I am a beginner with java, and I was wondering is if is possible to name and create an array from the value of a string.
Here is what I have:
public static void array(){
createArray(array1, 100, 100);
}
public static void createArray(String name, int r, int c) {
int[][] name = new int[r][c];
}
I hope that explains itself. Thanks
EDIT: The code above does not work. I just want to know if it is possible to do what is above
EDIT2: As a beginner with java, I am just watching tutorials, and creating programs with what I learned to make sure I understand what is being taught. I first created a program which creates s multidimensional arrays. It then calls a method which assigned values to the array, (row+1)*(column+1). This makes a table much like a multiplication table. Then it displays the table to the screen.
After I created that program, I wanted to be able to create arrays much like I assigned the values to it. So i asked this question...
Here is my code:
public static void array(){
int[][] array1 = new int[100][100];
int[][] array2 = new int[20][20];
setArrayValue(array1);
setArrayValue(array2);
drawArray(array1);
System.out.println();
drawArray(array2);
}
public static void setArrayValue(int x[][]){
for(int row = 0; row<x.length; row++){
for(int column=0; column<x[row].length; column++){
x[row][column]= (column+1)*(row+1);
}
}
}
public static void drawArray(int x[][]){
for(int row=0; row<x.length; row++) {
for(int column=0; column<x[row].length;column++){
System.out.print(x[row][column] + "\t");
}
System.out.println();
}
}
Your concept doesn't make sense.
You might want to use a Map<String, int[][]>, which will map names to arrays.
What you are trying to do is not possible in Java. In the createArray method, name is of type String and cannot be redeclared as an int array.
Perhaps you are interested in a Map that uses String objects as keys? The values could be int arrays (or any other object).
No, you can't do that.
Variable names are not variable in Java.
Furthermore, local variables even lose their names when the code is compiled. Variable names are just a help for the programmer to distinguish between variables.
Nop can't be done. Variable names need to be known before hand.
No, it is not possible. You might be able to accomplish your task with a TreeMap or another Map implementation instead.
Instead of saying
name = something;
You would say
map.put(name, something);
Instead of
name[0] + 7
You'd say
map.get(name)[0] + 7
As stated by others - this cannot be done. That is because Java compiler needs to know exact name of a variable at the compile time. This is mandatory, since otherwise Java compiler wouldn't know which variable you are addressing, so it couldn't perform, for instance, type-safety checks.
However, if you just wish to stamp your variable with some unique ID, I guess the solution is closest to what has been stated by SLaks. Simply use Map, and You should be good. Example below.
Map<String, int[][]> myMap = new HashMap<String, int[][]>();
myMap.put("someUniqueName", new int[][] {{0,0}, {1,1}});
and later on:
int[][] array = myMap.get("someUniqueName");
Hope that helps achieve what You want.
Strictly: almost ;-) You can add a dynamic field in a class, which could be an array, using AOP. But...
It's difficult.
This solution is too complicated in most cases. You could probably solve your real problem in a much easier way.
Some advice: start with the beginning... and try using List (interface) / ArrayList as much as possible unless you have some pretty good reason to use an array.
Related
I want to have a array of integers where the length is variable. The obvious choice is to use ArrayList but can I do this for primitive types such as
ArrayList<int> myArray=new ArrayList<int>();
I dont want to use
ArrayList<Integer>
because the Integer class is clumsy in terms of coding.
EDIT: From the answers below I think the solution is to write my own Integer class.
To answer the question below about "clumsy" let me give a specific, and I would of thought common use for integers namely using the last member of the array in any place you would want the integer. If I just call the array "name" then to get the actual integer that can be operated on I need
name.get(name.size()-1).intValue();
To me this seems like an awfully unwieldy expression for a simple integer - particularly if it appears in an expression twice. It also seems that (most of the) methods available for the Integer class are absolutely redundant. Take two examples
static int compare(int a, int b)
Quite unbelievably, according to the documentation, this method returns a-b!!
static Integer valueOf(int a)
returns an Integer instance of the integer a. Can someone give me a single example where
new Integer(a)
does not achieve exactly the same result?
Method 1: (not recommended)
You can do something like this, but this doubles the code and is not efficient:
int[] a;
//get size (from command line maybe ow whatever method you want)
You can set size 0 initially, and for ex. you are transferring values from arraylist so you will have to write:
while(itr.hasNext()){
size++;} //itr is an object of Iterator
int i=0;
a=new int[size];
// then loop again to store values
while(itr.hasNext()){
a[i]=itr.next();
i++;}
Method 2:
Or you may use ArrayList without making it clumsy as follows:
ArrayList al=new ArrayList();
then you may declare Integer objects as volatile and perform operations on them just as you do with the primitive types.
