三者都是取得物件的長度的方法

length :
Arrays (int[], double[], String[]) — 取得Array的長度

int[] intArray = new int[20]; 
int lengthArray = intArray.length;

length():
String related Object (String, StringBuilder, etc) — 取得字串的長度

String iAmString = "iAmString"; 
int lengthString = iAmString.length();

size():
Collection Object (ArrayList, Set, etc) — 取得集合物件相關大小

ArrayList<String> iAmArrayList = new ArrayList<String>();
    iAmArrayList.add("A1");
    iAmArrayList.add("A2");
int lengthArrayList = lengthArrayList.size();