How to build Arrays, push to them, and loop through them in Java/ Android

Question

I come from an action script back ground and i am baffled by how to use arrays in java.

In my main activity i created an empty array called mIcons like so

private Array mIcons;

Now i want to set the value of that array by using a method from my DataBaseHelper class which looks like this:

public Array getHomeScreenIcons() {

        Array iconList;

        // Select All Query
        String selectQuery = "SELECT  * FROM " + homeIcons;

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
            do {

iconList.push(Integer.parseInt(cursor.getString(0)));

            } while (cursor.moveToNext());
        }
        Log.v(TAG, "List Created"); 
        // return contact list

}

that bold line jumping out of the code is the problem so far.. how do i PUSH to my array

Then later i will want to run a for loop for the array from my main activity using.length

The best way to learn about the primitive language constructs is through a book or an online tutorial. This question involves such elementary syntax that it doesn't really belong on SO. — Kirk Woll
– Kirk Woll, Commented Mar 7, 2012 at 20:43

bbedward · Accepted Answer · 2012-03-07 20:42:03Z

7

Use an ArrayList paramaterized to any type you want

ArrayList<Integer> iconList = new ArrayList<Integer>();

add with

iconList.add(Integer.parseInt(cursor.getString(0)));

Iterate with

for (int i: iconList)
{
    // i is your entire array starting at index 0
}

or

for (int i=0; i< iconList.size(), i++)
{

}

answered Mar 7, 2012 at 20:42

bbedward

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4 Comments

devunwired Over a year ago

This answer is mostly correct, except that you cannot pass primitives as the type of a Collection. You will need ArrayList<Integer> iconList = new ArrayList<Integer>() instead.

erik Over a year ago

Wow, ok guys, first off thank you, secondly, can you explain to me the difference between Array and ArrayList? I was looking it up and apparently an Array has to have a predefined length? Obviously ArrayList is what i want to use, but if someone could explain to me why Array's have to have predefined lengths i would love you.. seems absolutely retarded to me?

bbedward Over a year ago

@erik They are very similar, see this link: max.cs.kzoo.edu/AP/Workshops/Java/ArraysArrayLists.html . Remember, you cannot use primitives in an ArrayList you have to use type Object

bbedward Over a year ago

@erik Just think if you need a fixed number of primitives, lets say 10 integers, it will be less resource-intensive to use an Array with int than it would be to use an ArrayList with the Integer object

broody · Accepted Answer · 2012-03-07 20:41:31Z

2

You're probably thinking about ArrayList

private ArrayList<String> iconList = ArrayList<String>();
iconList.add("Stuff");

and then later to loop through

for (int i=0; i<iconList.size(); i++) {
    String newStuff = iconList.get(i);
}

answered Mar 7, 2012 at 20:41

broody

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Comments

Axel Ljungkvist · Accepted Answer · 2012-03-07 20:52:31Z

You should probably hit up some basic java tutorials to get used to the array syntax and functionality. http://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html could be a good starting point.

Looking at your specific problem -

You generally don't want to use the Array class in the manner that you do. It's more of a helper class. Also, it seems that you are going for stack semantics, and you'd likely want to use a Stack instead.

First, arrays: you declare an array like so:

Type[] myArray = new Type[arraySize];

and then you access it with index like so:

Type myThingFromArray = myArray[myArrayIndex];

and you put things in it like so:

myArray[myTargetIndex] = myObjectOfTypeType;

Raw arrays in java have static size and are not easily growable. For most applications it would be a better idea to use a member of the Collections framework instead. If you're actively looking for a stack (as you mention pushing) then you could use Stack<Integer> and have all the regular stack operations.

Another benefit of using a modern collection class is that you can iterate through your collection using the for-each construct, which eliminates some regular for boilerplate. An example:

public ArrayList<Integer> iconList;
public Array getHomeScreenIcons() {

    Array iconList = new ArrayList<Integer>();

    // Select All Query
    String selectQuery = "SELECT  * FROM " + homeIcons;

    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);

    // looping through all rows and adding to list
    if (cursor.moveToFirst()) {
        do {
            iconList.add(Integer.parseInt(cursor.getString(0)));
        } while (cursor.moveToNext());
    }
    Log.v(TAG, "List Created"); 
    // return contact list

    //Iterate like so:
    for (Integer i : iconList){ 
        System.out.println("Here's all integers in the icon-list: " + i);
    }
}

Jan-Henk · Accepted Answer · 2012-03-07 20:46:46Z

0

You can define arrays in Java like this:

int[] intArray = new int[3]; // array for three ints
String[] stringArray = new String[10]; // array for 10 Strings

So for your code, you can do something like this:

if (cursor.moveToFirst()) {
    int[] iconList = new int[cursor.getCount()];
    int index = 0;
    do {
        iconList[index] = Integer.parseInt(cursor.getString(0));
        index++;
    } while (cursor.moveToNext());
}

After that you can loop over the array like this:

for (int icon : iconList) {
    // Do something with icon
}

answered Mar 7, 2012 at 20:46

Jan-Henk

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