Question

I have a design question: let me expalin in simple example:

Public class A()
{
public static HashMap map = new HashMap();
public static String url = "default";
static {
  getJson();
}

//url getters and setters are defined

public static getJson() {
//code which uses url to get json and populate hashmap
}
public string getresult(String key) {
//uses hashmap to send result.
}

I am using static initialization block because i want to get json only once.

public class B {

//here I want to change url and call getJson method. If i call A.setUrl() then before setting url, A.getJson () method is called as it is in static initialization block.how can i set url first and then call getJson().

//is this a bad design?

}

Answer 1

Yes, it is bad design:

1. It is impossible to customize where A gets its data from without modifying the definition of A. Among other things, this prevents unit testing (as you probably don’t want to fail the unit test if the network is unavailable …).
2. If initialization fails (for instance because the remote URL is currently unavailable), you can’t easily catch that exception, as you don’t know which access triggered loading. You can’t throw a checked exception from a static initializer. You can’t retry initialization either (all subsequent access immediately result in an exception).

If you must access A through a static field, I’d recommend:

public class A {
    private static Map<String, String> map;

    /** must be invoked before get is first called */
    public static void init(Map<String, String> newmap) {
        map = newmap;
    }

    public static String get(String key) {
        return map.get(key);
    }
}

This separates the concern of using the data from the concern of obtaining it, allowing each to be replaced and tested independently.

Also consider getting rid of static, as it enforces there is only ever one map in your entire application at the same time, which is quite inflexible. (See the second code sample in Ajay’s answer for how)