java - 相等检查后使用 Long 的 NullPointerException

Question

这让我很震惊。

如果您有一个 Java Long 变量，并且您使用 == 运算符检查原始值是否相等，则该值的运行时类型将更改为原始 long。

随后检查变量的空值然后抛出意外的 NullPointerException。

所以在测试类中:

public class LongDebug {

public static void main(String[] args) {
    Long validValue = 1L; 
    Long invalidValue = -1L;
    Long nullValue = null;

    System.out.println("\nTesting the valid value:");
    testExpectedBehaviour(validValue);
    testUnExpectedBehaviour(validValue);

    System.out.println("\nTesting the invalid value:");
    testExpectedBehaviour(invalidValue);
    testUnExpectedBehaviour(invalidValue);

    System.out.println("\nTesting the null value:");
    testExpectedBehaviour(nullValue);
    testUnExpectedBehaviour(nullValue);
}

/**
 * @param validValue
 */
private static void testExpectedBehaviour(Long value) {
    if (value == null || value == -1) System.out.println("Expected: The value was null or invalid");
    else System.out.println("Expected: The value was valid");
}

private static void testUnExpectedBehaviour(Long value) {
    try {
        if (value == -1 || value == null) System.out.println("Unexpected: The value was null or invalid");
        else System.out.println("Unexpected: The value was valid");
    } catch (NullPointerException e) {
        System.out.println("Unexpected: The system threw an unexpected NullPointerException");
    }
}
}

我得到的结果是:

Testing the valid value:
Expected: The value was valid
Unexpected: The value was valid

Testing the invalid value:
Expected: The value was null or invalid
Unexpected: The value was null or invalid

Testing the null value:
Expected: The value was null or invalid
Unexpected: The system threw an unexpected NullPointerException

这是符合规范还是 JDK 中的错误？

Answer 1

问题是:

value == -1 || value == null

表达式从左到右求值，因为 Long 必须先拆箱，JVM 将其转换为:

value.longValue() == -1 || value == null

和value.longValue()当 value 为 null 参数时抛出 NullPointerException。它永远不会到达表达式的第二部分。

它在顺序不同时有效:

value == null || value == -1

因为如果 value 为 null，第二部分(当 value 为 null) 由于 boolean 表达式 short-circuit evaluation 从未被执行.

Is this on spec or a bug in the JDK?

当然这不是错误。原始值包装器的拆箱方式符合规范(5.1.8. Unboxing Conversion):

• If r is a reference of type Long, then unboxing conversion converts r into r.longValue()

应用拆箱后，剩下的就是标准的Java。