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 1 public class TestArrayInstance {
2 2     public static void main(
3 3         String[] args
4 4     ) {
5 5         char[]      a1 = {'1', '2', '3'};
6 6         int[]       a2 = {1, 2, 3};
7 7         long[]      a3 = {1L, 2L, 3L};
8 8         float[]     a4 = {1.0F, 2.0F, 3.0F};
9 9         double[]    a5 = {1.0, 2.0, 3.0};
1010         Integer[]   a6 = {1, 2, 3};
1111         Long[]      a7 = {1L, 2L, 3L};
1212 
1313         String[]    a8 = {"1", "2", "3"};
1414         Object[]    a9 = {"1", 2L, 3};
1515 
1616         printCheck(a1, Character[].class);
1717         printCheck(a1, Object[].class);
1818         printCheck(a2, Integer[].class);
1919         printCheck(a2, Object[].class);
2020         printCheck(a2, Long[].class);
2121         printCheck(a2, long[].class);
2222         printCheck(a3, Long[].class);
2323         printCheck(a3, double[].class);
2424         printCheck(a4, Float[].class);
2525         printCheck(a4, Double[].class);
2626         printCheck(a4, double[].class);
2727         printCheck(a5, Double[].class);
2828         printCheck(a6, Long[].class);
2929         printCheck(a6, int[].class);
3030         printCheck(a6, long[].class);
3131         printCheck(a7, Double[].class);
3232         printCheck(a7, long[].class);
3333         printCheck(a7, int[].class);
3434         printCheck(a8, Object[].class);
3535         printCheck(a9, String[].class);
3636     }
3737 
3838     private static void printCheck(
3939         Object v,
4040         Class<?> c
4141     ) {
4242         System.out.println(paddingClassName(v.getClass()) + " instanceof " + paddingClassName(c) + (c.isInstance(v) ? " true" : " false"));
4343     }
4444 
4545     private static String paddingClassName(
4646         Class<?> c
4747     ) {
4848         return (c.getSimpleName() + "            ").substring(0, 12);
4949     }
5050 }

执行结果：

1char[]       instanceof Character[]  false
2char[]       instanceof Object[]     false
3int[]        instanceof Integer[]    false
4int[]        instanceof Object[]     false
5int[]        instanceof Long[]       false
6int[]        instanceof long[]       false
7long[]       instanceof Long[]       false
8long[]       instanceof double[]     false
9float[]      instanceof Float[]      false
10float[]      instanceof Double[]     false
11float[]      instanceof double[]     false
12double[]     instanceof Double[]     false
13Integer[]    instanceof Long[]       false
14Integer[]    instanceof int[]        false
15Integer[]    instanceof long[]       false
16Long[]       instanceof Double[]     false
17Long[]       instanceof long[]       false
18Long[]       instanceof int[]        false
19String[]     instanceof Object[]     true
20Object[]     instanceof String[]     false