Override getter for Kotlin data class

问题:

Given the following Kotlin class:

data class Test(val value: Int)

How would I override the Int getter so that it returns 0 if the value negative?

If this isn't possible, what are some techniques to achieve a suitable result?

回答1:

After spending almost a full year of writing Kotlin daily I've found that attempting to override data classes like this is a bad practice. There are 3 valid approaches to this, and after I present them, I'll explain why the approach other answers have suggested is bad.

1. Have your business logic that creates the data class alter the value to be 0 or greater before calling the constructor with the bad value. This is probably the best approach for most cases.

2. Don't use a data class. Use a regular class and have your IDE generate the equals and hashCode methods for you (or don't, if you don't need them). Yes, you'll have to re-generate it if any of the properties are changed on the object, but you are left with total control of the object.

   class Test(value: Int) {
     val value: Int = value
       get() = if (field < 0) 0 else field
   
     override fun equals(other: Any?): Boolean {
       if (this === other) return true
       if (other !is Test) return false
       return true
     }
   
     override fun hashCode(): Int {
       return javaClass.hashCode()
     }
   }
   

3. Create an additional safe property on the object that does what you want instead of having a private value that's effectively overriden.

   data class Test(val value: Int) {
     val safeValue: Int
       get() = if (value < 0) 0 else value
   }
   

A bad approach that other answers are suggesting:

data class Test(private val _value: Int) {
  val value: Int
    get() = if (_value < 0) 0 else _value
}

The problem with this approach is that data classes aren't really meant for altering data like this. They are really just for holding data. Overriding the getter for a data class like this would mean that Test(0) and Test(-1) wouldn't equal one another and would have different hashCodes, but when you called .value, they would have the same result. This is inconsistent, and while it may work for you, other people on your team who see this is a data class, may accidentally misuse it without realizing how you've altered it / made it not work as expected (i.e. this approach wouldn't work correctly in a Map or a Set).

回答2:

You could try something like this:

data class Test(private val _value: Int) {
  val value = _value
    get(): Int {
      return if (field < 0) 0 else field
    }
}

assert(1 == Test(1).value)
assert(0 == Test(0).value)
assert(0 == Test(-1).value)

assert(1 == Test(1)._value) // Fail because _value is private
assert(0 == Test(0)._value) // Fail because _value is private
assert(0 == Test(-1)._value) // Fail because _value is private

• In a data class you must to mark the primary constructor's parameters with either val or var.

• I'm assigning the value of _value to value in order to use the desired name for the property.

• I defined a custom accessor for the property with the logic you described.

回答3:

The answer depends on what capabilities you actually use that data provides. @EPadron mentioned a nifty trick (improved version):

data class Test(private val _value: Int) {
    val value: Int
        get() = if (_value < 0) 0 else _value
}

That will works as expected, e.i it has one field, one getter, right equals, hashcode and component1. The catch is that toString and copy are weird:

println(Test(1))          // prints: Test(_value=1)
Test(1).copy(_value = 5)  // <- weird naming

To fix the problem with toString you may redefine it by hands. I know of no way to fix the parameter naming but not to use data at all.

回答4:

This seems to be one (among other) annoying drawbacks of Kotlin.

It seems that the only reasonable solution, which completely keeps backward compatibility of the class is to convert it into a regular class (not a "data" class), and implement by hand (with the aid of the IDE) the methods: hashCode(), equals(), toString(), copy() and componentN()

class Data3(i: Int)
{
    var i: Int = i

    override fun equals(other: Any?): Boolean
    {
        if (this === other) return true
        if (other?.javaClass != javaClass) return false

        other as Data3

        if (i != other.i) return false

        return true
    }

    override fun hashCode(): Int
    {
        return i
    }

    override fun toString(): String
    {
        return "Data3(i=$i)"
    }

    fun component1():Int = i

    fun copy(i: Int = this.i): Data3
    {
        return Data3(i)
    }

}

回答5:

I know this is an old question but it seems nobody mentioned the possibility to make value private and writing custom getter like this:

data class Test(private val value: Int) {
    fun getValue(): Int = if (value < 0) 0 else value
}

This should be perfectly valid as Kotlin will not generate default getter for private field.

But otherwise I definitely agree with spierce7 that data classes are for holding data and you should avoid hardcoding "business" logic there.

回答6:

I found the following to be the best approach to achieve what you need without breaking equals and hashCode:

data class TestData(private var _value: Int) {
    init {
        _value = if (_value < 0) 0 else _value
    }

    val value: Int
        get() = _value
}

// Test value
assert(1 == TestData(1).value)
assert(0 == TestData(-1).value)
assert(0 == TestData(0).value)

// Test copy()
assert(0 == TestData(-1).copy().value)
assert(0 == TestData(1).copy(-1).value)
assert(1 == TestData(-1).copy(1).value)

// Test toString()
assert("TestData(_value=1)" == TestData(1).toString())
assert("TestData(_value=0)" == TestData(-1).toString())
assert("TestData(_value=0)" == TestData(0).toString())
assert(TestData(0).toString() == TestData(-1).toString())

// Test equals
assert(TestData(0) == TestData(-1))
assert(TestData(0) == TestData(-1).copy())
assert(TestData(0) == TestData(1).copy(-1))
assert(TestData(1) == TestData(-1).copy(1))

// Test hashCode()
assert(TestData(0).hashCode() == TestData(-1).hashCode())
assert(TestData(1).hashCode() != TestData(-1).hashCode())

However,

First, note that _value is var, not val, but on the other hand, since it's private and data classes cannot be inherited from, it's fairly easy to make sure that it is not modified within the class.

Second, toString() produces a slightly different result than it would if _value was named value, but it's consistent and TestData(0).toString() == TestData(-1).toString().