英文:
How to pass a Class as an function parameter
问题
知道这之前已经有人问过，但我无法完全理解/答案没有帮助到我，所以我会很感谢在这个问题上能得到一些帮助，因为我开始学习Java。
我想创建一个函数，用来计算一个类的实例数量并返回一个数字，例如我有：
public class A {}
public class A.a extends A {}
public class A.b extends A {}
我有一个包含A、A.a和A.b实例的A[]数组，我想在该数组中计算A、A.a或A.b的实例数量，因此我创建了以下函数：
int countA.a(A[] array) {
int counter = 0;
for (int i = 0; i < array.length; i++) {
if (array[i] instanceof A.a) {
counter++;
}
}
return counter;
}
int countA.b(A[] array) {
int counter = 0;
for (int i = 0; i < array.length; i++) {
if (array[i] instanceof A.b) {
counter++;
}
}
return counter;
}
我的问题是，我如何做类似于这样的事情：
int countInstances(A[] array, Class n) {
int counter = 0;
for (int i = 0; i < array.length; i++) {
if (n.isInstance(array[i])) {
counter++;
}
}
return counter;
}
感谢您的耐心！
英文:
I know this has been asked before, but I couldn't quite understand/the answers didn't help me, so I would appreciate some help on this matter, as I'm starting to learn Java.
I want to create a function that counts the instances of a class and returns me a number, for example i have:
Public class A{}
Public class A.a extends A{}
Public class A.b extends A{}
I have an array of A[] that has instances of A, A.a and A.b and I want to count the instances of A, A.a or A.b in that array, so i created these functions:
int countA.a(A[] array){
int counter = 0;
for(int i = 0; i < array.length; i++){
if(array[i] instanceof A.a){
counter++;
}
}
return counter;
}
int countA.b(A[] array){
int counter = 0;
for(int i = 0; i < array.length; i++){
if(array[i] instanceof A.b){
counter++;
}
}
return counter;
}
My question is, how can i do something like this:
int countInstances(A[] array, class n){
int counter = 0;
for(int i = 0; i < array.length; i++){
if(array[i] instanceof n){
counter++;
}
}
return counter;
}
Thanks for your patience!
答案1
得分: 3
你可以使用 Class 类：
int countInstances(A[] array, Class<? extends A> n){
而不是使用 instanceof，你应该在 Class 类中使用 isInstance 方法。
if(n.isInstance(array[i])){
counter++;
}
用法：
int count = countInstances(someArray, SubclassOfA.class);
英文:
You can use the Class class:
int countInstances(A[] array, Class<? extends A> n){
And instead of instanceof, you should use the isInstance method in the Class class.
if(n.isInstance(array[i])){
counter++;
}
Usage:
int count = countInstances(someArray, SubclassOfA.class);
答案2
得分: 0
你可以使用 A.class 获取类型的 Class，或者通过实例使用以下方式获取：
A myA = new A();
Class<?> clazzOfA = myA.getClass();
并将其作为类型为 Class<?> 的参数传递。
所以在你的示例中，类似于：
int instances = countInstances(myArray, A.b.class);
int countInstances(A[] array, Class<?> n) {
int counter = 0;
for (int i = 0; i < array.length; i++) {
if (array[i].getClass() == n) {
counter++;
}
}
return counter;
}
一些示例：
boolean hasSameClass(Object type, Class<?> n) {
return type.getClass() == n;
}
hasSameClass(new LinkedList(), ArrayList.class); // false
hasSameClass(new ArrayList(), LinkedList.class); // false
hasSameClass(new ArrayList(), ArrayList.class); // true
hasSameClass(new ArrayList(), List.class); // false
英文:
you can get the Class of a type by using A.class, or from an instance by using
A myA = new A();
Class<?> clazzOfA = myA.getClass();
and pass it as a parameter of type Class<?>
So in your example, something like:
int instances = countInstances(myArray, A.b.class);
int countInstances(A[] array, Class<?> n){
int counter = 0;
for(int i = 0; i < array.length; i++){
if(array[i].getClass() == n){
counter++;
}
}
return counter;
}
Some examples:
boolean hasSameClass(Object type, Class<?> n){
return type.getClass() == n;
}
hasSameClass(new LinkedList(), ArrayList.class); // false
hasSameClass(new ArrayList(), LinkedList.class); // false
hasSameClass(new ArrayList(), ArrayList.class); // true
hasSameClass(new ArrayList(), List.class); // false
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