java - 将字符串转换为整数时出现异常

Question

我是 Java 初学者。我使用 Netbeans。以下是我的代码片段，用于制作一个简单的计算器来添加两个数字。不包括按钮的自生成代码。

问题

当我尝试在函数 plus_buttonActionPerformed 和 equal_buttonActionPerformed 中将字符串 str 转换为整数 num1 时给出一个异常，指出:

 exception in thread awt-eventqueue-0 java.lang.numberformatexception for input string  

我通过将字符串打印在转换语句上方来确保该字符串不为空。代码有点长。赦免。我在这里做错了什么。

 public class calc extends javax.swing.JFrame {

/**
 * Creates new form calc
 */
public String str = " "; 
public String action = " ";
public int num1;
public int num2;
public int res;
public calc() {
    initComponents();
}
  private void button3ActionPerformed(java.awt.event.ActionEvent evt) {                                        
    str=str.concat("3");

    result.setText(str);

}                                       

private void button6ActionPerformed(java.awt.event.ActionEvent evt) {                                        
 str=str.concat("6"); 
 result.setText(str);

}                                       

private void button9ActionPerformed(java.awt.event.ActionEvent evt) {                                        
 str=str.concat("9");
   result.setText(str);
}                                       

private void resultActionPerformed(java.awt.event.ActionEvent evt) {                                       

   result.setText(str);// 
}                                      

private void button1ActionPerformed(java.awt.event.ActionEvent evt) {                                        
   str=str.concat("1");
   result.setText(str);//
}                                       

private void button2ActionPerformed(java.awt.event.ActionEvent evt) {                                        
     str=str.concat("2");
     result.setText(str);
}                                       

private void button4ActionPerformed(java.awt.event.ActionEvent evt) {                                        
    str=str.concat("4"); // 
    result.setText(str);
}                                       

private void button5ActionPerformed(java.awt.event.ActionEvent evt) {                                        
   str=str.concat("5");  // 
   result.setText(str);
}                                       

private void button7ActionPerformed(java.awt.event.ActionEvent evt) {                                        
 str=str.concat("7");   
 result.setText(str);//
}                                       

private void button8ActionPerformed(java.awt.event.ActionEvent evt) {                                        
 str=str.concat("8");  
 result.setText(str);//
}                                       

private void plus_buttonActionPerformed(java.awt.event.ActionEvent evt) {                                            
   System.out.println(str);
    num1=Integer.parseInt(str);
    System.out.println(num1);
    str=" ";

}                                           

private void equal_buttonActionPerformed(java.awt.event.ActionEvent evt) {                                             
    num2=Integer.parseInt(str);
    res=num1+num2;
    str=""+res;
    result.setText(str);
}                                            


public static void main(String args[]) {


    try {
        for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
            if ("Nimbus".equals(info.getName())) {
                javax.swing.UIManager.setLookAndFeel(info.getClassName());
                break;
            }
        }
    } catch (ClassNotFoundException ex) {
        java.util.logging.Logger.getLogger(calc.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(calc.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(calc.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(calc.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    }
    //</editor-fold>

    /* Create and display the form */
    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new calc().setVisible(true);
        }
    });
}

Answer 1

作为docs state ，在以下情况下会发生 NumberFormatException:

Thrown to indicate that the application has attempted to convert a string to one of the numeric types, but that the string does not have the appropriate format.

您尝试解析的字符串格式不是有效的整数。事实上，你的字符串的值是一个空格。

首先，您可以使用trim()删除字符边缘的空白。即使将其初始化为空字符串:String str = ""; 也会发生 NumberFormatException，因为它不是有效的整数。

因此，为其分配一个非空整数值，它将正确解析。