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Hope this is not a duplicate because I already looked up some thread incl. this one, but it didn't help me.

My program is reading in some arguments that are optional by the user. They can add a rule to the game but don't have to. I know that the rule will be containing 5 Numbers. I wanted to save them in a String Array with 5 spots so I can use them later. If the user won't enter a rule there will be a specific rule taken.

String[] rule = new String[5];
//reading in the program arguments and stuff..
//here I want to check whether the rule is taken from the user or not
//don't want to check with a boolean check
if (rule[0].equals("")) {
    String[] newRule = "270-90-315-45-90".split("-");
    for (int i = 0; i < newRule.length; i++) {
        rule[i] = newRule[i];
    }
}

Already tried this:

rule[0].equals("")
rule[0].equals(null)
rule[0].equals("null")
rule[0].matches("")
rule[0].matches("null")
rule.length == 0
rule.equals(null)

But I always get a NullPointerException or the if case will be skipped (length == 0)

Hopeyou can help me.

Community
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NhatNienne
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    We need to see how you populated the `rule` array. – Rohit Jain Mar 15 '15 at 16:00
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    Since you said you get a npe, try doing `rule[0] == null` – Constant Mar 15 '15 at 16:02
  • I didn't try the `rule[0] == null` because I kinda learned that you should compare Strings with `==` but with `equals`, but thanks it worked. – NhatNienne Mar 15 '15 at 16:09
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    Here you're not comparing two strings, you're comparing a string (object) to null. Quick lesson: you use == when comparing with null – Constant Mar 15 '15 at 16:15
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    so comparing two objects for equality : equals comparing an Object with null : == comparing any number: == – NhatNienne Mar 15 '15 at 16:21
  • Does [Java, check whether a string is not null and not empty?](https://stackoverflow.com/q/3598770/2157640) contain an answer to your question? – Palec Mar 15 '15 at 17:11
  • well it should although someone with the same problem would have to reverse the answers from this question – NhatNienne Mar 15 '15 at 17:14

6 Answers6

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You didn't have tried the obvious?

if (rule[0] == null) {
Smutje
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0

In the example you provide us with, rule[0] contains the value null which you can see yourself by adding the following line of code:

System.out.println(rule[0]==null);

returns true (try it !)

if (rule[0]==null) {

will return true and get inside the for loop if that is what you want.. See the following class which you can compile (using javac myEmpty.java) and run (using java myEmpty):

class myEmpty {
    public static void main(String[] args) {
        String[] rule = new String[5];
        System.out.println(Arrays.toString(rule));
        //reading in the program arguments and stuff..
        //here I want to check whether the rule is taken from the user or not
        //don't want to check with a boolean check
        System.out.println(rule[0] == null);
        if (rule[0] == null) {
            //if ("".equals(rule[0])) {
            String[] newRule = "270-90-315-45-90".split("-");
            System.out.println(Arrays.toString(newRule));
            for (int i = 0; i < newRule.length; i++) {
                rule[i] = newRule[i];
                System.out.println(rule[i]);
            }
        }
    }
}

YOUR if if (rule[0].equals("")) { fails simply because rule[0] does not contain the value "" which you are checking for ! Keep in mind that "" and null are not the same, and use your if clauses accordingly !

beresfordt
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adrCoder
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0

So you want to change indices entered by user only.

The best way to do this would be to initialize the array with default values, then override those specified by the user.

String[] rule = "270-90-315-45-90".split("-");
// Now read "program arguments and stuff"
spongebob
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0

Insted of using an fixed array, try using an ArrayList and add the values to the list. The ArrrayList-class will adjust the length of the list automaticly.

Example:

ArrayList<String> rules = new ArrayList<>();

if (rules.size() == 0) {
    String[] newRule = "270-90-315-45-90".split("-");

    //Instead of newRule.length as a comparison, use the
    //number of rules with a number. Or use a foreach-loop
    //if the number of rules may vary

    for (int i = 0; i < 5; i++) {
        rules.add(newRule[i]);
     }
}
Daniel Nordenström
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  • Rules have to be of the size 5. So I know that there will be this String format and if not I can easily catch that and throw an error message. Don't know if an ArrayList wouldn't be too much for just saving 5 little Strings which I know have to match strict formats – NhatNienne Mar 15 '15 at 16:22
  • Why does it have to be size 5? As I understand it you hopefully somehow have reasured is a maximum of 5 elements. You put them in a list, checks if the list is empy. If it is - you want to add your pre defined "standard" rules and if it already has rules - you start the program. Is that correct? Also, can the user input 3 rules or does it have to be 5? – Daniel Nordenström Mar 15 '15 at 16:51
  • no there has to be a rule with 5 numbers because those numbers are for 5 different field colors and each number shows how much ° the user is turning on this field (e.g. 45°) – NhatNienne Mar 15 '15 at 17:17
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The NullPointerException I imagine is coming from the fact you have created an array of strings but have not initialized them with any values. They will default to null if you don't give them a value...hence the NPE.

Depending on where you are getting your input you could do something like this; 

private static final String[] DEFAULT_RULES = {"270", "90", "315", "45","90" };
private static String[] rules;

public static void main(String[] args){

     if(!isValidRule(args)){
       // Potentially also check that the args are digits
       throw new IllegalArgumentException("Must contain 5 rules");
     }

     rules = (args.length > 0) ? args : DEFAULT_RULES; // Used brackets on the ternary operator here for readability. Not essential.

     ...

}

private static boolean isValidRule(String[] rules){
    return rules.length > 0 && rules.length != 5;
}

If you are working with some other non-static method thats taking the input, the same applies. You can perform your string split to get an array based on the delimiter you have specified and then do the same thing.

I don't imagine you would want to be passing around a string containing only hyphens if no rules are passed? Which is what you are hinting at by attempting to check if a string is empty after the split is performed.

Also if you want to check if the string contains characters or not use the isEmpty() method. It returns true if the length is 0 else it returns false. This already achieves what you are needlessly attempting.

kstandell
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-1

In each of your tried options

rule[0].equals("")
rule[0].equals(null)
rule[0].equals("null")
rule[0].matches("")
rule[0].matches("null")
rule.length == 0
rule.equals(null)

You already assumed that your element at 0 index is not null , and calling equals() and matches() method on element at 0th index causing NullPointerException

Thrown when an application attempts to use null in a case where an object is required. These include:

  1. Calling the instance method of a null object.
  2. Accessing or modifying the field of a null object.
  3. Taking the length of null as if it were an array.
  4. Accessing or modifying the slots of null as if it were an array.
  5. Throwing null as if it were a Throwable value.

Instead try like this 

if(rule[0] != null){
// Now you can call methods on your `0th` index element

}
Neeraj Jain
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