(java.lang.String) cannot be applied to (java.lang.Object)

问题:

Ive a Listner class called TopicS Im trying to call it from a gui called readMessages

When Im trying to run the class TopicS using the following method,

   private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                              System.out.println("test test test");      System.out.print("you pressed" +topicCombobox.getSelectedItem());     TopicS a = new TopicS();     a.addTopicToListner(topicCombobox.getSelectedItem()); }                  

It gives me error saying

addTopicListner(java.lang.String) in Topics Cannot be applied to (java.lang.Object)

When I change the String to Object I get other errors. The main method is included below, this works fine without GUI, but I need to add it to GUI. What I am trying to do is take value to combobox which is String array, and place that string into topic (where the (t) is now

 import java.util.Hashtable;  import javax.jms.JMSException; import javax.jms.Message; import javax.jms.MessageListener; import javax.jms.Session; import javax.jms.TextMessage; import javax.jms.Topic; import javax.jms.TopicConnection; import javax.jms.TopicConnectionFactory; import javax.jms.TopicSession; import javax.jms.TopicSubscriber; import javax.naming.Context; import javax.naming.InitialContext; import javax.naming.NamingException;  public class TopicS implements MessageListener {   private TopicConnection topicConnection;  private TopicSession topicSession;  public Topic topic;  private TopicSubscriber topicSubscriber;    public TopicS()             {}             public void addTopicToListner(String t){   try   {    // create a JNDI context    Hashtable properties = new Hashtable();    properties.put(Context.INITIAL_CONTEXT_FACTORY,"org.exolab.jms.jndi.InitialContextFactory");    properties.put(Context.PROVIDER_URL,"rmi://localhost:1099/");    Context context = new InitialContext(properties);     // retrieve topic connection factory    TopicConnectionFactory topicConnectionFactory =         (TopicConnectionFactory)context.lookup("JmsTopicConnectionFactory");    // create a topic connection    topicConnection = topicConnectionFactory.createTopicConnection();     // create a topic session    // set transactions to false and set auto acknowledgement of receipt of messages    topicSession = topicConnection.createTopicSession(false,Session.AUTO_ACKNOWLEDGE);     // retrieve topic    topic = (Topic) context.lookup(t);     // create a topic subscriber and associate to the retrieved topic    topicSubscriber = topicSession.createSubscriber(topic);     // associate message listener    topicSubscriber.setMessageListener(this);     // start delivery of incoming messages    topicConnection.start();   }   catch (NamingException e)   {    e.printStackTrace();   }   catch (JMSException e)   {    e.printStackTrace();   }  }   /* public static void main(String[] args)  //{    try   {    TopicS listener = new TopicS();    Thread.currentThread().sleep(2000);   }    catch (InterruptedException e)   {    e.printStackTrace();   }  }  */  // process incoming topic messages  public void onMessage(Message message)  {   try   {    String messageText = null;    if (message instanceof TextMessage)     messageText = ((TextMessage)message).getText();    System.out.println(messageText);   }   catch (JMSException e)   {    e.printStackTrace();   }  } } 

回答1:

That's because JComboBox.html.getSelectedItem() returns Object

public Object getSelectedItem() 

And your method expects a string

public void addTopicToListner(String t) 

If you're 100% sure the contents of your combobox are string you just have to cast it:

a.addTopicToListner( (String) topicCombobox.getSelectedItem()); 

And that's it.

This code sample reproduces exactly your compilation error:

class StringAndObject {     public void workWithString( String s ) {} // We just care about      public void workWithObject( Object o ) {} // the signature.       public void run() {          String s = ""; // s declared as String         Object o = s;  // o declared as Object          // works because a String is also an Object         workWithObject( s );         // naturally a s is and String         workWithString( s );           // works because o is an Object         workWithObject( o );         // compiler error....          workWithString( o );      }  } 

Output:

StringAndObject.java:19: workWithString(java.lang.String) in StringAndObject cannot be applied to (java.lang.Object)         workWithString( o );         ^ 1 error    

As you see, the last call (workWithString(o) ) doesn't compile even though it is a String object. It turns out the compiler only knows that o was declared as Object but it doesn't have a way to know if that object is a string or is something else ( a Date for instance ).

I hope this helps.

回答2:

JComboBox.getSelectedItem() returns type Object, not String. You can call toString() on its result to return the string representation of your object. It looks as if you're trying to return a type of Topic, which means you'll need to override the toString() method on Topic to return the value you want.

回答3:

Try the following code

topicCombobox.getSelectedItem() instanceof String ? (String)topicCombobox.getSelectedItem() : "Socks"; 

This is a temporary fix, because I don't know if the incoming getSelectedItem() is a String.
If you know it always will be just cast it

(String)topicCombobox.getSelectedItem() 

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