假设我正在使用 CustomDataModel 和 CustomTableListener 创建自定义 JTable。假设他们工作出色。
现在假设在实现中,保证每个 JTable 的第一行都填充有类型安全的数据,并且该数据永远不会被删除,只会被修改 - 并且永远不会设置为 null。(使用 JComboBoxes 作为编辑器,空字符串和呈现为空字符串的 0 是唯一的“空”选择)
现在;我想编写一个使用 getColumnClass 返回类型数据的方法。根据我所读到的内容,我必须结合使用以下方法:
class CustomDataModel extends AbstractTableModel {
...
//implement data struc and other methods (full testable code further down below in the SSCCE)
...
/**
* Guaranteed to return a class based on this table's construction
* @param c
* @return
*/
@Override
public Class getColumnClass(int c){
return getValueAt(0,c).getClass();
}
...
public <T> T returnType(int row, int column) {
//the following will not compile - and I'm stuck, don't know how to
//use getColumnClass to return type-specific data
return getColumnClass(column).cast(getValueAt(row,column));
}
}
NetBeans 告诉我强制转换调用返回 Object,但我确信cast(Object obj) 返回 T,其中 T 是强制转换中 this 的类型。
我越想越觉得我想要的东西是不可能的。这并不是真正必要的,但它会避免强制转换 - 尽管我认为如果我当前的实现“固定”并通过手动强制转换完成类型检索,这将强制执行强制转换。
现在;在 SSCCE 中,我使用 system.out.println 进行打印 - 它接受一个对象引用并调用其 toString() 方法,但我想要采取的方法或操作不一定希望绑定(bind)到对象参数。重点是直接获取存储的数据的类型;如果它存储为对象引用而不将其转换回其原始类型,我想这实际上是不可能的......除非使用泛型?我不知道 - 任何帮助表示赞赏!
SSCCE
//package utility;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.table.AbstractTableModel;
/**
* @author Sean
*/
public class UnitTesting extends JPanel{
public UnitTesting() {
super(new GridLayout(1,0));
JTable table = createAJTable();
table.getTableHeader().setReorderingAllowed(false);
table.setPreferredScrollableViewportSize(new Dimension(500, 70));
table.setFillsViewportHeight(true);
//Create the scroll pane and add the table to it.
JScrollPane scrollPane = new JScrollPane(table);
//Normally here JComboBoxes are set as the Editors - SSCCE will use dummy test data
//Add the scroll pane to this panel.
add(scrollPane);
//Test the data Types - I want to retrieve type-specific data to avoid casting
CustomModel dataModel = (CustomModel)table.getModel();
Object val;
for(int row = 0; row < dataModel.getRowCount(); row++){
//Row possibility
doSomeThings(row, dataModel);
for(int col = 0; col < dataModel.getColumnCount(); col++){
//Here's the old way of going about this. (also could invoke getColumnClass())
val = dataModel.getValueAt(row, col);
System.out.println(val + " : " + val.getClass());
//Overloaded possibility - needs typed data
// doSomeAction(dataModel.typedValueAt(row, col));
}
}
}
private JTable createAJTable() {
return new JTable(new CustomModel(new String[]{"Col1", "Col2", "Col3", "Col4", "Col5"}));
}
private void doSomeAction(String str){
//Do things with strings
}
private void doSomeAction(int number){
//Do things with integers
}
private void doSomeThings(int row, CustomModel dataModel) {
String col1Data, col2Data, col5Data;
int col3Num, col4Num;
//Retrieve data and store it in typed local variable (need casting or typed retrieval)
//Do things with the Strings and Integers together
}
/**
* Create the GUI and show it. For thread safety,
* this method should be invoked from the
* event-dispatching thread.
*/
private static void createAndShowGUI() {
//Create and set up the window.
JFrame frame = new JFrame("TableDemo");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//Create and set up the content pane.
UnitTesting newContentPane = new UnitTesting();
newContentPane.setOpaque(true); //content panes must be opaque
frame.setContentPane(newContentPane);
//Display the window.
frame.pack();
frame.setVisible(true);
}
public static void main(String[] args) {
/*
* Set Look and feel here; taken out for SSCCE
*/
//Schedule a job for the event-dispatching thread:
//creating and showing this application's GUI.
javax.swing.SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
createAndShowGUI();
}
});
}
class CustomModel extends AbstractTableModel {
//Dummy data for the SSCCE
private List<String> columnHeadings;
private List<List<Object>> data;
CustomModel(String[] cols){
data = new ArrayList<>();
columnHeadings = Arrays.asList(cols);
ArrayList<Object> testRow = new ArrayList<>();
testRow.add("String");
testRow.add("Str");
testRow.add(0);
testRow.add(5);
testRow.add("Strong");
data.add(testRow);
}
//This is the method I want to create
// public <T> T typedValueAt(int row, int column) {
// return getColumnClass(column).cast(getValueAt(row,column));
// }
@Override
public int getRowCount() {
return data.size();
}
@Override
public int getColumnCount() {
return columnHeadings.size();
}
@Override
public String getColumnName(int columnIndex) {
return columnHeadings.get(columnIndex);
}
@Override
public Class<?> getColumnClass(int c) {
return getValueAt(0,c).getClass();
}
@Override
public boolean isCellEditable(int rowIndex, int columnIndex) {
//For the SSCCE we'll just test with one row of uneditable data
return false;
}
@Override
public Object getValueAt(int rowIndex, int columnIndex) {
return data.get(rowIndex).get(columnIndex);
}
@Override
public void setValueAt(Object aValue, int rowIndex, int columnIndex) {
//Normally check for row existence and populate necessary rows - but for SSCCE just this will suffice
data.get(rowIndex).set(columnIndex, aValue);
}
}
}
最佳答案
首先,getColumnClass(int)
应该返回一个带有泛型类型信息的类,否则该函数的静态返回类型实际上将是 Class<Object>
因此它是 cast(Object)
函数将具有返回类型 Object。但是,这并不能解决你的问题。问题的根源在于getValueAt(int,int)
的AbstractTableModel
除 Object 之外不携带任何静态类型信息(即其返回类型为 Object)。
仔细想想,getValueAt(int,int)
无法返回除 Object 之外的任何静态类型,因为它适用于表中的每一列,并且每一列都可以具有不同的类型。事实上,您可以在运行时添加在编译时甚至没有想到的列。如何静态确定这些列的类型?
您很可能不得不求助于使用 instanceof
运算符和运行时强制转换或重新实现 JTable 框架的大部分,以便为每列添加静态类型信息(并且很可能失去在运行时添加列的能力)。
关于java - JTable 的自定义 DataModel 返回类型安全值,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/20101766/