如何將對象添加到數據結構中的數組列表？

Question

歡迎來到我的難題;）在Java中我有一個問題..我有我的具體清單實施商店對象

我能做些什麼來解決這個問題？任何身體都可以幫助我並拯救我的一天？

public class Alist implements ListInterface { 

     private Object entry[]; // array of list entries 
    static public int length; // current number of entries in list 
     private static final int MAX_SIZE = 50; // max length of list 
public Alist() 
     { length=0; 
     entry= new Object [ MAX_SIZE ]; 
     }// end default constructor 

public Alist (int maxSize) 
     { length=0; 
     entry= new Object[ maxSize ]; 
     }// end user constructor 

public boolean add (Object newEntry) 
     { 
     Boolean isSuccessful =true; 
     if(!isFull()) 
     { 
      entry [length]=newEntry; 
      length++; 
     } 
     else 
     isSuccessful =false; 
     return isSuccessful; 
     }//end add 

public boolean add (int newPosition, Object newEntry) 
     { boolean isSuccessful =true; 
      if((! isFull() && newPosition >=0)&&(newPosition<=length)) 
      { if (newPosition < length)  // optional 
       makeRoom (newPosition);// private method that helps to shift items 
                    //in order to make space for the new entry; 
       entry [newPosition]=newEntry; 
       length++; 
      } 
      else 
      isSuccessful =false; 
      return isSuccessful; 
    }//end add 

private void makeRoom (int newPosition) 
     { for (int index=length; index> newPosition; index--) 
      entry [index]=entry[index-1]; 
     }//end makeRoom 

public Object remove (int givenPosition) 
     { Object result=null; // returned value 
      if(! isEmpty() &&(givenPosition >=0)&&(givenPosition<length)) 
       { result= entry [givenPosition]; 
        if (givenPosition <length-1)  // optional 
        removeGap (givenPosition); // private method that helps to shift 
                      // items in order to remove space 
                      // after removing the entry; 
        length--; 
       } 
      return result; 
     }//end remove 

private void removeGap (int givenPosition) 
     { for (int index= givenPosition; index <length-1;index++) 
       entry [index]=entry[index+1]; 
     }//end removeGap 

public boolean replace (int givenPosition, Object newEntry) 
     { boolean isSuccessful = true; 
     if (! isEmpty() &&(givenPosition >= 0) && (givenPosition < length)) 
      entry [givenPosition] = newEntry; 
     else 
      isSuccessful = false; 
     return isSuccessful; 
     } // end replace 

public Object getEntry (int givenPosition) 
     { 
     Object result = null; // result to return 
     if (! isEmpty() &&(givenPosition >= 0) && (givenPosition < length)) 
      result = entry [givenPosition]; 
     return result; 
     } // end getEntry 

public boolean contains (Object anEntry) 
     { boolean found = false; 
     for (int index = 0; !found && (index < length); index++){ 
      if (anEntry.equals (entry [index])) 
      found = true; 
     } // end for 
     return found; 
     } // end contains 

public int getLength() 
     { return length; 
     }//end getLength 

public void clear() 
     { length=0; 
     } 

public boolean isEmpty() 
     { boolean itEmpty =false; 
      if (length ==0)      // return length==0; 
      itEmpty =true; 
      return itEmpty; 
     }//end isEmpty 

public boolean isFull() 
     { boolean itFull =false;   // return length = = entry. length; 
     if (length== entry. length) 
      itFull =true; 
     return itFull; 
     }// end isFull 

public void display() 
     { for (int index=0;index< length; index++) 
      System.out.println (entry [index]); 
     } 
}//end of class AList 

在應用層

..當我將數據存儲到我的數組這樣

static Alist Amman = new Alist(); 


public static void main(String[] args) { 

    blablablablabla 

Amman.add(x); 

出現錯誤說：

Exception in thread "main" java.lang.NullPointerException 
     at Cleint.main(Cleint.java:93) 

Answer 1

你可能訪問列表的無效位置，嘗試在方法isFull（）中使用（entry.length - 1）。另外，您可以使用> =而不是==，只是爲了安全編程。

public boolean isFull() 
    {   
    return length >= (entry.length - 1); 
    }