Return value of assignment operation in Java

Question

I encountered a statement in Java

while ((line = reader.readLine()) != null) {
    out.append(line);
}

How do assignment operations return a value in Java?

The statement we are checking is line = reader.readLine() and we compare it with null.

Since readLine will return a string, how exactly are we checking for null?

Answer 1

The assignment operator in Java returns the assigned value (like it does, e.g., in c). So here, readLine() will be executed, and it's return value stored in line. That value is then checked against null, and if it is null, the loop will terminate.

Answer 2

Assignment expressions are evaluated to their assignment value.

(test = read.readLine())

>>

(test = <<return value>>)

>>

<<return value>>

Answer 3

(line = reader.readLine()) != null

means

1. the method readLine() is invoked.
2. the result is assigned to variable line,
3. the new value of line will be proof against null

maybe many operations at once...

Answer 4

reader.readLine() reads and returns a line for you. Here, you assigned whatever returned from that to line and check if the line variable is null or not.