java复制那些事

在开发中，复制是我们经常遇到的。

今天用arraylist来对比一下java中collections的几种复制：

1. =
2. clone()
3. new arraylist(arraylist)
4. Collections.copy

我们可以写一个这样的程序

public class COPY {static ArrayList<Integer> arrayList1;static ArrayList<Integer> arrayList2;static ArrayList<Integer> arrayList3;public static void main(String[] args) {arrayList1 = new ArrayList<>();arrayList2 = new ArrayList<>();for (int i = 0; i < 10; i++) {arrayList1.add(i);}//=arrayList2 = arrayList1;if (arrayList2 == arrayList1) {System.out.println("==指向对象");} //copyarrayList1 = new ArrayList<>();arrayList2 = new ArrayList<>();arrayList2 = (ArrayList<Integer>) arrayList1.clone();if (arrayList2 != arrayList1) {System.out.println("clone()创建新对象");}else {System.out.println("clone()指向对象");}//new Arraylist(old_arraylist)arrayList1 = new ArrayList<>();arrayList2 = new ArrayList<>();arrayList3 = arrayList2;for (int i = 0; i < 10; i++) {arrayList1.add(i);}arrayList2 = new ArrayList<>(arrayList1);if (arrayList2 == arrayList1) {System.out.println("new Arraylist(old_arraylist)指向对象");}else {System.out.println("new Arraylist(old_arraylist)不指向对象");}if (arrayList2 == arrayList3) {System.out.println("new Arraylist(old_arraylist)只复制数值");}else {System.out.println("new Arraylist(old_arraylist)创建新的对象");}arrayList1 = new ArrayList<>();arrayList2 = new ArrayList<>();arrayList3 = arrayList2;for (int i = 0; i < 10; i++) {arrayList1.add(i);}for (int i = 10; i < 20; i++) {arrayList2.add(i);}//注意，copy需要两个collection对象size相同Collections.copy(arrayList2, arrayList1);System.out.println(arrayList2);if (arrayList2 == arrayList1) {System.out.println("Collections.copy指向对象");}else {System.out.println("Collections.copy不指向对象");}if (arrayList3 == arrayList2) {System.out.println("Collections.copy只复制数值");}else {System.out.println("Collections.copy创建新的对象");}}}

他的输出结果是这样的：

==指向对象clone()创建新对象new Arraylist(old_arraylist)不指向对象new Arraylist(old_arraylist)创建新的对象[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]Collections.copy不指向对象Collections.copy只复制数值