[英]How do I make my code work with Generics?
I am currently learning about Generics, first I made the Class which makes a stackArrayList with Integers, after that I wanted it to be used by every Number so I did put the Number in everywhere in the Methods, it kind of worked but I think this is not quiet correct, my biggest Problem is that I cant use any addition or subtraction with the Class Number.我目前正在学习泛型,首先我制作了一个类,它用整数制作了一个 stackArrayList,之后我希望它被每个数字使用,所以我确实把数字放在方法的任何地方,它有点工作,但我认为这个不是安静正确,我最大的问题是我不能对班级编号使用任何加法或减法。
So I tried implement the N extends Number but that does not work either and with that I can't even put a Number which returns from the Method pop() inside of a variable, telling me that pop() must return an int then.因此,我尝试实现 N extends Number 但这也不起作用,因此我什至不能将从 Method pop() 返回的 Number 放入变量中,告诉我 pop() 必须返回一个 int 然后。
Can someone explain to me please, what I did there?有人可以向我解释一下,我在那里做了什么吗? I can grasp the logic behind of it but cannot understand it to the fullest.
我能理解它背后的逻辑,但不能完全理解它。
public class StackArrayList<N extends Number> {
private ArrayList<N> stackList = new ArrayList<>();
public void push (N value) {
if(value == null) {
System.out.println("value null not allowed");
return;
}
stackList.add(0,value);
}
public N pop() {
N popInt = stackList.get(0);
stackList.remove(0);
return popInt;
}
public N addFirstTwo() {
if(stackList.size()==1) {
return stackList.get(0);
}
if (stackList.size() == 0) {return null;}
if(stackList.get(0) instanceof Integer) {
N summe;
summe =
push(summe);
return summe;
}else if(stackList.get(0) instanceof Double){
summe = pop().doubleValue()+pop().doubleValue();
push(summe);
return summe;
}else if(stackList.get(0) instanceof Float){
summe = pop().floatValue()+pop().floatValue();
push(summe);
return summe;
}else if(stackList.get(0) instanceof Long){
summe = pop().longValue()+pop().longValue();
push(summe);
return summe;
}
push(summe);
return summe;
}
}
The problem is that Number
does not support the +
operator, even though all its implementations ( Double
, Float
etc.) do support it.问题是
Number
不支持+
运算符,即使它的所有实现( Double
、 Float
等)都支持它。
What you can do is to define an inner interface to your class which exposes a sum
method:您可以做的是为您的类定义一个内部接口,该接口公开一个
sum
方法:
interface SumPerformer<N> {
N sum(N n1, N n2);
}
Then, you will declare a field into your StackArrayList<N extends Number>
class that holds an implementation of this interface:然后,您将在
StackArrayList<N extends Number>
类中声明一个字段,该类包含此接口的实现:
private final SumPerformer<N> sumPerformer;
And so, you will use this implementation to return the sum of the two first numbers inside your addFirstTwo()
method:因此,您将使用此实现返回
addFirstTwo()
方法中前两个数字的总和:
public N addFirstTwo() {
if (stackList.isEmpty()) {
return null;
}
if (stackList.size() == 1) {
return stackList.get(0);
}
return sumPerformer.sum(stackList.get(0), stackList.get(1));
}
When it's time to initialize your StackArrayList
with a concrete implementation of Number
, you will simply pass it the implementation's sum
by method reference:当需要使用
Number
的具体实现初始化StackArrayList
时,您只需通过方法引用将实现的sum
传递给它:
StackArrayList<Double> stackArrayListDoubles = new StackArrayList<>(Double::sum);
StackArrayList<Long> stackArrayListLongs = new StackArrayList<>(Long::sum);
StackArrayList<Integer> stackArrayListInts = new StackArrayList<>(Integer::sum);
Full code:完整代码:
public static final class StackArrayList<N extends Number> {
private final List<N> stackList = new ArrayList<>();
private final SumPerformer<N> sumPerformer;
public StackArrayList(SumPerformer<N> sumPerformer) {
this.sumPerformer = sumPerformer;
}
public N pop() {
N popInt = stackList.get(0);
stackList.remove(0);
return popInt;
}
public void push(N value) {
if (value == null) {
System.out.println("value null not allowed");
return;
}
stackList.add(0, value);
}
public N addFirstTwo() {
if (stackList.isEmpty()) {
return null;
}
if (stackList.size() == 1) {
return stackList.get(0);
}
N sum = sumPerformer.sum(stackList.get(0), stackList.get(1));
push(sum);
return sum;
}
interface SumPerformer<N> {
N sum(N n1, N n2);
}
}
Doesn't the Variable need to be inside the <> ?
