构造函数中的NullPointerException

我已经将我的问题缩小到了我的GUI中的一行Java代码。构造函数中的NullPointerException

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

每次我点击我已附加到的按钮。这不是按钮，没有它就运行良好。

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)

我不能为我的生活找出问题。

来源

2013-01-13 user1974447

看来Shop1是null。尝试添加空值检查：

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

另外，请注意遵循Java命名约定，不要使用大写字母来启动变量。而不是Shop1，请使用shop1。

来源

2013-01-13 17:14:10

如果你在'new Product（proid，proName，proPrice，proQis）'构造函数中做了一些特殊的事情，变量'proid'，'proName'，'proPrice'，'proQis'也可以为null，它们可以导致相同的NPE。 – gaborsch

@GaborSch +1我同意，但在大多数情况下，构造函数仅用于分配实例字段。特别是，对于Java bean类（即'Product'）。 –

是的，我同意，但无论如何，这个（非常）小的机会，我们没有看到构造函数的内部。 – gaborsch

构造函数中的NullPointerException

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