Java：無法獲得價值

Question

常量類：

public class Constants { 
    FileHandle h = new FileHandle();
    String[] LIST_DATA3 = h.getA().split(Pattern.quote("."));
    public static  int NEW_ELEMENT_ID =0;
}

FileHandle類：

public class FileHandle {

   String a;
   String b;
   String c;
   String d;

  public void openFile() throws FileNotFoundException {        
    File dir = new File("DB");
    if (!dir.exists()){
    dir.mkdirs();
    }
   // System.out.println(dir.getAbsolutePath());
    if (dir.isDirectory()) {
        for (File file : dir.listFiles()) {
            try (Scanner s = new Scanner(file)) {
                if (s.hasNext()) {
                    this.a = (a == null) ? s.next() :a + "."+s.next();                        
                    b = (b == null) ? s.next() :b + "." + s.next();
                    c = (c == null) ? s.next() :c + "." + s.next();
                    d = (d == null) ? s.next() :d + "." + s.next();

                }else{
                s.close();
                }
            }  
        }System.out.println(a); 

    }
  }
  public String getA(){
      if(this.a !=null){
      System.out.println(this.a);
      }

   return this.a;
}
}

錯誤：

Exception in thread "main" java.lang.NullPointerException
at databasesearch.Constants.<init>(Constants.java:16)
at databasesearch.Frame.CreatingGUI(Frame.java:63)
at databasesearch.DatabaseSearch.main(DatabaseSearch.java:23)
C:\Users\D1sturbance\AppData\Local\NetBeans\Cache\8.1\executor-snippets\run.xml:53: Java returned: 1
BUILD FAILED (total time: 1 second)

框架63線：

    SwingJList<String> myJList = new SwingJList<>(Arrays.asList(con.LIST_DATA3));

我的問題（我認為，如果我錯了，請告訴我）：獲取NPE，因為Constants假設getA（）返回的是非null-它甚至沒有調用openFile（），因此在這種情況下它始終為null。 我該如何解決？

Answer 1

在您的DatabaseSearch類中編寫以下代碼：

String aaaa;
public DatabaseSearch(){

FileHandle h = new FileHandle();
    try {    
        h.openFile();
    } catch (FileNotFoundException ex) {
        Logger.getLogger(DatabaseSearch.class.getName()).log(Level.SEVERE, null, ex);
    }
 pirmas = h.a;
 System.out.println(pirmas+"cia");
}