[Java] What is ArrayList?

How to declare / initialize ArrayList

ArrayList is declared as follows. ArrayList Variable name = new ArrayList (initial capacity); * Initial capacity can be omitted

ArrayListSample.java

ArrayList<Integer> arrayList = new ArrayList<Integer>();  //No initial capacity specified
ArrayList<Integer> arrayList = new ArrayList<Integer>(3); //Specify the initial capacity
//ArrayList<int> arrayList = new ArrayList<int>();        //Compile error

Why primitive types are bad

Take a look at the contents of the ArrayList

ArrayList.class

public class ArrayList<E> extends AbstractList<E>　… {
    private static final int DEFAULT_CAPACITY = 10;
    private static final Object[] EMPTY_ELEMENTDATA = {};
    transient Object[] elementData;      //← Actual condition of ArrayList data
    private int size;
    //constructor
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }
//Omission

The identity of ArrayList was a convenient operation class centered on __Object [] __. Only __Object type data types can be used as ArrayList. __ Since the primitive type is not a data type that inherits the Object type, can it not be used?

Primitive type

//8 types
byte, short, boolean, char, int, long, float, double

What is the initial capacity specified for?

What happens when you do new, add, get, remove

ArrayList<Integer> arrayList = new ArrayList<Integer>(3);
arrayList.add(new Integer(10); //a
arrayList.add(new Integer(11); //b
arrayList.add(new Integer(12); //c
arrayList.get(1);
arrayList.remove(1);

When the number of elements exceeds the initial capacity

ArrayList<Integer> arrayList = new ArrayList<Integer>(3);
arrayList.add(new Integer(10); //a
arrayList.add(new Integer(11); //b
arrayList.add(new Integer(12); //c
arrayList.add(new Integer(13); //d

How much the movement of the array affects

If not specified

ArrayListSample.java

ArrayList<Integer> arrayList = new ArrayList<Integer>();
long array1Start = System.currentTimeMillis();
for(int i=0; i < 10000000;i++) {              //← 10 million
	arrayList.add(new Integer(10));
}
long array1End = System.currentTimeMillis();
System.out.println("processing time:" + (array1End - array1Start) + " ms");

result

Processing time: 6505 ms

If specified

ArrayListSample.java

ArrayList<Integer> arrayList = new ArrayList<Integer>(10000000);
long array1Start = System.currentTimeMillis();
for(int i=0; i < 10000000;i++) {               //← 10 million
	arrayList.add(new Integer(10));
}
long array1End = System.currentTimeMillis();
System.out.println("processing time:" + (array1End - array1Start) + " ms");

result

Processing time: 5630 ms

There was a difference of about 1 second.

Conclusion

For large capacity, specify the expected capacity