在哪里可以找到“==”等于操作数的 Java 文档？

Question

I believe "==" is for reference comparison (address comparison). So, I believe this should print out false, but I have an error, Incomparable types: java.lang.Integer and java.lang.Double. When comparing addresses, why would type matter? It's not the same address, so print false. Again, I'm just looking for the documentation. It's a hypothetical question.

Integer seven = 7;
Double sevenPointTwo = 7.2;
System.out.println("seven == sevenPointTwo is " + (seven == sevenPointTwo) );

Answer 1

The place you find this defined is the specification , which says:

15.21.3. Reference Equality Operators == and !=

If the operands of an equality operator are both of either reference type or the null type, then the operation is object equality.

It is a compile-time error if it is impossible to convert the type of either operand to the type of the other by a casting conversion (§5.5). The run-time values of the two operands would necessarily be unequal (ignoring the case where both values are null).

At run time, the result of == is true if the operand values are both null or both refer to the same object or array; otherwise, the result is false.

The result of != is false if the operand values are both null or both refer to the same object or array; otherwise, the result is true.

While == may be used to compare references of type String, such an equality test determines whether or not the two operands refer to the same String object. The result is false if the operands are distinct String objects, even if they contain the same sequence of characters (§3.10.5). The contents of two strings s and t can be tested for equality by the method invocation s.equals(t).

(their emphasis)

Equality of objects is about more than where they are in memory. Comparing one type of object to another type of object is never going to yield a true result (see their emphasized part), so rather than have an == that will never be true, the compiler tells you that it's an error.

The compiler does this with other errors that can be detected at compilation time, such as code you can't reach.