Long to String return null (jFrame)

Question

我正在尝试在 jFrame 中使用 currentTimeMillis 创建一个计时器，但是当我单击 returns 已用时间的按钮时，返回值为 null，控制台中都没有（请参阅这个error image)的值是正常的。目前，就像您在代码中看到的那样，我正在使用 toString，尝试使用 valueOf，但收到错误 "long cannot be converted to string"。他是我代码的'piece'：

public class MainFrame extends javax.swing.JFrame {

public long startTime;
long elapsedSeconds;
long elapsedTime;    
public long secondsDisplay; // Segundos |
public long elapsedMinutes; // Minutos   } os três principais
public long elapsedHours;   // Horas    |
String ShowSeg = Long.toString(secondsDisplay);
String ShowMin = Long.toString(elapsedMinutes);
String ShowHour = Long.toString(elapsedHours);
/**
 * Creates new form SSMFrame
 */
public MainFrame() {
    initComponents();
}
@SuppressWarnings("unchecked")
//*A long code resided here*//                     
private void StoCountActionPerformed(java.awt.event.ActionEvent evt) {                                         
elapsedTime = System.currentTimeMillis() - startTime;
elapsedSeconds = elapsedTime / 1000;
secondsDisplay = elapsedSeconds % 60;
elapsedMinutes = elapsedSeconds / 60;
elapsedHours = elapsedMinutes / 60;
    ShowETseg.setText(ShowSeg);
    ShowETmin.setText(ShowMin);
    ShowEThour.setText(ShowHour);

Answer 1

获取时间后需要设置字符串变量的值
所以把你的代码改成这样：

public class MainFrame extends javax.swing.JFrame {

    public long startTime;
    long elapsedSeconds;
    long elapsedTime;    
    public long secondsDisplay; // Segundos |
    public long elapsedMinutes; // Minutos   } os três principais
    public long elapsedHours;   // Horas    |


//initialize the string variables to empty strings 
    String ShowSeg = "";//
    String ShowMin = "";//
    String ShowHour = "";//
    /**
     * Creates new form SSMFrame
     */
    public MainFrame() {
        initComponents();
    }
    @SuppressWarnings("unchecked")
    //*A long code resided here*//                     
    private void StoCountActionPerformed(java.awt.event.ActionEvent evt) {                                         
    elapsedTime = System.currentTimeMillis() - startTime;
    elapsedSeconds = elapsedTime / 1000;
    secondsDisplay = elapsedSeconds % 60;
    elapsedMinutes = elapsedSeconds / 60;
    elapsedHours = elapsedMinutes / 60;


//set the strings to their long equivalents
ShowSeg = Long.toString(secondsDisplay);
ShowMin = Long.toString(elapsedMinutes );
ShowHour = Long.toString(elapsedHours);


        ShowETseg.setText(ShowSeg);
        ShowETmin.setText(ShowMin);
        ShowEThour.setText(ShowHour);

Long to String return null (jFrame)

Long to String return null (jFrame)

java

string

jframe

long-integer