条件赋值中的数字推广如何在Java中工作

Question

Found some strange things in java.

Code:

    System.out.println(System.getProperty("java.version"));
    System.out.println((true) ? (int)2.5 : 3.5);
    System.out.println((true) ? (int)2.5 : 3);
    System.out.println((true) ? (int)2.5 + "" : 3.5);

Result:

1.8.0_40
2.0
2
2

What is it? Why integer value returns only if value for false is not a double or if string value added to value for true? Is it a bug?

Answer 1

This specification documents the numeric promotion in Java. In section 2, it says that if either operand is of type double then the result is of type double. In this specification (which was added by @Januson), it is mentioned that:

the type of the conditional expression is the promoted type of the second AND third operands

Now, if we apply those two documents to our examples, we get the following:

System.out.println(System.getProperty("java.version"));

This is the version (release) of Java. It is a String .

System.out.println((true) ? (int)2.5 : 3.5);

In this case the compiler looks at the both parts of the conditional and decides the data type. (int)2.5 is an integer and 3.5 is a double . The result will be a double . So the integer part of 2.5 (ie, 2) will be cast to a double .

System.out.println((true) ? (int)2.5 : 3);

In this case, the compiler looks at 2 integers so the result is an integer . The first case gives an integer and the second case is also an integer so the result will be in integer .

System.out.println((true) ? (int)2.5 + "" : 3.5);

In this case, the compiler looks at the first case and takes the integer part which is 2. Now, because this number is appended to a string, it becomes a String . The result will also be a string. If you try to return 3.5, this will be returned as a String and not a double .

Answer 2

Because of the way ternary operator resolves its return type. Check its specification for more information.

First expression returns double because 2.5 is first converted to int 2 but since third argument is double and int is convertible to double then return type for whole expression is double 2.0

Second return symply int 2 because 2.5 to int is 2 and return type is int.

Third first converts 2.5 to 2, then to String "2" and return type is String.