Description Simon Archer 2003-01-01 13:04:00 EST

It would be useful for Eclipe to include a Compiler Problem Marker to flag the 
case where the assignment operator in a boolean expression was mistakenly used 
when the identity operator was intended.  Consider the following legal Java 
code:

public class AssignmentTest {
  private boolean test() {
    return true;
  }
	
  public void start() {
    boolean value = false;
	
    if (value = test()) {
      System.out.println("Passed");
    }
  }
}


In the start() method did the author really mean to use the assignment 
operator?  Or perhaps he intended to use the identity operator:

    if (value == test()) {
      System.out.println("Passed");
    }


Unfortunately, just like C, Java's assignment operator returns a value that can 
lead to problematic code as shown above.  I would argue that in most cases the 
author intended to use the identity operator.  I would like to suggest that 
Eclipse includes a Compiler Problem Marker to flag this problem and that the 
default value be "Warning".  This marker should apply to complex boolean 
expressions such as if(...), while(...), do while(...).  There may well be 
other places where this marker can be applied.

The following code should probably NOT be flagged by the marker:

    // Added parens with explicit boolean identity test.
    if ((value = test()) == true) {
      System.out.println("Passed");
    }


And possibly even the following:

    // Added parens with implicit boolean identity test.
    if ((value = test())) {
      System.out.println("Passed");
    }


BTW, a well known coding conventions that helps prevents this problem involves 
reordering the boolean expression as follows:

    if (test() = value) {  // COMPILE ERROR!
      System.out.println("Passed");
    }

Compile Error: "The left-hand side of an assignment must be a variable"