Method 3: (not recommended)
Or simply write:
ArrayList al=new ArrayList();//ignore the warning about <E>
int x=2;
al.add(2);
Method 4: (recommended)
If I were you I would use ArrayList<Integer>.
UPDATE: Another thing that might work is that you may initially create an ArrayList<Integer> and store values there and later convert it to int[]
This SO answer tells about the conversion. Quoted the code form there:
public static int[] convertIntegers(List<Integer> integers)
{
int[] ret = new int[integers.size()];
for (int i=0; i < ret.length; i++)
{
ret[i] = integers.get(i).intValue();
}
return ret;
}
Hope this helps.
No it's not possible to use primitive types as generic type.
Well I would recommend you do use ArrayList and avoid primitive types in this case.
You can't change the size of an array once created. You have to allocate it bigger than you think you'll ever need
or
Accept the overhead of having to reallocate it to a new larger array and copy the data from the old to the new:
System.arraycopy(oldItems, 0, newItems, 0, 10);
But Much simpler to go with ArrayList.
I have following lines of code:
private ArrayList<wordClass>[] words;
and
public class wordClass {
public String wordValue = null;
public int val = 0;
public boolean used = false;
}
Is there anyway I can access wordValue, val, and used via words? Like words[5].val? I know I can do that if they are just in an array of wordClass, but I want a dynamic array to make it easier to add and subtract from the array.
And yes, I know the values should be private. Just don't want to write getters and setters yet.
Thanks.
Do you really want an Array of an ArrayList?
It doesn't seem correct.
In Arrays, you use [] to access (words[0]).
In ArrayLists, you should use words.get(0).
The way you have coded, you should use: words[0].get(0).val to get the very first value.
But I recommend you to review your words definition.
ArrayList Documentation: http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html
Regards,
Bruno
Your code is a bit off for a dynamic array (Java has immutable arrays), so you need an ArrayList. Also, Java uses Capital Letters for class names (please follow the convention) -
// like this, changing wordClass to WordClass. Also, using the diamond operator
private ArrayList<WordClass> words = new ArrayList<>();
To access your WordClass fields you can use something like -
for (WordClass wc : words) {
if (wc.used) {
System.out.println(wc.wordValue + " = " + wc.val);
}
}
Note, you still need to create WordClass instances and place them into the words List.
Write wrapper classes for each value. e.g. What you call "getters".
Then call:
words[1].getWordValue() ==> None
Voila
i have an array of strings which i want to convert to int, pretty simple and straightforward here is the code :
public static void main(String[] args) {
String myarray[]=readfile("[pathtothefile]");
int mynums[] = new int[myarray.length];
for (int i=0;i<myarray.length;i++){
mynums[i]=Integer.parseInt(myarray[i]);
}
System.out.print(Arrays.toString(mynums));
}
But the Problem here is, if i initialize "mynums" like this: mynums[]=null; i get NullPointerException on the following line:
"mynums[i]=Integer.parseInt(myarray[i]);"
what i have to do to solve it is
int mynums[] = new int[myarray.length];
here someone explained why it happens but i dont know how to initialize now! i mean sometimes i dont know how big my array can get and i just want to initialize it. is it even possible?
In Java everything is a pointer behind the scenes. So when you do mynums[]=null, you are pointing to a null. So what is null[i]? That is where your NPE comes from. Alternatively when you point it to an array, then you are actually accessing the i'th element of the array.
You have to first initialize the array because it allocates memory depending on the array size. When you want to add for example an integer to an array it writes the int into previously allocated memory.
The memory size won't grow bigger as you add more items.( Unless you use Lists or Hashmaps, ... but it's not true for generic arrays)
If you don't know how big your array will be, consider using SparseIntArray. which is like Lists and will grow bigger as you add items.
Briefly, in java an array is an object, thus you need to treat it like an object and initialize it prior to doing anything with it.
Here's an idea. When you're initializing something as null, you're simply declaring that it exists. For example ... if I told you that there is a dog, but I told you nothing about it ... I didn't tell you where it was, how tall it was, how old, male/female, etc ... I told you none of its properties or how to access it, and all I told you was that there IS a dog (whose name is Array, for sake of argument), then that would be all you know. There's a dog whose name is Array and that is it.
Typically, arrays are used when the size is already known and generally the data is meant to be immutable. For data that are meant to be changed, you should use things like ArrayList. These are intended to be changed at will; you can add/remove elements at a whim. For more information about ArrayList, read up on the links posted above.
Now, as for your code:
public static void main(String[] args) {
ArrayList<int> myInts = new ArrayList<int>();
// define a new null arraylist of integers.
