java logical not operator [duplicate]

问题

This question already has answers here:

What does an exclamation mark mean in Java? (6 answers)

Closed last year.

This is a silly question but I just want to understand it clearly before I start using it.

if (!ServiceHelpers.DISCOVER) {
  ServiceHelpers.discover(MainActivity.this, peerList);
}

I would like to know what !ServiceHelpers.DISCOVER mean?

回答1:

! operator inverts the value of a boolean.

In this case the boolean is ServiceHelpers.DISCOVER.

If it's value is true the ! operator will make it false or vice-versa.

回答2:

! (logical not)

Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false.

From Javadocs,

!Logical complement operator;

inverts the value of a boolean

if (!ServiceHelpers.DISCOVER) 

is same as

if( Not True)

meaning, ServiceHelpers.DISCOVER is not True the enter inside If

回答3:

! is a logical not operator in java.

Code:

 boolean a = true; // --> variable 
   if(! a)
   {
      System.out.println("Not A i.e. A if false");
   }

if a = true, then (!a) evaluates to (not a) i.e (not true) i.e (false) and if condition is false, then it doesn't evalutes the if condition.

来源：https://stackoverflow.com/questions/33932074/java-logical-not-operator