变量不需要在 <> 内吗?
No. The <>
(called diamond operator) is used to infer a specific type to a class which takes a "generic" type (you can read more about generics ).不。
<>
(称为菱形运算符)用于将特定类型推断为采用“泛型”类型的类(您可以阅读 有关泛型的更多信息)。
Before Java 8, you needed to specify the type both in the declaration and the instantiation of the variable.在 Java 8 之前,您需要在变量的声明和实例化中指定类型。 For example, the interface
List<T>
is implemented by the class ArrayList<T>
.例如,接口
List<T>
由类ArrayList<T>
。
Before Java 8, if you wanted a list of Integer
you needed to do:在 Java 8 之前,如果您想要一个
Integer
列表,您需要执行以下操作:
List<Integer> list = new ArrayList<Integer>();
After Java 8, you can simply state:在 Java 8 之后,您可以简单地声明:
List<Integer> list = new ArrayList<>();
Indeed, the type Integer
will be inferred to ArrayList
by the declaration List<Integer>
.实际上,类型
Integer
将通过声明List<Integer>
推断为ArrayList
。
This is why, when I initialize my StackArrayList<>
, I'm not passing Double
to it because I already pass it to the declaration, so the variable already knows what is its type.这就是为什么当我初始化我的
StackArrayList<>
,我没有将Double
传递给它,因为我已经将它传递给了声明,所以变量已经知道它的类型是什么。
shouldnt it just pass down the Number i initialize the Object with ?
它不应该只是传递我用它初始化对象的数字吗?
Careful, you're mixing up stuff.小心,你把东西弄混了。 The reason why you shouldn't pass
Double
is explained above.上面解释了为什么不应该通过
Double
的原因。 However, here I am doing something else.但是,在这里我正在做其他事情。
If you check the new constructor of StackArrayList
, I've added a member which is private final SumPerformer<N> sumPerformer;
如果您检查
StackArrayList
的新构造StackArrayList
,我添加了一个成员,它是private final SumPerformer<N> sumPerformer;
. .
This member is final
, it means it should be initialized when you initialize the class, and I do that by passing it into the constructor.这个成员是
final
,这意味着它应该在你初始化类时被初始化,我通过将它传递给构造函数来实现。
But so what am I passing to the constructor?但是我传递给构造函数的是什么? I am passing a concrete implementation of the abstract interface that I define.
我正在传递我定义的抽象接口的具体实现。
Specifically, I have declared the interface SumPerformer<N>
to expose only one method which is called sum
, returns a N
and takes in input two distinct N
.具体来说,我已经声明了接口
SumPerformer<N>
只公开一个称为sum
方法,返回一个N
并接受输入两个不同的N
。
A concrete implementation of this interface, hence, should be a method with the same signature.因此,该接口的具体实现应该是具有相同签名的方法。
And this method, is the sum()
method of all Number
children ( Double
, Integer
...)而这个方法,是所有
Number
孩子( Double
, Integer
...)的sum()
方法
If you go into the class Double
, you will see that there is this method:如果你进入类
Double
,你会看到有这个方法:
public static double sum(double a, double b) {
return a + b;
}
The above method's signature matches with your interface when the type N extends Number
is Double
.当类型
N extends Number
是Double
时,上述方法的签名与您的接口匹配。
So, in your class, you simply call the interface .sum()
to sum up your two numbers, and you leave the implementation details (the a + b
, basically) to the concrete implementation which is not done by you, but by Java itself in the JDK's classes that extend Number
.因此,在您的课程中,您只需调用接口
.sum()
来总结您的两个数字,并将实现细节(基本上是a + b
)留给具体实现,这不是由您完成的,而是由 Java 完成的本身在扩展Number
的 JDK 类中。
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