單擊按鈕時出現NullPointerException

-3

我在程序中出現nullpointerexception錯誤，但在代碼中沒有錯誤。我不明白我錯在哪裏。有人可以檢查我嗎？我的程序工作正常，直到我按下「查看購物車」按鈕，nullpointerexception出來。提前致謝！單擊按鈕時出現NullPointerException

public class ShoppingCart extends Activity implements OnClickListener{ 

private List<Product> mCartList; 
private ProductAdapter mProductAdapter; 

Button btn = (Button) findViewById(R.id.Checkout); 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.shoppingcart); 

mCartList = ShoppingCartHelper.getCartList(); 

// Make sure to clear the selections 
for (int i = 0; i < mCartList.size(); i++) { 
      mCartList.get(i).selected = false; 
    } 

// Create the list 
final ListView listViewCatalog = 
    (ListView) findViewById(R.id.ListViewCatalog); 
mProductAdapter = new ProductAdapter(mCartList, getLayoutInflater(),true); 
listViewCatalog.setAdapter(mProductAdapter); 

listViewCatalog.setOnItemClickListener(new OnItemClickListener() { 

@Override 
public void onItemClick(AdapterView<?> parent, View view,int position, long id) { 
Intent productDetailsIntent = new Intent(getBaseContext(), ProductDetails.class); 

    productDetailsIntent.putExtra(ShoppingCartHelper.PRODUCT_INDEX, position); 
      startActivity(productDetailsIntent); 
     } 
      }); 
        } 

@Override 
protected void onResume() { 
    super.onResume(); 

    // Refresh the data 
    if (mProductAdapter != null) { 
     mProductAdapter.notifyDataSetChanged(); 
    } 

    double subTotal = 0; 
    for (Product p : mCartList) { 
     int quantity = ShoppingCartHelper.getProductQuantity(p); 
     subTotal += p.price * quantity; 
    } 

TextView productPriceTextView = (TextView)findViewById(R.id.TextViewSubtotal); 
    productPriceTextView.setText("Subtotal: $" + subTotal); 
      } 

    @Override 
    public void onClick(View v) { 
     btn.setOnClickListener(new View.OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      myClick(v); /* my method to call new intent or activity */ 
       } 

      public void myClick(View v) { 
       Intent intent = new Intent(null, MainActivity.class); 
       startActivity(intent);// for calling the activity 
      }; 
     }); 

    } 
      }

來源

2014-01-05 KnockFall

你確定你能夠得到btn的參考。 – Bennet

*「但我的代碼中沒有錯誤」* ...您的計算機不同意。 –

'Button btn =（Button）findViewById（R.id.Checkout）;'setContentView'錯誤之前。看下面的答案會解決它。 – Raghunandan

當前您在調用setContentView之後，正在初始化Button級別而不是任何方法。這樣做：

Button btn; // declare here 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.shoppingcart); 
    btn = (Button) findViewById(R.id.Checkout);//<<initialize after setContentView 
//...your code 
}

來源

2014-01-05 04:59:24

啊！我現在知道了。非常感謝你的幫助！ – KnockFall

單擊按鈕時出現NullPointerException

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