问题
I can't understand how an object is created implicitly.
Example:
String s = "implicit instantiation";
Can I make my own class whose objects can be created implicitly?
回答1:
No, String instantiation is handled implicitly by the compiler. Only the String and Array classes have this property.
String greeting = "Hello world!";
char[] helloArray = { 'h', 'e', 'l', 'l', 'o', '.' };
Autoboxing allows you to implicitly instantiate objects of primitive wrapper types, but that's also a special case handled by the compiler. You can't create your own classes with this ability.
Boolean b = false;
Integer i = 0;
Double pi = 3.1416;
回答2:
Unfortunately you just can not do that!
opposite to C or C++ you can not overload any operator in java language, so there is no possible way to do something like
Foo myFoo = 1
in the case of the string class:
String s = "implicit instantiation"
that is sugar sintax for the developers, behind the scenes is the compiler doing the "dirty" work and doing something like (remember there is a string pool):
String s = new String("implicit instantiation")
The same applies for some other Types like Arrays, or wrapper for numbers...
回答3:
For every instance of an object you need a Constructor and a constructor its a a special method for construct and initialize methods. Example:
String s; // Is not initialized and it's nos constructed.
So how do you construct a new object in java? Easy with the new operator you create a New Object!
s = new String("qwe"); // New object constructed
But here is something that a lot of newbies get confussed. Why i can do this:
String s= "asdfasd;"
Because String is a special case in Java and you don't need to add a new operator like all the primitive variables that are classes. Example:
Integer i = 3;
Double d = 3.3d;
and So on.
来源：https://stackoverflow.com/questions/43372097/how-java-create-objects-implicitly-like-in-case-of-string-class