What are the 3 dots in parameters?/What is a varia

问题:

This question already has an answer here:

• Can I pass an array as arguments to a method with variable arguments in Java? 5 answers
• Java, 3 dots in parameters 9 answers

I am wondering how the parameter of ... works in Java. For example:

public void method1(boolean... arguments)
{
  //...     
}

Is this like an array? How I should access the parameter?

回答1:

Its called Variable arguments or in short var-args, introduced in Java 1.5. The advantage is you can pass any number of arguments while calling the method.

For instance:

public void method1(boolean... arguments) throws Exception {
    for(boolean b: arguments){ // iterate over the var-args to get the arguments.
       System.out.println(b);
    }
 }

The above method can accept all the below method calls.

method1(true);
method1(true, false);
method1(true, false, false);

回答2:

As per other answer, it's a "varargs" parameter. Which is an array.

What many people don't realise is two important points:

• you may call the method with no parameters: method1();
• when you do, the parameter is an empty array

Many people assume it will be null if you specify no parameters, but null checking is unnecessary.

---

You can force a null to be passed by calling it like this:

method1((boolean[])null);

But I say if someone does this, let it explode.