Why does javac insert Objects.requireNonNull(this) for final fields?

Since the field is not only final, but a compile-time constant, it will not get accessed when being read, but the read gets replaced by the constant value itself, the iconst_5 instruction in your case.

But the behavior of throwing a NullPointerException when dereferencing null, which would be implied when using a getfield instruction, must be retained¹. So when you change the method to

int sumA() {
  Temp t = this;
  return 1 + t.field;
}

Eclipse will insert an explicit null check too.

So what we see here, is javac failing to recognize that in this specific case, when the reference is this, the non-null property is guaranteed by the JVM and hence, the explicit null check is not necessary.

¹ see JLS §15.11.1. Field Access Using a Primary:

• If the field is not static:

  • The Primary expression is evaluated. If evaluation of the Primary expression completes abruptly, the field access expression completes abruptly for the same reason.
  • If the value of the Primary is null, then a NullPointerException is thrown.
  • If the field is a non-blank final, then the result is the value of the named member field in type T found in the object referenced by the value of the Primary.

  …