    }
    c.setRowData(rowData);
  }
}

类ClassWithTable

class ClassWithTable {
  private JTable table;
  private Object[][]rowData;
  private final String[] colNames = new String[]{null,null};

  public void initComponents() {
    table = new JTable(getRowData(),colNames); //in this line I get the exception
    System.out.println(table.getValueAt(1, 1));
  }

  public void setRowData(Object[][]rowData){this.rowData = rowData;}
  private Object[][] getRowData(){return this.rowData;}  
  public JTable getTable(){return table;}
}

JTableInstantiation 类

public class JTableInstantiation {
  public static void main (String[] af){
    ClassWithTable c = new ClassWithTable();
    Parsing p = new Parsing();
    p.doAction();

  }
}

当我运行它时，我得到一个 java.lang.NullPointerException 。我发现里面的表ClassWithTable无法实例化，尽管 rowData取所有正确的值。您能向我解释一下为什么会发生这种情况吗？我究竟做错了什么？预先感谢您。

异常表:

Exception in thread "main" java.lang.NullPointerException
at javax.swing.JTable$1.getColumnName(JTable.java:685)
at javax.swing.JTable.addColumn(JTable.java:2801)
at javax.swing.JTable.createDefaultColumnsFromModel(JTable.java:1287)
at javax.swing.JTable.tableChanged(JTable.java:4386)
at javax.swing.JTable.setModel(JTable.java:3688)
at javax.swing.JTable.<init>(JTable.java:632)
at javax.swing.JTable.<init>(JTable.java:573)
at javax.swing.JTable.<init>(JTable.java:684)
at drafts.ClassWithTable.initComponents(JTableInstantiation.java:28)
at drafts.Parsing.doAction(JTableInstantiation.java:10)
at drafts.JTableInstantiation.main(JTableInstantiation.java:41)

Answer 1

跟踪您的字段。您有两个ClassWithTable对象。

JTableInstantiation 类

ClassWithTable vEntity = new ClassWithTable();

您这里有 vEntity。然后你就可以:

  Parsing p = new Parsing();
  p.doAction();

看看里面有什么 Parsing

private ClassWithTable c = new ClassWithTable();

您尚未对 vEntity 执行任何操作根本不。您需要分配 vEntity p.getClassWithTable() 的值或通过 vEntity至p .