java 数组的协变和逆变

先说结论：

1. 基元类型数组不允许协变和逆变，无法编译通过。
2. 引用类型数组允许协变和逆变，逆变时会检查实际类型，如果不相符则抛出java.lang.ClassCastException。

下面是验证代码。

 1 public class TestArrayInstance {
 2     public static void main(
 3         String[] args
 4     ) {
 5         char[]      a1 = {'1', '2', '3'};
 6         int[]       a2 = {1, 2, 3};
 7         long[]      a3 = {1L, 2L, 3L};
 8         float[]     a4 = {1.0F, 2.0F, 3.0F};
 9         double[]    a5 = {1.0, 2.0, 3.0};
10         Integer[]   a6 = {1, 2, 3};
11         Long[]      a7 = {1L, 2L, 3L};
12 
13         String[]    a8 = {"1", "2", "3"};
14         Object[]    a9 = {"1", 2L, 3};
15 
16         printCheck(a1, Character[].class);
17         printCheck(a1, Object[].class);
18         printCheck(a2, Integer[].class);
19         printCheck(a2, Object[].class);
20         printCheck(a2, Long[].class);
21         printCheck(a2, long[].class);
22         printCheck(a3, Long[].class);
23         printCheck(a3, double[].class);
24         printCheck(a4, Float[].class);
25         printCheck(a4, Double[].class);
26         printCheck(a4, double[].class);
27         printCheck(a5, Double[].class);
28         printCheck(a6, Long[].class);
29         printCheck(a6, int[].class);
30         printCheck(a6, long[].class);
31         printCheck(a7, Double[].class);
32         printCheck(a7, long[].class);
33         printCheck(a7, int[].class);
34         printCheck(a8, Object[].class);
35         printCheck(a9, String[].class);
36     }
37 
38     private static void printCheck(
39         Object v,
40         Class<?> c
41     ) {
42         System.out.println(paddingClassName(v.getClass()) + " instanceof " + paddingClassName(c) + (c.isInstance(v) ? " true" : " false"));
43     }
44 
45     private static String paddingClassName(
46         Class<?> c
47     ) {
48         return (c.getSimpleName() + "            ").substring(0, 12);
49     }
50 }

执行结果：

char[]       instanceof Character[]  false
char[]       instanceof Object[]     false
int[]        instanceof Integer[]    false
int[]        instanceof Object[]     false
int[]        instanceof Long[]       false
int[]        instanceof long[]       false
long[]       instanceof Long[]       false
long[]       instanceof double[]     false
float[]      instanceof Float[]      false
float[]      instanceof Double[]     false
float[]      instanceof double[]     false
double[]     instanceof Double[]     false
Integer[]    instanceof Long[]       false
Integer[]    instanceof int[]        false
Integer[]    instanceof long[]       false
Long[]       instanceof Double[]     false
Long[]       instanceof long[]       false
Long[]       instanceof int[]        false
String[]     instanceof Object[]     true
Object[]     instanceof String[]     false