Java Language Tutorial => Pitfall: combining assignment and...

Example

Occasionally we see StackOverflow Java questions (and C or C++ questions) that ask what something like this:

i += a[i++] + b[i--];

evaluates to ... for some known initial states of i, a and b.

Generally speaking:

for Java the answer is always specified¹, but non-obvious, and often difficult to figure out
for C and C++ the answer is often unspecified.

Such examples are often used in exams or job interviews as an attempt to see if the student or interviewee understands how expression evaluation really works in the Java programming language. This is arguably legitimate as a "test of knowledge", but that does not mean that you should ever do this in a real program.

To illustrate, the following seemingly simple example has appeared a few times in StackOverflow questions (like this one). In some cases, it appears as a genuine mistake in someone's code.

int a = 1;
a = a++;
System.out.println(a);    // What does this print.

Most programmers (including Java experts) reading those statements quickly would say that it outputs 2. In fact, it outputs 1. For a detailed explanation of why, please read this Answer.

However the real takeaway from this and similar examples is that any Java statement that both assigns to and side-effects the same variable is going to be at best hard to understand, and at worst downright misleading. You should avoid writing code like this.

^{1 - modulo potential issues with the Java Memory Model if the variables or objects are visible to other threads.}

PDF - Download Java Language for free

Previous Next

Java Language

Fastest Entity Framework Extensions

Example

Got any Java Language Question?

Java Language

Java Language Common Java Pitfalls Pitfall: combining assignment and side-effects

Fastest Entity Framework Extensions

Example

Got any Java Language Question?