// I'm going to assume that readfile() is a way for you get the file
// into myarray. I'm not quite sure why you would need the [], but I'll
// leave it.
String myarray[] = readfile("[pathtothefile]");
for (int i = 0; i < myarray.length; i++) {
//adds the value you've specifed as an integer to the arraylist.
myInts.add(Integer.parseInt(myarray[i]));
}
for (int i = 0; i < myInts.size(); i++) {
//print the integers
System.out.print(Integer.toString(myInts.get(i)));
}
}
What if you don't use an array but an ArrayList? It grows dynamically as you add elements.
In java how to use class variables in methods?
this is the code that I have
public class ExamQ3a {
String[] descriptionArr = new String[50];
static int[] codeArr = new int[50];
public static void displayItems(int count, int[] codeArr,
String[] descriptionArr) {
count = this.codeArr.length;
for (int i = codeArr.length; i < codeArr.length; i--) {
}
}
}
The line that is being highlighted here is the count = this.codeArr.length; the error that I am getting is that the non-static variables cannot be referenced from a static context. But I already made the variable static. So what gives?
So as per request only! not that I want to ask the whole question, just to know why I want to use static, this is a practice question
You are to develop a simple application system to manage the inventory
in a company. The system should be able to maintain a list of up to 50
items. Each item has a unique integer code and a description.
(a) Write Java statements that declare and create two arrays to store the
code and the description of the items.
(b) Write Java method with the following method signature:
public static void displayItems(int count, int[] codeArr, String[] descriptionArr)
This method displays the code and description of all items in the company
in tabular form with appropriate column heading.
Parameters: codeArr: the array that stores the codes of the items
descriptionArr: the array that stores the descriptions of the items
count: the number of items in the system
There is no this in the static world. Get rid of it. To explain, this refers to the current instance, and when you're dealing with static methods or variables, you're dealing with items associated with the class, not with any one particular instance. So change the code to:
count = codeArr.length;
Edit 1
As an aside, you don't want to bunch up your closing braces like } } } which makes your code very difficult to read and follow. White space is free, so might as well use it judiciously to improve code readability.
Edit 2
You state:
so how would I reference the array codeArr to the class variable codeArr?
You're inside of the class, and there's no need to use the class variable name here since it is assumed to be used. Just use the static variable or method name and you should be golden.
Edit 3
Your use of static for this type of variable gives the code a bad smell. I'm thinking that your entire program would be much better off if this were an instance variable and not a static variable. For more discussion on this, you may tell us why you decided to make the variable static.
Is you're going to reference a static variable having the same name as a method parameter you prefix the static variable with the name of the class. In this case it would be ExamQ3a.codeArr.
The other way to handle this is to pick different names for your method parameters, or start using a common prefix for instance/static variables.
Another point to note is that, in the following piece of code statement1 will never be executed:
for (int i = codeArr.length; i < codeArr.length; i--) { statement1; }
it should be either
int length = codeArr.length;
for (int i = 0; i < length; i++) { ... }
or
int length = codeArr.length;
for (int i = (length-1); i > -1 ; i--) { ... }
I have an ArrayList of my own class Case. The class case provides the method getCaseNumber() I want to add all of the cases casenumber to a String[] caseNumber. I've tried this
public String[] getCaseNumberToTempList(ArrayList<Case> caseList) {
String[] objectCaseNumber = null;
for(int i = 0; i < caseList.size(); i++) {
objectCaseNumber[i] = caseList.get(i).getCaseNumber();
}
return objectCaseNumber;
}
But my compiler complaints about that the objectCaseNumber is null at the point insid the for-loop. How can I manage to complete this?
Well, you need to create an array to start with, and initialize the variable with a reference to the array. (See the Java tutorial for arrays for more information.) For example:
String[] objectCaseNumber = new String[caseList.size()];
Alternatively, build a List<String> (e.g. using ArrayList) instead. That's more flexible - in this case it's simple as you know the size up front, but in other cases being able to just add to a list makes life a lot simpler.
In idiomatic Java, you wouldn't use ArrayList as a parameter type. Use List.
Slightly more overhead, but simpler and more readable code is to accumulate in another List and then convert into an arrray:
public String[] getCaseNumberToTempList(List<Case> caseList) {
final List<String> r = new ArrayList<String>();
for (Case c : caseList) r.add(c.getCaseNumber());
return r.toArray(new Case[0]);
}
In your code it does make sense to insist on ArrayList due to performance implications of random access via get, but if you use this kind of code (and I suggest making a habit of it), then you can work with any List with the same results.
Well, as I think you may have misunderstood Arrays as a primitive type. Arrays in java are objects and they need to be initialized before you access it.