嘗試在com.comp.Project上的空對象引用上調用虛擬方法'...'

Question

調試時出現錯誤，我不知道爲什麼。

錯誤：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void layout.CardFragment.setCard(com.mycomp.MyCards.Card)' on a null object reference at com.mycomp.MyCards.MainActivity.onCreate(MainActivity.java:16) 

我認爲這是說，我試圖通過進入cardFragment.setCard(Card card)的Card對象爲null。我已經檢查過，它不是空的。

MainActivity.java

1 package com.mycomp.MyCards; 
2 
3 import android.support.v7.app.AppCompatActivity; 
4 import android.os.Bundle; 
5 
6 public class MainActivity extends AppCompatActivity { 
7  public Card cardApples; 
8 public CardFragment cardFragment = (CardFragment) 
9    getSupportFragmentManager().findFragmentById(R.id.frgCard); 
10 
11  @Override 
12  protected void onCreate(Bundle savedInstanceState) { 
13   super.onCreate(savedInstanceState); 
14   setContentView(R.layout.activity_main); 
15   createDummyCards(); 
16 ---> cardFragment.setCard(cardApples); 
17  } 
18 
19  public void createDummyCards() { 
20   cardApples = new Card(new SideText("Apple"), new SideText("Red")); 
21  } 
22 } 

Card.java

package com.mycomp.MyCards; 

public class Card { 
    public Side[] sides; 

    public Card(Side...sides) { 
     this.sides = sides; 
    }  
} 

Side.java

package com.mycomp.MyCards; 

public class Side {} 

SideText.java

package com.mycomp.MyCards; 

public class SideText extends Side{ 
    public String text; 

    public SideText(String text) { 
     this.text = text; 
    } 
} 

CardFragment.java

package com.midassoft.testingerror; 

import android.content.Context; 
import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.TextView; 

public class CardFragment extends Fragment { 
    private Card card; 
    private int sideIndex; 

    private static TextView lblText; 

    @Override 
    public void onAttach(Context context) { 
     super.onAttach(context); 
    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
      Bundle savedInstanceState) { 
     View view = inflater.inflate(R.layout.fragment_card, container, false); 
     lblText = (TextView) view.findViewById(R.id.lblText); 
     return view; 
    } 

    public void setCard(Card card) { 
     this.card = card; 
     sideIndex = 0; 
     showSide(); 
    } 

    private void showSide() { 
     Side side = card.sides[sideIndex]; 
     switch (side.getClass().getName()) { 
      case "SideText": 
       lblText.setText(((SideText) side).text); 
       break; 
      case "SideMultipleChoice": 
       ///do stuff 
       break; 
      case "SideTrueFalse": 
       ///do stuff 
       break; 
      default: break; 
     } 
    } 
} 

Answer 1

嘗試以下之一：

public CardFragment cardFragment; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    createDummyCards(); 
    cardFragment = (CardFragment) 
     getSupportFragmentManager().findFragmentById(R.id.frgCard); 
    cardFragment.setCard(cardApples); 
}