作业分配Java

Question

class string1 {
    static public void main(String[] ar) {
        String s1 = "test";
        String s2 = new String();
        if(s2=null) { // line 0
            System.out.println(s1);
            System.out.println(s2.length()); //Line 1
        }
    }
}

What i expected to happen was

a) s2 will be set to null because its an assignment operation as I am not using ==

b) And I will get a NPE at Line 1 during Runtime

and instead I got the following output as shown below.

Output is

if(s2=null)
     ^
  required: boolean
  found:    String
1 error

Can someone explain why I am getting compile error ?

Answer 1

Java will not implicitly cast to a boolean like some languages will. You must put an expression that evaluates to a boolean inside of conditional:

if ((s2 = null) != null) { }

This condition of course makes no sense, but it's the equivalent of what you're attempting to do. It's worth noting by the way that assignments should rarely be done in a condition.

Answer 2

You should do like this:

if( (s2 = null) != null)
{
  // do stuff
}

The reason you are getting this error because you have to explicitly put the code for boolean evaluation in conditions as Java doesn't implicitly cast it boolean .

Answer 3

An if-then statement is executed by first evaluating the Expression.

From, JLS - Chapter14 : Blocks and Statements

14.9.1. The if-then Statement

An if-then statement is executed by first evaluating the Expression. If the result is of type Boolean, it is subject to unboxing conversion (§5.1.8).

If evaluation of the Expression or the subsequent unboxing conversion (if any) completes abruptly for some reason, the if-then statement completes abruptly for the same reason.

Otherwise, execution continues by making a choice based on the resulting value:

If the value is true, then the contained Statement is executed; the if-then statement completes normally if and only if execution of the Statement completes normally.

If the value is false, no further action is taken and the if-then statement completes normally.

Answer 4

You should note the fact that in java there are only two boolean values : true and false unlike some other languages which use 0 as false and anything else as true .

Can someone explain why I am getting compile error ?

In an if statement the condition can either be true or false which are returned by comparison operations and by the boolean variables.

In,

if(s2=null)

the s2=null doesn't evaluate to any boolean value ie, true or false therefore the compiler error :

required: boolean
  found:    String

Answer 5

The if condition evaluates only boolean conditions.

And the assignment operator does not give a boolean result here.

That is the reason for getting exception.

If you use if ((s2 = null) != null) instead of if(s2=null) it will work.

Answer 6

Besides what everyone else said, s2 will never be null - you just assigned a new String to it! Are you sure you're not conflating string length/existence tests from other languages?