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");
		} 

		//copy
		arrayList1 = 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