英文:
How to override a java class property variable in kotlin
问题
这是代码的翻译部分：
// Java
public class myBaseClass {
protected String myVar = "hello";
public myBaseClass() {
}
protected void myOneMethod() {
System.out.println(myVar);
}
}
// Kotlin
class myChildClass() : myBaseClass() {
override var myVar = "hi"
override fun myOneMethod() {
System.out.println(myVar)
}
}
记得检查一下代码，看看是否有任何问题。
英文:
I am trying to override a java property variable in my kotlin class but keep getting the error "myVar overrides nothing". The example of how to override a kotlin class is here but it doesn't say anything about overriding java properties. Here is the code:
//Java
public class myBaseClass{
protected String myVar = "hello";
public myBaseClass(){
}
protected void myOneMethod(){
System.out.println(myVar);
}
}
//Kotlin
class myChildClass() : myBaseClass() {
override var myVar = "hi"
override fun myOneMethod()
{
System.out.println(myVar)
}
}
答案1
得分: 5
因为Java没有属性（properties），你需要将字段设为私有，并提供一个getter和setter。然后在Kotlin中，你可以重写getter和setter。虽然不太优雅，但这也是在Java子类中的实现方式。你需要另一个私有属性来为子类实现创建新的后备字段。
public class MyBaseClass {
private String myVar = "hello";
public MyBaseClass(){
}
protected String getMyVar() {
return myVar;
}
protected void setMyVar(String myVar) {
this.myVar = myVar;
}
protected void myOneMethod(){
System.out.println(getMyVar());
}
}
class MyChildClass : MyBaseClass() {
private var overriddenMyVar: String? = "hi"
override fun getMyVar(): String? {
return overriddenMyVar
}
override fun setMyVar(myVar: String?) {
overriddenMyVar = myVar
}
}
fun main() {
val x : MyBaseClass = MyChildClass()
x.myOneMethod() // 输出 "hi"
}
英文:
Since Java doesn't have properties, you need to make the field private and provide a getter and setter. Then in Kotlin, you can override the getter and setter. It's not pretty, but this is how you would have to do it in a Java subclass as well. You need another private property to create the new backing field for your subclass implementation.
public class MyBaseClass {
private String myVar = "hello";
public MyBaseClass(){
}
protected String getMyVar() {
return myVar;
}
protected void setMyVar(String myVar) {
this.myVar = myVar;
}
protected void myOneMethod(){
System.out.println(getMyVar());
}
}
class MyChildClass : MyBaseClass() {
private var overriddenMyVar: String? = "hi"
override fun getMyVar(): String? {
return overriddenMyVar
}
override fun setMyVar(myVar: String?) {
overriddenMyVar = myVar
}
}
fun main() {
val x : MyBaseClass = MyChildClass()
x.myOneMethod() // Prints "hi"
}
答案2
得分: 3
在Java中，字段无法以任何方式被覆盖。由于myVar是Java中的一个字段，它无法在Kotlin中被覆盖。
英文:
Fields in Java cannot be overridden in any way. Since myVar is a field in Java, it cannot be overridden in Kotlin.
答案3
得分: 2
尝试像这样：
//Kotlin
class KotlinImplementation : MyBaseClass() {
init {
myVar = "ji"
}
}
//Java
public class MyBaseClass {
protected String myVar = "hello";
public MyBaseClass() {
}
protected void myOneMethod() {
System.out.println(myVar);
}
}
英文:
Try like that:
//Kotlin
class KotlinImplementation: MyBaseClass() {
init {
myVar = "ji"
}
}
//Java
public class MyBaseClass {
protected String myVar = "hello";
public MyBaseClass(){
}
protected void myOneMethod(){
System.out.println(myVar);
}
}
</details>
# 答案4
**得分**: 0
看起来是Java互操作性方面的问题。
以下是一个解决方法：
1. 创建一个辅助的Java类，隐藏有问题的`protected`字段：
```lang-java
public class myBaseClassKtInterop extends myBaseClass {
private String myVar;
}
- 在Kotlin中扩展它；还要考虑为方法添加
public修饰符，覆盖protected的方法（如果不这样做，方法将保持protected；在这种情况下，默认的公共可见性不起作用）：
class myChildClass : myBaseClassKtInterop() {
var myVar = "hi"
public override fun myOneMethod() {
println(myVar)
}
}
现在两种访问myVar的方式都可行：
myChildClass().myOneMethod() //hi
println(myChildClass().myVar) //hi
还要注意，如果在myBaseClass中有另外的方法访问myVar，它们会使用值"hello"。考虑同时覆盖它们，或直接将myVar的可见性在myBaseClass中改为private（如果可能的话）；在这种情况下，你将不需要这个辅助类。
英文:
Looks like an issue in Java interop.
Here is a hack to overcome it:
- Create an auxilary Java class, hiding problematic
protectedfield:
public class myBaseClassKtInterop extends myBaseClass {
private String myVar;
}
- In kotlin extend it; also consider adding
publicmodifier for method, overring theprotectedone (if you don't do it, method will remainprotected; default public visibility is not working in this case):
class myChildClass : myBaseClassKtInterop() {
var myVar = " hi"
public override fun myOneMethod() {
println(myVar)
}
}
Now both ways to access myVar are working:
myChildClass().myOneMethod() //hi
println(myChildClass().myVar) //hi
Also note that if there were another methods, accessing myVar in myBaseClass, they will be working with "hello" value. Consider overriding all of them too, or change myVar visibility to private in myBaseClass directly (if it is possible), in this case you will not need this auxilary class.
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