Learning about constants

[André Asantos]

I have already read about constants I have saw an example using PI calculate:

final float pi=4.141592;

But when do I need to use, how to use and how to invoke a constant?

[Rob Spoor]

I hope that's not your final definition of pi, because it's off by 1.

You use constants for anything that, well, needs to stay constant. This could be anything, from a maximum value (like Integer.MAX_VALUE) to a hard coded name for something (like BorderLayout.CENTER).

[Joanne Neal]

They are also normally defined as static, so that you can access them using the class name and don't need to create an instance of the class

class MyConstants
{
  public static final float pi=3.141592;
}

class MyCircle
{
    private int radius;

    public float getArea()
    {
        return MyConstants.pi * radius * radius;
    }
}

[Jim Hoglund]

Be careful about the static part since you may want a different final value
in each object, to track its instance number, for example, or capture its
time of creation.

Jim ... ...

[pete stein]

Jim Hoglund wrote:Be careful about the static part since you may want a different final value
in each object, to track its instance number, for example, or capture its
time of creation.

But wouldn't it then not be a constant?

[John de Michele]

pete stein wrote:But wouldn't it then not be a constant?

Not necessarily. Static means that the variable or reference would be associated with the class, and not an individual object. Making the variable or ref final would mean that you could have a constant for each object.

John.

[Campbell Ritchie]

André Asantos wrote: . . .

final float pi=4.141592;

. . .

4.141592f, surely?

[Rob Spoor]

No, 3.141592f. It's pi for pete's sake. You guys suck at math!

[Joanne Neal]

Rob Prime wrote:No, 3.141592f. It's pi for pete's sake. You guys suck at math!

I got it right in my post. Do I get a gold star ?

[Campbell Ritchie]

And I thought it was pie

[fred rosenberger]

Rob Prime wrote:No, 3.141592f. It's pi for pete's sake. You guys suck at math!

well...it's APPROXIMATELY 3.141592f.

I assume nobody has mentioned Math.PI since the OP is trying to figure out how to do this on his own.

FWIW, that is defined as 3.141592653589793d

[Rob Spoor]

Joanne Neal wrote:

Rob Prime wrote:No, 3.141592f. It's pi for pete's sake. You guys suck at math!

I got it right in my post. Do I get a gold star ?

You, always. I'll just take one of Campbell's.

[marc weber]

Rob Prime wrote:I hope that's not your final definition of pi, because it's off by 1...

Actually, it's only off by 4.141592 - pi, which is in the general neighborhood of 0.9999993464102067615373566167205. Far less than 1.

[Campbell Ritchie]

fred rosenberger wrote: . . . I assume nobody has mentioned Math.PI since the OP is trying to figure out how to do this on his own. . . .

No, I thought it was because he insisted on using a float.

[Tracy Tse]

well , in summary, if you want a class-wide constants ,then simply declare it as static final ,otherwise eliminate static modifier.

[Abhishk Gupta]

wooohh....That was quite a discussion but allow me to divert it

John de Michele wrote:

pete stein wrote:But wouldn't it then not be a constant?

Not necessarily. Static means that the variable or reference would be associated with the class, and not an individual object. Making the variable or ref final would mean that you could have a constant for each object.

John.

Does this mean, I can change the value of pi with each new object?

 
public class TestClass {
     private final float pi=3.141592f;
          public static void main(String[] args) {
	TestClass tc = new TestClass();
	tc.pi=10;//This would give compilation error
          }
    }

Please elaborate what you mean when you say "you may want a different final value
in each object"

Abhee

[Rob Spoor]

Abhishk Gupta wrote:Does this mean, I can change the value of pi with each new object?

Yes and no.
Yes, each new object can have its own value of pi. No, you cannot change it after it has been set, and it needs to be set either when declared or in the constructor:

public class TestClass
{
    private final float pi; // not yet initialized so it must be done in the constructor

    public TestClass(float pi) { this.pi = pi; }

    public static void main(String[] args)
    {
        TestClass tc = new TestClass(10);
        System.out.println(tc.pi); // 10
    }
}

If you initialize it when declaring it then all instances will have the same value and you should turn it into a static field. If you forget to initialize it in the constructor (actually each constructor) the code will not compile.

[Campbell Ritchie]

Rob Prime wrote: . . . Yes, each new object can have its own value of pi. . . .

Even 4.141592f

[Abhishk Gupta]

wow...why it didn't strike me

Anyways Congrats folks
finally your cummulative efforts changed the value of pi

[Ernest Friedman-Hill]

It's 3.14159265358979323..., so in fact the correct number is 3.141593.

I'm just sayin'...

[Campbell Ritchie]

Ernest Friedman-Hill wrote: . . . I'm just sayin'...

I'm just annoying