是否可以在java中定义使用原语的自定义类型？

Question

For example the following is syntactically correct code

Double number = 10.0;

Is it possible to define my own class such as Price

Price myPrice = 10.0;

Actually compiles ?

Answer 1

Auto-boxing and auto-unboxing only works with primitives. The concept you are talking about is similar to C++ conversions. Unfortunately, there is no such thing in Java. The best you can do is

Price myPrice = new Price(10.0);

Answer 2

No, you can't define your own primitive types for numerical quantities.

Declaring Price myPrice means that the variable myPrice will be of type Price and will be used to as its instance.

You can have following valid.

Suppose you declare variable myPrice of type Price . Some instance variables can be accessed via that myPrice reference.

Price myPrice = new Price();
myPrice.value = 10.0;
myPrice.currency = "Dollar"; 
etc ....

Answer 3

You can't extend Double class, since it is final.

But you can extend Number class and implement Comparable interface - the way actual Double class is created .

Answer 4

No. It's hard-wired into the language.

Only primitives may be instantiated without the new keyword, except String , which although not a primitive, may be assigned using primitive syntax. ie both new String("foo") and "foo" will do it (note though that these are not exactly the same).