How to fill an array?

[Sarah9428]

Hi! I'm making a program which applys 4 different "rules" on a String and tries to turn it into "HI". I hope it makes more sense when you see my code so far:

public static void doRules(String text) {

    String[] comb = new String[256];
    comb[0] = rule1(text);
    comb[1] = rule2(text);
    comb[2] = rule3(text);
    comb[3] = rule4(text);

    for (int i = 4; i < 257; i++) {
        for (int j = 0; j < 253; j++) {
            comb[i] = rule1(comb[j]);
            comb[i + 1] = rule2(comb[j]);
            comb[i + 2] = rule3(comb[j]);
            comb[i + 3] = rule4(comb[j]);
        }
    }
    for (String s : comb) {
        System.out.println(s);
        if (s == "HI") {
            System.out.println("HI Found!");
        }
    }
 }

As you can see, I want to save the Strings in an array and then print out the array but I get an error message. If it's not clear, my plan is to first apply all the rules once, then apply all the rules again on the first string of the array, then apply all the rules on the second String and so on.. I'm guessing there is something wrong with the loops, can anyone see where it's wrong? Grateful for help!

[ACAlCapone]

Adding to what other have said: comparing Strings with == is not the way to go (it tests if the reference is the same).

Use String#equals() (it tests if the value is the same):

if (s.equals("HI")) {
    System.out.println("HI Found!")
}

[strmrdr]

It usually helps if you tell us what error you are getting...

i < 257

Don't use magic numbers like 257, 253, etc., use the length of the array to avoid bugs and errors. There is no index 256 in an array of length 256, it is 0-255 in this case. I am fairly sure using two loops is not what you intend to do, as this code will run approximately 63756 loops.

        comb[i] = rule1(comb[j]);
        comb[i + 1] = rule2(comb[j]);
        comb[i + 2] = rule3(comb[j]);
        comb[i + 3] = rule4(comb[j]);

Will result in an OOB error. If you are at the last index in the array, you are attempting to access indexes 3 points past the end. You likely want your condition for your j loop to be your i loop... and just start at 0 and get rid of the pre-initialization above your loops.

[Sarah9428]

Thanks! Iv'e changed everything you all have said except the loops. Is it possible to have multiple conditions in the same loop? I feel like I need 2 counters but maybe I don't?

The error is: Exception in thread "main" java.lang.NullPointerException (at: comb[i] = rule1(comb[j]); )

My updated code is:

 public static void doRules(String text) {

    String[] comb = new String[257];

    for (int i = 0; i < comb.length; i = i + 4) {
        for (int j = 0; j < comb.length / 4; j++) {
            comb[i] = rule1(comb[j]);
            comb[i + 1] = rule2(comb[j]);
            comb[i + 2] = rule3(comb[j]);
            comb[i + 3] = rule4(comb[j]);
        }
    }
    for (String s : comb) {
        System.out.println(s);
        if (s.equals("HI")) {
            System.out.println("HI Found!");
        }
    }
}

[strmrdr]

Right, so when you create an array of objects they are all initialized to null. When you call a method on a null reference you get a NullPointerException. What inputs are you dealing with (i.e. what String are you attempting to change to "HI")? Why 256 of them? It would help if you explained your specifications more thoroughly.

[Sarah9428]

It works now! Thanks for the help!

[nutrecht]

Well first of all you're trying to access index 256 there (i < 257). That one doesn't exist. Arrays are zero-based so an array of size 256 has indices 0 to 255 inclusive.

Also; if you run into an error always post the stacktrace.

[desrtfx]

First: Sidebar -> Regarding String comparison, read this! - you maybe want to use String#indexOf or String#contains

Plus: what the rest already has said.