Question

Possible Duplicate:
What is the reason behind “non-static method cannot be referenced from a static context”?

I am new to Java. I have following sets of code as below.

class Default
{
    private short s;
    private int i;
    private long l;
    private float f;
    private double d;
    private char c;
    private String str;
    private boolean b;

    public static void main (String args[ ])
    {

        Default df = new Default();

        System.out.println("\n Short = "+s);

        System.out.println ("\n int i =" + i);
        System.out.println ("\n long l =" + l );
        System.out.println ("\n float f =" + f);
        System.out.println ("\n double d =" + d);
        System.out.println ("\n char c =" + c);
        System.out.println ("\n String s =" + str);
        System.out.println("\n boolean b =" + b);
    }
}

This produces an error message as the subject of this question but following code works perfectly.

class Default
{
    private short s;
    private int i;
    private long l;
    private float f;
    private double d;
    private char c;
    private String str;
    private boolean b;

    public static void main (String args[ ])
    {

        Default df = new Default();

        System.out.println("\n Short = "+df.s);

        System.out.println ("\n int i =" + df.i);
        System.out.println ("\n long l =" + df.l );
        System.out.println ("\n float f =" + df.f);
        System.out.println ("\n double d =" + df.d);
        System.out.println ("\n char c =" + df.c);
        System.out.println ("\n String s =" + df.str);
        System.out.println("\n boolean b =" + df.b);
    }
}

This gives the desired result. What is the difference in these two set of code.

Answer 1

You have an instantiated object of Default named df which is calling those variables. Since the variables you created are not static, they must be apart of some object that has been created.

Default df = new Default();
df.i //<- is now a part of the instantiated object df.

You use the keyword static if it is not used with an object. So you could just say:

private static char c;

And then you can call char c anytime, which will be null because you haven’t given it a value yet.

You use static when you will be using that variable or method without an object.