空指針異常

Question

我想通過調用另一個類（庫）的printlibrary方法來顯示我的ArrayList在動作偵聽器方法中的元素。我一直在這條線上得到一個空指針異常library.addItem(libraryItem);我不知道如何解決這個問題 我已經使用if語句形式的驗證來查看庫是否爲null，並且使用system.out來查看按鈕動作列表是否爲空實際上工作和按鈕確實工作

 { 

    {System.out.println("hello world"); 
     String t = title.getText();  
     int y = Integer.parseInt(year.getText()); 
     int q = Integer.parseInt(quantity.getText()); 

       if(library != null) 
       { 
     LibraryItem libraryItem = new LibraryItem(t,y,q); 
     library.addItem(libraryItem); 
       } 
       else 
       { 
        System.out.println("its null"); 
       } 


library.printLibrary(); 
    } 

這裏是我的LIBRARYFRAME類的休息！

import javax.swing.JFrame; 
    import javax.swing.JButton; 
    import java.awt.GridLayout; 
    import java.awt.event.ActionListener; 
    import java.awt.event.ActionEvent; 
    import javax.swing.JTextField; 
    import javax.swing.JLabel; 



    public class LibraryFrame extends JFrame implements ActionListener 
{ 
    JLabel label, label2, label3; 
    JTextField title, year, quantity; 
    JButton button, button2; 

     private Library library; 



public LibraryFrame() 
{ 
    this.setLayout(new GridLayout(4,2,10,10)); //2nd set of co-ord (10,10) is for row and coloumn spacing. 

    label = new JLabel("Title"); 
    title = new JTextField(20); 

    label2 = new JLabel("Year"); 
    year = new JTextField(20); 

    label3 = new JLabel("Quantity"); 
    quantity = new JTextField(20); 

    button = new JButton("Add"); 
    button2 = new JButton("Search"); 
    button.addActionListener(this); 


    this.add(label); 
    this.add(title); 
    this.add(label2); 
    this.add(year); 
    this.add(label3); 
    this.add(quantity); 
    this.add(button); 
    this.add(button2); 

} 


public void actionPerformed(ActionEvent ev) 
{ 



    {System.out.println("hello world"); 
     String t = title.getText();  
     int y = Integer.parseInt(year.getText()); 
     int q = Integer.parseInt(quantity.getText()); 

       if(library != null) 
       { 
     LibraryItem libraryItem = new LibraryItem(t,y,q); 
     library.addItem(libraryItem); 
       } 
       else 
       { 
        System.out.println("its null"); 
       } 


library.printLibrary(); 
    } 

} 







public static void main (String args[]) 
{ 

    LibraryFrame frame = new LibraryFrame(); 
    frame.pack(); 
    frame.setVisible(true); 




} 

} 

這是我的庫類

import java.util.ArrayList; 


       public class Library { 

     private ArrayList<LibraryItem> items; 




public Library() 
{ 
    items = new ArrayList<LibraryItem>(); 
} 



public void addItem(LibraryItem newItem) 
{ 
    items.add(newItem); 



} 

public LibraryItem searchForItem (String name) 

{ 
    for(LibraryItem searchForItem: items) 
    { 
     if(searchForItem.getName().equals(name)) 

    return searchForItem; 


} 
    return null; 
} 

public void printLibrary() 
{ 


    for(int i = 0; i< items.size(); i++) 
    { 

     System.out.println(items.get(i)); 
    } 


} 

}

這是我的庫項目CLASS`

public class LibraryItem 
{ 

    private String name; 
    private int year, quantity; 

     LibraryItem(String nameIn, int yearIn, int quantityIn) 
     { 
      name = nameIn; 
      year = yearIn; 
      quantity = quantityIn; 



     } 



    public boolean rent() 
    { 


     if(quantity > 0) 

     { 
      quantity--; 

     } 
      return true; 
    } 






        public String toString() 
      { 
       return name + " " +"has"+ " " +quantity+ "available"; 


       } 

        public String getName() 
      { 
        return name; 

      } 


      public int getQuantity() 
      { 

      return quantity; 
     } 

       public int getYear() 
      { 

        return year; 
      } 


    } 

這是我的成員類`

    public class Member 
      { 

    private String name ; 
     private int dayOfBirth, monthOfBirth, yearOfBirth; 



    Member(String nameIn, int dayOfBirthIn, int monthOfBirthIn, int yearOfBirthIn) 
    { 
     name = nameIn; 
     dayOfBirth = dayOfBirthIn; 
     monthOfBirth = monthOfBirthIn; 
     yearOfBirth = yearOfBirthIn; 



    } 



      public String toString() 
    { 
    return name + " " +dayOfBirth+ " " +monthOfBirth+ " " +yearOfBirth; 

     } 




} 

`

Answer 1

在嘗試使用它之前，您似乎沒有將庫分配給任何內容。