Array Vector Set & Map

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Java集合的一些基础知识。

Array

An array stores a sequence of values

  • same type
  • primary type and String
  • fixed size

3 steps to create an array:

int[] a;						// 1. declare
a = new int[9];					// 2. create
for (int i = 0; i < 9; i++) {	// 3. store values
    a[i] = i*10;
}

example:

String[] b = new String[]{"1", "2", "3"};
String[] c = {"1", "2", "3"};

Two-dimensional arrays, e[1][2] = 23:

int[][] e = {	{11,12,13},
                {21,22,23},
                {31,32,33}	};

Vector

  • can grow or shrink
  • access its data using index
  • similar to ArrayList but it is synchronised
  • implements List
1
Vector<String> v = new Vector<>();	// initial capacity is 10.

Vector VS ArrayList

Synchronization

  • Vector is synchronized, only one thread at a time can access the code (thread-safe)
  • ArrayList is not synchronized, multiple threads can work on arrayList at the same time. (not thread-safe)

Size

  • ArrayList increments 50% of the current array size
  • Vector increments 100%.

Traversal

  • Vector: Enumeration and Iterator
  • ArrayList: Iterator

Usage

ArrayList is newer and faster. usually choose ArrayList

Set

  • unordered collection of objects

  • duplicate values cannot be stored

      Set<String> s = new HashSet<>();
  s.add("1");
  s.add("2");
  s.add("3");
  s.add("3");
  System.out.println(s);	// [1, 2, 3]
  s.get(0)	// error

Map

Not a subtype of Collection

  • mapping between key and value
  • no duplicate keys allowed
  • HashMap had no order, while TreeMap and LinkedHashMap have