Why unboxing of method call parameters does not lead to a helpful NullPointerException

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I investigated helpful NullPointerExceptions from Java 14 and tested different scenarios. Surprisingly, unboxing seems to lead to helfpul NPEs in some scenarios, whereas other scenarios do not contain any error message:

// NPE with message 'Cannot invoke "java.lang.Integer.intValue()" because "index" is null'
Integer index = null;
int i = index;

// NPE with message null
List.of('a', 'b', 'c').indexOf(index);

I wonder, why the second example does not lead to the same message as the first one. I read JEP 358 and learned that the message is comuputed based on the byte code instructions. Are they different for these two scenrios? I expected both snearios to call Integer.intValue() to unbox the value.

(Tested with Oracle JDK 17.0.4.1)

java
1 Answers

The reason is because the cause is different. The List created by List.of doesn't allow nulls, and when indexOf receives a null, it simply throws a NullPointerException without a message:

@Override
public int indexOf(Object o) {
    if (!allowNulls && o == null) {
        throw new NullPointerException();
    }
    Object[] es = elements;
    for (int i = 0; i < es.length; i++) {
        if (Objects.equals(o, es[i])) {
            return i;
        }
    }
    return -1;
}

(from the java.util.ImmutableCollections.ListN class)

On the other hand, in the case of int i = index; it is compiled to the equivalent of int i = index.intValue();, and the JVM generates the message of the NullPointerException based on the method calls and variables involved.

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