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Oracel Java Tutorial on Collections

ArrayList

1. You can point an element of Array or ArrayList to null, but remember that a null reference cannot invoke any method. For example, if you have an object set of ArrayList<ArrayList<Point>> whose first element is null. Then you cannot add points (objects of Point) to the first element (which is of type ArrayList<Point>) of set by invoking set.get(0).add. To achieve the goal, you have to first create a new object of ArrayList<Point>, point the first element of set to it, and then add points by invoking set.get(0).add.

2. Elements added into an ArrayList or returned by get from an ArrayList are by reference.

3. To insert an element into an ArrayList with size n (using the add method), the legal range of index is 0 to n. The add method without specifying location add the element to the end of the ArrayList.

4. An Array or ArrayList in Java is essentially a sequence of addresses. Since an address can point to anything, Array or ArrayList in Java can be used as container for anything. Specially, you can use an array as a container for ArrayLists and you can also use an ArrayList as a container for Arrays. For example

   ArrayList<double[]> list;
   

   and :::java ArrayList[] array;

   are both legitimate in Java.

Capacity

1. The clear method of ArrayList does not affect its capacity.

2. The capacity of an ArrayList can be 0.

3. You might be curious that why the methods boolean add(E e) and void add(int index, E e) have different signatures. The reason is that boolean add(E e) is inherited from the Collection interface. For other collections such as set and map, etc., it is beneficial to know whether an elements to be added to it already exists or not. The return value (indicating whether the ...) is then very helpful. For the list specific method void add(int index, E e), there is no need to define the signature as boolean because a list does not force uniqueness and whether an element will be added has nothing to do with whether an element of the same value already exists in the list or not.

Array

1. The Arrays class contains many userful static methods for array operations, but generally speaing, they can only be applied to 1-d array directly. To applied to multi-dimensional arrays, you have to iterate into the array until you have 1-d arrays. For example, if you want to compare whether two 1-d integer arrays are equal or not, you can use the static equals in the Arrays. To compare whether two 2-d interger arrays are equal or not, you can compare whether the two arrays have the same length, and whether each corresponding elements (which are 1-d arrays) of the two arrays are equal (using the static Arrays.equals method). Applying Arrays.equals on a 2-d intger arrays directly compares whether the two 2-d arrays contain same references to 1-d arrays. The static Arrays.equals method is overloaded for double arrays, but it is no a good idea to compare whether two double arrays are equal using Arrays.equals because it does not allow an error tolerance.

2. A 2-d array in java is just an array of arrays, so even if you define

   int[][] a = new int[2][3];
   

   you can still point a[i] to another 1-d array. This means that you can change the length of each element of the 2-d arraya whenever you want.

3. To copy arrays quicly, you can use System.arraycopy which is implemented in native code. It is preferred to other ways such as clone, user-defined method and so on. Notice that System.arraycopy performs shallow copys for arrays of objects, i.e., only references of objects will be copied. If you want to make a deep copy of an array of objects, you have to do it by yourself.

4. A container in Java can be initialized using curly braces (similar to the universal initializer list in C++11), and in this case you do not have to specified the dimensions of the container. To initialized all elements of a 1-dimensional array to a single value, you can the static Arrays.fill method. Note that arrays in Java are automatically initialized to default values, so you need not to initialize an array if the default value is what you want.