Java 尝试使用if语句确定等级

创建一个允许用户输入数字0-100的项目。然后，该数字将用于确定该人员的等级。但是，如果用户未输入等级或等级大于100，该怎么办

我尝试了几种不同的编码选项

    private void DetemineGrade() {
    // Determines the grade

    String strOutputLine;
    String strGrade;

    // Number Formatting
    NumberFormat nf = NumberFormat.getNumberInstance();

    nf.setMinimumFractionDigits(2);
    nf.setMaximumFractionDigits(2);

    dblPointsTotal = Double.parseDouble(txtTotal.getText());

    strGrade = this.txtTotal.getText();
    if (strGrade == null || strGrade.trim().equals(""))
    {
        strOutputLine = ("A grade needs to be entered");
        this.txtMessage.setText(strOutputLine);
    }
    else if (dblPointsTotal < 60)
    {
        strOutputLine = ("Your grade is an F");
        this.txtMessage.setText(strOutputLine);
    }   
    else if (dblPointsTotal < 70)
    {
        strOutputLine = ("Your grade is a D");
        this.txtMessage.setText(strOutputLine);
    }       
    else if (dblPointsTotal < 80)
    {
        strOutputLine = ("Your grade is a C");
        this.txtMessage.setText(strOutputLine);
    }           
    else if (dblPointsTotal < 90)
    {
        strOutputLine = ("Your grade is a B");
        this.txtMessage.setText(strOutputLine);
    }               
    else if (dblPointsTotal >= 90)
    {
        strOutputLine = ("Your grade is an A");
        this.txtMessage.setText(strOutputLine);
    }

---

可能尝试使用异常处理，这样如果输入了无效的“数字”（或根本没有输入），程序将捕获异常并告诉用户他们的输入（或缺少输入）无效。（有关API，请参见）

正如错误所说，您正在将空字符串传递到

Double.parseDouble（txtotal.getText（））

处的

parseDouble

，并且无法将空字符串解析为数字。您正在尝试转换空字符串后检查它是否为空。只有当你确定输入不是空的时候，你才应该转换它。所以现在我所有的东西都工作了，只是它仍然会给出相同的错误。我应该在哪里放置带有strGrade的部件，这样dblPointsTotal就不会检查是否为空。很抱歉，我是java新手，对于代码中的某些内容，我仍然有很多问题

Exception in thread "AWT-EventQueue-1" java.lang.NumberFormatException: empty String
at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
at sun.misc.FloatingDecimal.parseDouble(Unknown Source)
at java.lang.Double.parseDouble(Unknown Source)
at DetermineGrade.DetemineGrade(DetermineGrade.java:100)
at DetermineGrade.actionPerformed(DetermineGrade.java:79)
at java.awt.Button.processActionEvent(Unknown Source)
at java.awt.Button.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.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)