Find out what variable is throwing a NullPointerException programmatically

Question

I know I can find out if a variable is null in Java using these techniques:

• if (var==null) -> too much work
• try { ... } catch (Null

Answer 1

You might consider, as announced here, the JDK 14 which should include the JEP 358:

JEP 358: Helpful NullPointerExceptions

Suppose an NPE occurs in this code:

a.b.c.i = 99;

The filename and line number do not pinpoint exactly which variable was null.
Was it a or b or c?

A similar problem occurs with array access and assignment. Suppose an NPE occurs in this code:

a[i][j][k] = 99;

The filename and line number do not pinpoint exactly which array component was null.
Was it a or a[i] or a[i][j]?

Description:

If the more complex statement a.b.c.i = 99; throws an NPE, the message would dissect the statement and pinpoint the cause by showing the full access path which led up to the null:

Exception in thread "main" java.lang.NullPointerException: 
        Cannot read field "c" because "a.b" is null
    at Prog.main(Prog.java:5)

Again: to be tested with JDK 14.

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Holger adds in the comments:

For the expression a[i][j][k], there’s also the possibility that either, i, j, or k has type Integer and is null, so unboxing fails.

In real life scenarios, the expression to the right of the = might bear potential for NPE too.

I tried with jdk-14.0.1

it works; it produces a message like

Cannot invoke "java.lang.Integer.intValue()" because "i" is null then.

When the method has been compiled without debug information, it will use something like "" instead of "i", but that’s unavoidable.