Rudimentary java.util.prefs question

[Eric Barnhill]

Hi, sorry to be asking the simplest possible question about this API, but I am getting a complier error right off the bat and not sure why.

Java Preferences API Overview states:

The following examples illustrate how you might obtain the Preferences objects (system and user) pertaining to the enclosing class. These examples would work only inside instance methods.

It then cites:

 package com.acme.widget;
import  java.util.prefs.*;

public class Gadget {
    // Preference keys for this package
    private static final String NUM_ROWS = "num_rows";
    private static final String NUM_COLS = "num_cols";

    void foo() {
        Preferences prefs = Preferences.userNodeForPackage(this);

        int numRows = prefs.getInt(NUM_ROWS, 40);
        int numCols = prefs.getInt(NUM_COLS, 80);

        ...
    }
} 

The above example obtains per-user preferences. If a single, per-system value were desired, the first line in foo would be replaced by: Preferences prefs = Preferences.systemNodeForPackage(this);

So I added the following method to my class ViscoSimGUI:

 void setPreferences() {
		
		Preferences p = Preferences.systemNodeForPackage(this);
		
	}
 

But I immediately get the error

The method systemNodeForPackage(Class<?>) in the type Preferences is not applicable for the arguments (ViscoSimGUI)

It's definitely an instance method as the constructor is

public ViscoSimGUI() {
       (...)
    }

I figure I am missing something too obvious for even the API overview, and would appreciate any help. Thanks, Eric

[Maneesh Godbole]

Preferences p = Preferences.systemNodeForPackage(this);  

The method systemNodeForPackage(Class<?>) in the type Preferences is not applicable for the arguments (ViscoSimGUI)

Hint: What do you get if you say this.getClass()?

[Eric C Barnhill]

That removes the compiler error, thank you. Also works to use ViscoSimGUI.class .

I am trying to think why -- maybe this was referring to an instance of the class when it needs to refer to the class itself here. However I still don't see why the API overview's example would work, or some other examples I see on the web.

I just tried taking off the "extends JFrame", also removing the constructor...I don't know what is different in mine.

Thanks for the help though.

[Stuart A. Burkett]

Eric Barnhill wrote:It then cites:

 package com.acme.widget;
import  java.util.prefs.*;

public class Gadget {
    // Preference keys for this package
    private static final String NUM_ROWS = "num_rows";
    private static final String NUM_COLS = "num_cols";

    void foo() {
        Preferences prefs = Preferences.userNodeForPackage(this);

        int numRows = prefs.getInt(NUM_ROWS, 40);
        int numCols = prefs.getInt(NUM_COLS, 80);

        ...
    }
} 

No it doesn't. The actual code is

public class Gadget {
    // Preference keys for this package
    private static final String NUM_ROWS = "num_rows";
    private static final String NUM_COLS = "num_cols";

    void foo() {
        Preferences prefs = Preferences.userNodeForPackage(Gadget.class);

        int numRows = prefs.getInt(NUM_ROWS, 40);
        int numCols = prefs.getInt(NUM_COLS, 80);

        ...
    }
}

this and Gadget.class are not the same things. this is a reference to the current instance of the Gadget class, whereas Gadget.class is an instance of the Class class.

[Eric Barnhill]

You are right, I was looking at an outdated overview page from a previous Java version.

This discussion helped me understand the keyword this a lot better though.

Thank you for your help.