String s = null ； String s = new String()； String s = "; what is the difference between string s?

1.String s =null ;

No memory resources have been requested , It just declares the reference of the object , The object is not really instantiated , Just put the stack memory String The reference points to a null, Indicates that a reference variable is declared and the reference is initialized , But the reference does not point to any object . But it can be passed as a parameter or used for other purposes , But you can't call its method as an object .

2.String s ="" and String s = new Stirng（）;

It's really creating objects , And put the object in heap memory .String s=""; Indicates that a memory resource has been requested , But the resource space value is empty . This statement represents a declaration and reference to an object , Only this object is 0 Bytes . So now that we have an object , You can call the methods of the object .

3.String s ;

This statement indicates that only a reference variable is declared , But the reference is not initialized , So for variables s Any operation of ( In addition to initializing assignments ) Will throw an exception .