java - JFreeChart 从另一个 JDialog 接收方程产生 "AWT-EventQueue-0"java.lang.NullPointerException

Question

问题是，在将整个方程发送到 JFreeChart here 时遇到问题我试图仅发送 x^3、x^2... 等参数。

仍然出现错误:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at calc.kalkulator.OknoFunkcji.getDane(OknoFunkcji.java:24)
at calc.kalkulator.OknoFunkcji.<init>(OknoFunkcji.java:37)
at calc.kalkulator.PanelFunkcji.actionPerformed(PanelFunkcji.java:103)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.WaitDispatchSupport$2.run(Unknown Source)
at java.awt.WaitDispatchSupport$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.awt.WaitDispatchSupport.enter(Unknown Source)
at java.awt.Dialog.show(Unknown Source)
at java.awt.Component.show(Unknown Source)
at java.awt.Component.setVisible(Unknown Source)
at java.awt.Window.setVisible(Unknown Source)
at java.awt.Dialog.setVisible(Unknown Source)
at calc.kalkulator.mainApp.actionPerformed(mainApp.java:299)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.AbstractButton.doClick(Unknown Source)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(Unknown Source)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

为什么我不能使用解析的 double ？当我使用代码中设置的变量作为某些值时，它可以工作...这是我的 JDialog 代码 (PanelFunkcji.java):

    package calc.kalkulator;

 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;

 import javax.swing.JButton;
 import javax.swing.JComboBox;
 import javax.swing.JDialog;
 import javax.swing.JFrame;
 import javax.swing.JLabel;
 import javax.swing.JTextField;

 class PanelFunkcji extends JDialog implements ActionListener {
private JLabel lA, lB, lC, lD;
private JTextField tA, tB, tC, tD;
private JButton bOK, bCancel;
private OknoFunkcji oknoFunkcji;
private JComboBox comboBox;
public double dA, dB, dC, dD;






public PanelFunkcji(JFrame owner) {
    super(owner, "Wprowadzanie funkcji", true);
    setSize(300, 300);
    setLayout(null);

    comboBox = new JComboBox();
    comboBox.setBounds(10, 10, 270, 20);
    add(comboBox);
    comboBox.setVisible(false);

    bOK = new JButton("OK");
    bOK.setBounds(10, 40, 130, 20);
    add(bOK);
    bOK.addActionListener(this);

    bCancel = new JButton("Anuluj");
    bCancel.setBounds(150, 40, 130, 20);
    add(bCancel);
    bCancel.addActionListener(this);

    lA = new JLabel("A = ");
    lA.setBounds(10, 70, 40, 20);
    add(lA);

    tA = new JTextField("0");
    tA.setBounds(50, 70, 60, 20);
    add(tA);

    lB = new JLabel("B = ");
    lB.setBounds(10, 100, 40, 20);
    add(lB);

    tB = new JTextField("0");
    tB.setBounds(50, 100, 60, 20);
    add(tB);

    lC = new JLabel("C = ");
    lC.setBounds(10, 130, 40, 20);
    add(lC);

    tC = new JTextField("0");
    tC.setBounds(50, 130, 60, 20);
    add(tC);

    lD = new JLabel("D = ");
    lD.setBounds(10, 160, 40, 20);
    add(lD);

    tD = new JTextField("0");
    tD.setBounds(50, 160, 60, 20);
    add(tD);

}

public double getA() {
    //if (Double.parseDouble(tA.getText()) != null)
    return Double.parseDouble(tA.getText());
    //else return 0;
}

public double getB() {
    return Double.parseDouble(tB.getText());
}

public double getC() {
    return Double.parseDouble(tC.getText());
}

public double getD() {
    return Double.parseDouble(tD.getText());
}

@Override
public void actionPerformed(ActionEvent e) {
    Object z = e.getSource(); 
    if(z == bOK) {
        if (oknoFunkcji == null) {
            oknoFunkcji = new OknoFunkcji("Wykres");
        }
        oknoFunkcji.setVisible(true);
        setVisible(false);
    }
    else if (z == bCancel) {
        setVisible(false);          
    }

}



}

这是我修改后用于我的应用程序的示例:

 package calc.kalkulator;

import javax.swing.JPanel;

import org.jfree.chart.ChartFactory;
import org.jfree.chart.ChartPanel;
import org.jfree.chart.JFreeChart;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.chart.plot.XYPlot;
import org.jfree.data.function.Function2D;
import org.jfree.data.general.DatasetUtilities;
import org.jfree.data.xy.XYDataset;
import org.jfree.ui.ApplicationFrame;

class OknoFunkcji extends ApplicationFrame {

private static PanelFunkcji panelFunkcji;
private static double a;
private static double b;
private static double c;
private static double d;

public void getDane () {
    a = panelFunkcji.getA();
    b = panelFunkcji.getB();
    c = panelFunkcji.getC();
    d = panelFunkcji.getD();
}


public OknoFunkcji(String title) {
    super(title);
    JPanel chartPanel = createDemoPanel();
    //chartPanel.setPreferredSize(new java.awt.Dimension(500, 270));
    chartPanel.setSize(500, 270);
    setContentPane(chartPanel);
    getDane();




}

/**
 * Tworzy chart.
 * 
 * @param dataset - zbiór danych.
 * 
 * @return Zwraca instancję charta.
 */
private static JFreeChart createChart(XYDataset dataset) {
    // create the chart...
    JFreeChart chart = ChartFactory.createXYLineChart(
        " ",       // chart title
        "X",                      // x axis label
        "Y",                      // y axis label
        dataset,                  // data
        PlotOrientation.VERTICAL,  
        true,                     // include legend
        true,                     // tooltips
        false                     // urls
    );

    XYPlot plot = (XYPlot) chart.getPlot();
    plot.getDomainAxis().setLowerMargin(0.0);
    plot.getDomainAxis().setUpperMargin(0.0);
    return chart;
}

/**
 * Creates a sample dataset.
 * 
 * @return A sample dataset.
 */
public static XYDataset createDataset() {
    XYDataset result = DatasetUtilities.sampleFunction2D(new X2(), 
            -10.0, 10.0, 40, "f(x)");
    return result;
}

public static JPanel createDemoPanel() {
    JFreeChart chart = createChart(createDataset());
    return new ChartPanel(chart);
}

static class X2 implements Function2D {

    public double getValue(double x) {
        return a*x*x*x + b*x*x + c*x + d;
    }

}



}

出于某种奇怪的原因，无论我做什么，我都会得到 NullPointerException...

Answer 1

错误在这段代码中:

private static PanelFunkcji panelFunkcji;   //1
private static double a;
private static double b;
private static double c;
private static double d;

public void getDane()
{
    a = panelFunkcji.getA();   //9
    b = panelFunkcji.getB();
    c = panelFunkcji.getC();
    d = panelFunkcji.getD();
}

public OknoFunkcji(String title)
{
    super(title);
    JPanel chartPanel = createDemoPanel();
    //chartPanel.setPreferredSize(new java.awt.Dimension(500, 270));
    chartPanel.setSize(500, 270);
    setContentPane(chartPanel);
    getDane();    //22
}

您在第 1 行声明了变量 panelFunkcji，并在第 9 行使用它。但是，您从未实例化该变量。这就是导致 NullPointerException 的原因。

要修复它，请添加以下行

panelFunkcji = new PanelFunkcji(chartPanel);

OknoFunkcji 构造函数第 22 行之前的任何位置。