CODEWITHSUNDEEP
javaoopinterface
bcr
bcr

Reputation: 1328

Difference betwen constant declared in interface and class?

I ran into a peculiar issue I was curious about (and would like to find a workaround for).

If I declare a variable NODES in my class:

private static final String NODES = "nodes";

Everything works ok.

However, if I declare the variable in an interface:

public interface MSMConstants {
    public static final String NODES = "nodes";
}

and implement that interface in the class that uses the variable NODES, I get a null pointer exception upon my first use of NODES.

Any ideas on what's going on here?

Lines that are crashing:

lr_fontSize.setSelectedItem(((VisualItem)m_vis.getVisualGroup(NODES).tuples().next()).getFont().getSize());

SearchQueryBinding sq = new SearchQueryBinding((Table) m_vis.getGroup(NODES), "label",
        (SearchTupleSet) m_vis.getGroup(Visualization.SEARCH_ITEMS));

The second is inside a constructor; the first is also inside a constructor, but for an object that is instantiated from the second.

Stack trace was requested:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at prefuse.data.query.SearchQueryBinding.<init>(SearchQueryBinding.java:53)
    at edu.stanford.folding.msmexplorer.MSMExplorer.<init>(MSMExplorer.java:703)
    at edu.stanford.folding.msmexplorer.MSMExplorer.graphView(MSMExplorer.java:1538)
    at edu.stanford.folding.msmexplorer.MSMExplorer.graphView(MSMExplorer.java:1526)
    at edu.stanford.folding.msmexplorer.MSMExplorer$1.actionPerformed(MSMExplorer.java:202)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2028)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2351)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
    at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:272)
    at java.awt.Component.processMouseEvent(Component.java:6375)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
    at java.awt.Component.processEvent(Component.java:6140)
    at java.awt.Container.processEvent(Container.java:2083)
    at java.awt.Component.dispatchEventImpl(Component.java:4737)
    at java.awt.Container.dispatchEventImpl(Container.java:2141)
    at java.awt.Component.dispatchEvent(Component.java:4565)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4619)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4280)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4210)
    at java.awt.Container.dispatchEventImpl(Container.java:2127)
    at java.awt.Window.dispatchEventImpl(Window.java:2482)
    at java.awt.Component.dispatchEvent(Component.java:4565)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:684)
    at java.awt.EventQueue.access$000(EventQueue.java:85)
    at java.awt.EventQueue$1.run(EventQueue.java:643)
    at java.awt.EventQueue$1.run(EventQueue.java:641)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
    at java.awt.EventQueue$2.run(EventQueue.java:657)
    at java.awt.EventQueue$2.run(EventQueue.java:655)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:654)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Upvotes: 0

Views: 543

Answers (2)

Bobulous
Bobulous

Reputation: 13169

Well, I'm not sure why it would generate a NullPointerException, but if you've got two static variables with the same name, you should be prefixing them with either the class name or the interface name, so that Java knows which static variable you mean.

Can you change your code to use either MSMConstants.NODES or MyClassName.NODES (for example) and see whether that fixes the problem?

Upvotes: 0

Andreas Dolk
Andreas Dolk

Reputation: 114767

There is no technical difference. We can declare final static constants on interfaces and classes and use them. Both approaches that you've shown should work.

But constant declarations don't match too well with the concept of an interface. I know, that a lot of people use interfaces as "repositories for constants" that's definitely not their purpose.

One reason to do that was that it provided a simple way to "inherit" constant definitions through "implementing" those constant holder interface.

But now we have static imports and though there's no need to "abuse" the interfaces anymore.

Upvotes: 2

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