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Why? null is not a common object,then what it is?

 
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public class Test {
void doo(Object obj){
System.out.println("object");
}
void doo(String s){
System.out.println("string");
}
public static void main(String[] args) {
new Test().doo(null);
}
}
 
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Please have a look at the following discussion which handles the same topic:
https://coderanch.com/t/239328/java-programmer-SCJP/certification/null-argument-method-being-passed
 
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null is a literal which is used to give a value to a reference that does not refer to anything.

The compiler has to choose which of those doo() methods to call. Because String is a subclass of Object, String is said to be "more-specific" than object. The compiler will call the doo()method with the most specific formal parameter type.
In this case doo(String) rather than doo(Object) will be chosen.
-Barry
 
Sam Wang
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I know.Thanks.
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