構造函數中的NullPointerException

我已經將我的問題縮小到了我的GUI中的一行Java代碼。構造函數中的NullPointerException

Shop1.addProduct(new Product(proid, proName, proPrice, proQis));

每次我點擊我已附加到的按鈕。這不是按鈕，沒有它就運行良好。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
    at ShopFrame$AddListener.actionPerformed(ShopFrame.java:56) 
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) 
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source) 
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source) 
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source) 
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source) 
    at java.awt.Component.processMouseEvent(Unknown Source) 
    at javax.swing.JComponent.processMouseEvent(Unknown Source) 
    at java.awt.Component.processEvent(Unknown Source) 
    at java.awt.Container.processEvent(Unknown Source) 
    at java.awt.Component.dispatchEventImpl(Unknown Source) 
    at java.awt.Container.dispatchEventImpl(Unknown Source) 
    at java.awt.Component.dispatchEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) 
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source) 
    at java.awt.Container.dispatchEventImpl(Unknown Source) 
    at java.awt.Window.dispatchEventImpl(Unknown Source) 
    at java.awt.Component.dispatchEvent(Unknown Source) 
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source) 
    at java.awt.EventQueue.access$200(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
    at java.awt.EventDispatchThread.run(Unknown Source)

我不能爲我的生活找出問題。

來源

2013-01-13 user1974447

看來Shop1是null。嘗試添加空值檢查：

if(Shop1 != null) Shop1.addProduct(new Product(proid, proName, proPrice, proQis));

另外，請注意遵循Java命名約定，不要使用大寫字母來啓動變量。而不是Shop1，請使用shop1。

來源

2013-01-13 17:14:10

如果你在'new Product（proid，proName，proPrice，proQis）'構造函數中做了一些特殊的事情，變量'proid'，'proName'，'proPrice'，'proQis'也可以爲null，它們可以導致相同的NPE。 – gaborsch

@GaborSch +1我同意，但在大多數情況下，構造函數僅用於分配實例字段。特別是，對於Java bean類（即'Product'）。 –

是的，我同意，但無論如何，這個（非常）小的機會，我們沒有看到構造函數的內部。 – gaborsch

構造函數中的NullPointerException

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