object array assignment problem

Question

I have a problem where each element of my array seem to be reassigned.

class Car {
    private static int nom = 0;
    private static String whee = "";
    public void setCar(int r, String s) {
        this.nom = r;
        this.whee = s;
    }
}

class Rawr {
    private Car[] jar = new Car[3];

    public Mar() {
        jar[0] = new Car();
        jar[1] = new Car();
        jar[2] = new Car();
        jar[0].setCar(2, "yar");
        jar[1].setCar(3, "tar");
        jar[2].setCar(4, "sars");
    }
}

If I printed it like jar[0].nom + jar[0].whee + jar[1].nom + jar[2].whee + jar[3].whee, the output would be

4 sars 4 sars sars

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– T.J. Crowder, Commented Feb 4, 2011 at 13:39

dogbane · Accepted Answer · 2011-02-04 13:42:38Z

6

It's because your variables are static i.e. they belong to the class, rather than to an instance. Take a look at Java Tutorials | Understanding Instance and Class Members for more information about what this means.

You should remove the static keyword, so that they become instance variables.

answered Feb 4, 2011 at 13:42

dogbane

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Comments

T.J. Crowder · Accepted Answer · 2011-02-04 13:43:10Z

5

Change

private static int nom = 0;
private static String whee = "";

to

private int nom = 0;
private String whee = "";

static means the variable is shared by all instances. (The fact you can use this to refer to static variables is a Java oddity.)

answered Feb 4, 2011 at 13:43

T.J. Crowder

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3 Comments

Peter Lawrey Over a year ago

As a rule, don't set static fields in a constructor. Try making the fields final. This can help you avoid many mistakes.

T.J. Crowder Over a year ago

@Peter: Yes, very good rule. But in this case, I think he doesn't want them to be statics anyway, so...

Peter Lawrey Over a year ago

Whenever I have seen static fields set in a constructor it has been the source of a bug or confusion. ;)

JB Nizet · Accepted Answer · 2011-02-04 13:50:31Z

0

Your nom and whee fields are static. This means that they are tied to the class, and not to the object (instance) of the class.

Thus, when you assign a new value to this.nom, in reality, you assign a the value to Car.nom. The compiler allows referring to static variables through an object, but it's very bad practice. You should always refer to static fields by their class : Car.nom, Car.whee. This makes it clear that the nom and whee are static, and thus shared by all instances of the class. In this case, these fields should not be static : each Car instance has its own name and whee (whatever it might be).

answered Feb 4, 2011 at 13:50

JB Nizet

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Comments

Peter Lawrey · Accepted Answer · 2011-02-04 13:52:12Z

0

A better way to structure your code is as follows.

class Car {
    private final int nom;
    private final String whee;
    public Car(int nom, String whee) {
        this.nom = nom;
        this.whee = whee;
    }
    public String toString() { return num + " " + whee; }
}

class Rawr {
    private final Car[] jar = {new Car(2, "yar"), new Car(3, "tar"), new Car(4, "sars")};
    public String toString() {
        return Arrays.toString(jar);
    }
}

answered Feb 4, 2011 at 13:52

Peter Lawrey

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