Assignment operation returns Object or Null in Java

Question

While digging in ArrayList code I spotted this code fragment

  if ((lst = list) != null && (a = lst.elementData) != null) { ... }

Now I really don't know what this expression is called, talking about (lst = list) != null, and why it is used like this. It every assignment in java returns an assigned object or what?

Assignment is an *expression* whose value is the assigned value. — chrylis -cautiouslyoptimistic-, Jul 27 '19 at 22:40
Instead of doing `lst = list;` before the if clause, you can do it in one go like this and save a line of code. — RaminS, Jul 27 '19 at 22:43

score 1 · Answer 1 · answered Jul 27 '19 at 22:55

You are basically assigning the value of list in lst and then doing other checks. The above code basically means

    lst = list;
    if ((lst != null && (a = lst.elementData) != null) { ... }

why it is used like this

Probably just to save a line of code. It definitely reduces the readability a little bit.

Side note : This approach is very useful when you are doing something inside a loop.

try (BufferedReader br = new BufferedReader(new FileReader(file))) {
    String line;
    while ((line = br.readLine()) != null) {
       // process the line.
    }
}

Here in the while loop the value of line keeps getting updated after each iteration of while loop.

Assignment operation returns Object or Null in Java

1 Answers1