学习OOP，怎么了？

Question

I can't get why the Exception always fires when the value of $lf is 1 or 2.

class Demo {
    public function __construct($lv) {
        if ($lv != 1 || $lv != 2) {
            throw new Exception('Wrong access level!');
        }    
    }

    $objDemo = new Demo(1);

What I want is for an exception to be thrown if the value of $lv isn't 1 or 2.

Answer 1

Your issue is not OOP-related.

You're passing the value of 1 into your $lv variable. That makes for the following situation:

$lv = 1;

if ( $lv != 1 || $lv != 2 ){
    throw new Exception('Wrong access level!');
}

We can separate what's in the if statement:

• The first expression ($lv != 1) will be evaluated to false because 1 does equal 1.
• The second expression ($lv != 2) will be evaluated to true because 1 does not equal 2.

In this case:

false || true

Evaluates to true.

What you want instead is to change the or operator to an and operator:

if ( $lv != 1 && $lv != 2 ){
    throw new Exception('Wrong access level!');
}

Or, equivalently:

if ( !( $lv == 1 || $lv == 2 )){
    throw new Exception('Wrong access level!');
}