“+” operator for Java-classes

问题:

I have a class like this:

private static class Num {
    private int val;

    public Num(int val) {
        this.val = val;
    }
}

Is it possible to add to objects of the class by using the "+"-operator?

Num a = new Num(18);
Num b = new Num(26);
Num c = a + b;

回答1:

No, you can't. + is overloaded only for numbers, chars and String, and you are not allowed to define any additional overloadings.

There is one special case, when you can concatenate any objects' string representation - if there is a String object in the first two operands, toString() is called on all other objects.

Here's an illustration:

int i = 0;
String s = "s";
Object o = new Object();
Foo foo = new Foo();

int r = i + i; // allowed
char c = 'c' + 'c'; // allowed
String s2 = s + s; // allowed
Object o2 = o + o; // NOT allowed
Foo foo = foo + foo; // NOT allowed
String s3 = s + o; // allowed, invokes o.toString() and uses StringBuilder
String s4 = s + o + foo; // allowed
String s5 = o + foo; // NOT allowed - there's no string operand

回答2:

No, because James Gosling said so:

I left out operator overloading as a fairly personal choice because I had seen too many people abuse it in C++.

Source: http://www.gotw.ca/publications/c_family_interview.htm

Reference: Why doesn't Java offer operator overloading?

回答3:

No. Java does not support operator overloading (for user-defined classes).

回答4:

There is no operators overloading in java. The only thing which is supported for objects, is string concatenations via "+". If you have a sequences of objects joined via "+" and at least one of them is a String, then the result will be inlined to String creation. Example:

Integer a = 5;
Object b = new Object();

String str = "Test" + a + b;

will be inlined to

String str = new StringBuilder("Test").append(a).append(b).toString();

回答5:

No, it is not possible, as Java doesn't support operator overloading.