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

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)

CodePudding user response：

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.