class can't be instantiated

问题:

I have a problem with java applet and graphics. I'm trying to run it in Eclipse and it fails. Im new in java and i hope you can help me. I have two files: Say.java and SayWhat.java. Say.java:

public class Say {
    SayWhat word = new SayWhat("Hello World");

}

SayWhat.java:

import java.applet.Applet;
import java.awt.Graphics;

@SuppressWarnings("serial")
public class SayWhat extends Applet {
     Graphics g;
     String what;
    public SayWhat(String what) {
        this.what=what;
    }
    public void paint(Graphics g){
        g.drawString(what, 20, 20);
    }
}

Error that appears is:

load: SayWhat.class can't be instantiated.
java.lang.InstantiationException: SayWhat
    at java.lang.Class.newInstance0(Unknown Source)
    at java.lang.Class.newInstance(Unknown Source)
    at sun.applet.AppletPanel.createApplet(Unknown Source)
    at sun.applet.AppletPanel.runLoader(Unknown Source)
    at sun.applet.AppletPanel.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Can you please tell me what am i doing wrong?

回答1:

An applet needs to have a public no-arg constructor (either by having an explicit public no-arg constructor, or by having no explicit constructors at all; in the latter case, the compiler will supply a public no-arg constructor as a default). Your class's sole constructor takes an argument:

public SayWhat(String what) {

so the class can't be instantiated without that argument, so it can't be used as an applet.

回答2:

The Class SayWhat should have a public constructor with no arguments.

回答3:

Check the documentation for java.lang.InstantiationException

There are two possible causes:

1) Both code and object attributes are specified in the tag:

<APPLET code=MyApplet object=MyApplet.ser width=100 height=100>
</APPLET>

The Sun JRE can access either the code or the object attribute, but not both.

2) A code attribute is specified in the tag, and an object attribute is specified in a tag :

<APPLET code=MyApplet width=100 height=100>
<PARAM name="object" value="someValue">
</APPLET>

public class MyApplet extends java.applet.Applet
{
        public void init()
        {
               String value = getParameter("object");
        }
        ....
}

EDIT: Add a default constructor, as below:

public SayWhat() {}