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。

public class TestArrayInstance {
    public static void main(
        String[] args
    ) {
        char[]      a1 = {'1', '2', '3'};
        int[]       a2 = {1, 2, 3};
        long[]      a3 = {1L, 2L, 3L};
        float[]     a4 = {1.0F, 2.0F, 3.0F};
        double[]    a5 = {1.0, 2.0, 3.0};
        Integer[]   a6 = {1, 2, 3};
        Long[]      a7 = {1L, 2L, 3L};

        String[]    a8 = {"1", "2", "3"};
        Object[]    a9 = {"1", 2L, 3};

        printCheck(a1, Character[].class);
        printCheck(a1, Object[].class);
        printCheck(a2, Integer[].class);
        printCheck(a2, Object[].class);
        printCheck(a2, Long[].class);
        printCheck(a2, long[].class);
        printCheck(a3, Long[].class);
        printCheck(a3, double[].class);
        printCheck(a4, Float[].class);
        printCheck(a4, Double[].class);
        printCheck(a4, double[].class);
        printCheck(a5, Double[].class);
        printCheck(a6, Long[].class);
        printCheck(a6, int[].class);
        printCheck(a6, long[].class);
        printCheck(a7, Double[].class);
        printCheck(a7, long[].class);
        printCheck(a7, int[].class);
        printCheck(a8, Object[].class);
        printCheck(a9, String[].class);
    }

    private static void printCheck(
        Object v,
        Class<?> c
    ) {
        System.out.println(paddingClassName(v.getClass()) + " instanceof " + paddingClassName(c) + (c.isInstance(v) ? " true" : " false"));
    }

    private static String paddingClassName(
        Class<?> c
    ) {
        return (c.getSimpleName() + "            ").substring(0, 12);
    }
}

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