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[英]I need help with a program ia m writing in JAVA ( nullpointer exception)
[英]I need help. I'm writing a program that has a GUI
用戶將輸入一個介於1到99之間的數字。我已經編寫了大多數代碼,只是想不出要使用“計算”按鈕進行計算。 我真的可以使用一些逐步的幫助,我願意了解是否有人可以幫助我。 我是一個初學者。
package minimumchris;
import javax.swing.JButton;
import javax.swing.JTextField;
public class useri extends javax.swing.JFrame {
public useri() {
initComponents();
}
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {
jScrollPane1 = new javax.swing.JScrollPane();
jLabel1 = new javax.swing.JLabel();
jTextField1 = new javax.swing.JTextField();
jLabel2 = new javax.swing.JLabel();
jLabel3 = new javax.swing.JLabel();
jLabel4 = new javax.swing.JLabel();
jLabel5 = new javax.swing.JLabel();
jButton1 = new javax.swing.JButton();
jButton2 = new javax.swing.JButton();
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
jLabel1.setText("enter a number between 1-99");
jLabel2.setText("quarters");
jLabel3.setText("dimes");
jLabel4.setText("nickels");
jLabel5.setText("pennies");
jButton1.setText("calculate");
jButton1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
jButton1ActionPerformed(evt);
}
});
jButton2.setText("clear");
org.jdesktop.layout.GroupLayout layout = new org.jdesktop.layout.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
.add(layout.createSequentialGroup()
.add(layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
.add(layout.createSequentialGroup()
.addContainerGap()
.add(jLabel1, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, 197, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE)
.addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
.add(jTextField1, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, 47, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE))
.add(layout.createSequentialGroup()
.add(66, 66, 66)
.add(layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
.add(jLabel5)
.add(jLabel4)
.add(jLabel3)
.add(jLabel2))
.add(0, 0, Short.MAX_VALUE)))
.addContainerGap())
.add(layout.createSequentialGroup()
.addContainerGap()
.add(jButton1, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, 195, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE)
.addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED)
.add(jButton2, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, 177, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE)
.addContainerGap(16, Short.MAX_VALUE))
);
layout.setVerticalGroup(
layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
.add(layout.createSequentialGroup()
.addContainerGap()
.add(layout.createParallelGroup(org.jdesktop.layout.GroupLayout.TRAILING)
.add(jTextField1, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE)
.add(jLabel1))
.add(32, 32, 32)
.add(jLabel2)
.add(18, 18, 18)
.add(jLabel3)
.addPreferredGap(org.jdesktop.layout.LayoutStyle.UNRELATED)
.add(jLabel4)
.addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED)
.add(jLabel5)
.add(29, 29, 29)
.add(layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING, false)
.add(jButton2, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, 73, Short.MAX_VALUE)
.add(jButton1, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
.addContainerGap(32, Short.MAX_VALUE))
);
pack();
}// </editor-fold>
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
int cntrquart = 0;
int cntrdimes = 0;
int cntrpennies = 0;
int cntrnickels = 0;
JTextField jTextField=new JTextField(); // initialize textFild
JButton calculate =jButton1;
int quarters = Integer.parseInt(jTextField.getText());
while (quarters >= 25)
{
quarters = quarters - 25;
cntrquart++;
}
//dimes
int dimes = quarters;
while (dimes >= 10)
{
dimes = dimes - 10;
cntrdimes++;
}
//nickels
int nickels = dimes;
while (nickels >= 5)
{
nickels = nickels - 5;
cntrnickels++;
}
//pennies
int pennies = nickels;
while (pennies >= 1)
{
pennies = pennies - 1;
cntrpennies++;
}
jLabel2.setText("Quarter" + cntrquart );
jLabel3.setText("Dimes:" + cntrdimes );
jLabel4.setText("Nickels:" + cntrnickels );
jLabel5.setText("Pennies:" + cntrpennies );
}
/**
* @param args the command line arguments
*/
public static void main(String args[]) {
/* Set the Nimbus look and feel */
//<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
/* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
* For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
*/
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(useri.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
java.util.logging.Logger.getLogger(useri.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
java.util.logging.Logger.getLogger(useri.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(useri.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}
//</editor-fold>
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
new useri().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JButton jButton1;
private javax.swing.JButton jButton2;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JLabel jLabel3;
private javax.swing.JLabel jLabel4;
private javax.swing.JLabel jLabel5;
private javax.swing.JScrollPane jScrollPane1;
private javax.swing.JTextField jTextField1;
// End of variables declaration
}
問題是按下“計算”時您正在使用全新的JTextField:
JTextField jTextField=new JTextField(); // useless
JButton calculate =jButton1; // useless
int quarters = Integer.parseInt(jTextField.getText()); // should use jTextField1 (probably)
就像RC所說的那樣:
JTextField jTextField=new JTextField();
您正在創建一個不需要的新JTextField對象。 由於您擁有的變量是類實例,因此可以這樣直接引用它們:
int quaters = Integer.parseInt(jTextField1.getText());
創建新變量意味着不共享該變量的完整的新參數集,並且不共享實例特定的數據(例如JTextField
框內的內容)。 由於(大概)用文本填充了JTextField jTextField1
,因此只能通過在jTextField1
對象本身上調用getText()
來檢索文本。
除了重點,我可能建議使用更直觀的變量名,並且僅復制代碼的相關部分。 這有助於我們更快地找到您的問題。 還請針對問題本身設置問題標題,“我需要幫助”不會助您一臂之力。
最后一點,如果您使用簡單的除法/ mod操作替換循環,則可以進一步優化和簡化代碼。 代替:
while (quarters >= 25){
quarters = quarters - 25;
cntrquart++;
}
int dimes = quarters;
//...
做:
cntrlquart = quarters / 25; //Integer division
int dimes = quarters % 25; //Mod operation, remainder of dividing by 25.
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