Objective-C, How to convert this Java statement into Objective-C code?

Question

I have a very simple Java code like this. I don't have any idea how to do this in Objective-C. Especially, the static part which calls the getLocalAddress() method and assign it into the static string variable. I know how to set a static variable and a static method in Objective but I don't know how to implement that static { } part in Java.

public class Address {

     public static String localIpAddress;

    static {
        localIpAddress =  getLocalIpAddress();
    }

    public Address() {

    }

    static String getLocalIpAddress() {
         //do something to get local ip address
     }
}

I added this in my .h file

 #import <Foundation/Foundation.h>

 extern NSString *localIpAddress;

 @class WifiAddrss;

 @interface Address : NSObject {

 }

 @end

And my .m file looks like

 #import "Address.h"
 #import "WifiAddress.h"
 
 @implementation Address

 +(void)initialize{
     if(self == [Address class]){
         localIpAddress = [self getLocalIpAddress];
     }
 }

 +(NSString *)getLocalIpAddress{
      return address here
 }

 -(id)init{    
     self = [super init];
     if (self == nil){
         NSLog(@"init error");
     }
     
     return self;
 }
 @end

And Now I am getting a linking error and it complains about "extern NSString *localIpAddress" part. If I change the extern to static, it works fine. But what I wanted to do is that I want make the scope of "localIpAddress" variable as global. Since if I put "static" in front of a variable in Objective-C then the variable is only visible in the class. But this time, I want to make that as a global variable. So my question is how to make "localIpAddress" variable as a global variable which is initialized once when the first time Address class is created..

Answer 1

Look here to emulate Java's static block in Objective C: Static constructor equivalent in Objective-C?

Answer 2

You can implement this as a class method, storing the IP address in a static variable.

Address.h

@interface Address : NSObject
+ (NSString *)localIPAddress;
@end

Address.m

#import "Address.h"

static NSString *localIPAddress = nil;

@implementation Address

+ (NSString *)localIPAddress {
  if (!localIPAddress) {
    localIPAddress = @"127.0.0.1";
  }

  return localIPAddress;
}

@end

Answer 3

The linker error occurs because you have an extern declaration but no definition. To fix the particular error, you need to put in your .m file something like:

NSString *localIpAddress = @"some value";

What extern does is tell the compiler that, by the time link time comes around, there will be a variable called localIpAddress, even though there isn't now. The line above in the .m file creates some space in the data segment for the NSString* and then initialises it to point at a constant string.

A better pattern for Objective-C programs is to expose the variable via a class method e.g.

static localIpAddress; // static limits visibility to this file.

-(void) initialize
{
  if (self == [Address class]) 
  {
    localIPAddress = /* whatever */;
  }
}

+(NSString*) localIpAddress
{
    return localIpAddress;
}

Or I prefer

+(NSString*) localIpAddress
{
    static NSString* localIpAddress = nil;
    if (localIpAddress == nil)
    {
        localIpAddress = /* Do whatever you need to get it */ ;
    }
    return localIpAddress;
}

Answer 4

It sounds like you only want localIPAddress to be created the first time an instance is created, but not via a class method. This is a rather odd scenario, but nonetheless try this:

static NSString *localIPAddress = nil;

@implementation Address

- (id)init {
    if ((self = [super init])) {
        if (!localIPAddress) {
            localIPAddress = //getAddress
        }
    }
    return self;
}

@end

Of course this is a very unusual setup. You would generally either get the local ip address each time you create the object, or you would use Jonathan's code to get it via a class method.