Difference of null and another proposition using this null object

Question

If I have a code, for example:

if(window!=null && window.isOpen()){
   //todo
}

If window for null, Will he still try to call the second proposition or not check anymore? Because if he tries to call, then will give a NullPointerException.

Answer 1

The second will not be called. That’s called short Circuit Evaluation (in Portuguese). When the expression already gets a final guarantee value, there is no reason to keep checking the rest and the execution ends. In the case how the relational operator is an "AND" and the two operands need to be true to result in true, if the first is already false the expression is known to be false.

This is equivalent to:

if (window != null) {
    if (window.isOpen()) {
       //todo
    }
}

I put in the Github for future reference.

but it’s a more concise way.

This is the correct technique, and quite used, to ensure that the object is not null. First it checks if it is null and then an operator "E" does what you want, which will only be executed if the first one is true. You can be creative with a lot of stuff in there and use the same technique for a lot of things.

There are cases where you use the ||. in that case if the first true the second will not be executed, since in an operator "OR" just one true being need not check the other operand.

Answer 2

In order to enter the if, both conditions need to be true, if the first is false(that is to say, windows == null), he will ignore the second and will not enter the if.

Take this example:

String b = null;

if(b != null && b.equals("")){
    System.out.println(b);
}else{
    System.out.println("b não atende as condições do if");
}

The output will be just the display of the message b não atende as condições do if, for b is null.

See working on ideone.

Answer 3

I do not know if it proceeds, but my college teacher said that if you use & (a &) it checks both, and if you use && (two &) it check the first and if it already gives negation it does not check the second.

Reference: http://diegonascimentojava.blogspot.com.br/2013/09/operadores-logicos-and-or-xor.html