java - 为什么我们有时需要在静态方法声明中使用 ？

Question

对于以下方法，我收到以下错误“无法对非静态类型 E 进行静态引用”:

public static void reverse(E[] a) {
       Stack<E> buffer = new Lab8<>(a.length); 

       for (int i=0; i < a.length; i++)
          buffer.push(a[i]);

       for (int i=0; i < a.length; i++)
          a[i] = buffer.pop();
}

现在我知道执行以下操作将修复错误，但我不明白 的意义(为什么我们在这种情况下需要它)，并且在方法中定义它没有意义像那样？我确实知道如何使用它来声明泛型类和实例，但这是我第一次看到它以这种方式使用...

public static <E> void reverse(E[] a) {
       Stack<E> buffer = new Lab8<>(a.length); 

       for (int i=0; i < a.length; i++)
          buffer.push(a[i]);

       for (int i=0; i < a.length; i++)
          a[i] = buffer.pop();
}

这是完整的引用代码:

 package labs;

import java.util.Arrays;

public class Lab8<E> implements Stack<E> {

 public static final int CAPACITY=1000;
 private E[] data;
 private int t = -1;
 public Lab8() { this(CAPACITY); }
 public Lab8(int capacity) {
// default array capacity
// generic array used for storage
// index of the top element in stack
// constructs stack with default capacity // constructs stack with given capacity // safe cast; compiler may give warning
   data = (E[ ]) new Object[capacity];

 }
 public int size() { return (t + 1); }
 public boolean isEmpty() { return (t == -1); }
 public void push(E e) throws IllegalStateException {
      if (size() == data.length) throw new IllegalStateException("Stack is full");
      data[++t] = e;
 }

 public E top() {
     if (isEmpty()) return null;
        return data[t];
 }

 public E pop() {
     if (isEmpty( )) return null;
     E answer = data[t];
     data[t] = null;
     t--;
     return answer;
 }

 public static void reverse(E[] a) {
        Stack<E> buffer = new Lab8<>(a.length); 

        for (int i=0; i < a.length; i++)
          buffer.push(a[i]);

        for (int i=0; i < a.length; i++)
          a[i] = buffer.pop();
 }

 public static void main(String[] args) {
            Integer[] a = {4, 8, 15, 16, 23, 42}; // autoboxing allows this 
            String[] s = {"maryam", "ricardo", "Mostafa", "Ahmend", "Hitler"};
            System.out.println("a = " + Arrays.toString(a)); 
            System.out.println("s = " + Arrays.toString(s)); 
            System.out.println("Reversing...");
            reverse(a);
            reverse(s);
            System.out.println("a = " + Arrays.toString(a)); System.out.println("s = " + Arrays.toString(s));
 }
}

Answer 1

如果你不把那个 <E>在方法声明中，编译器将无法理解你是想创建一个泛型方法，还是试图引用一个名为 E 的实际类并且只是忘记导入它。在没有类型参数的情况下，编译器只能假设第二个选项并提示，因为它找不到 E随时随地上课。