如何在枚舉中定義泛型成員變量？

Question

我想引用一個枚舉方法來檢索算法的 class 以便我可以延遲加載算法的新實例以用於策略設計模式。

在此示例中，我使用枚舉來引用計算 Fibonacci 數的三個不同策略類： RecursiveFibonacciGenerator 、 IterativeFibonacciGenerator和MemoizedFibonacciGenerator （所有這些都繼承自FibonacciGenerator ）。

代碼（帶有意圖注釋的生成錯誤的行）如下：

package com.example.strategy;

public class Fibonacci {
    private enum Algorithm {
        RECURSIVE (RecursiveFibonacciGenerator.class),
        ITERATIVE (IterativeFibonacciGenerator.class),
        MEMOIZED (MemoizedFibonacciGenerator.class);

        private final Class<T> algorithmClass; // Declare class of same type as constructor
        private final T instance; // Declare instance of class defined in constructor
        private <T extends FibonacciGenerator> Algorithm(Class<T> algorithmClass) {
            this.algorithmClass = algorithmClass;
        }

        public T getInstance() {
            if (this.instance == null) {
                this.instance = this.algorithmClass.newInstance();
            }
            return this.instance;
        }
    }

    public Integer getTerm(Integer termNumber) {
        profileGenerator(termNumber, Algorithm.RECURSIVE);
        profileGenerator(termNumber, Algorithm.ITERATIVE);
        return profileGenerator(termNumber, Algorithm.MEMOIZED);
    }

    private Integer profileGenerator(Integer termNumber, Algorithm algorithm) {
        System.out.print("Computing term using " + algorithm.toString() + " algorithm... ");
        Long startTimeMilliseconds = System.currentTimeMillis();
        Integer term = algorithm.getInstance().generateTerm(termNumber);
        Long endTimeMilliseconds = System.currentTimeMillis();
        Long computationTimeMilliseconds = endTimeMilliseconds - startTimeMilliseconds;
        System.out.println("term computed in " + computationTimeMilliseconds + " milliseconds");
    }
}

我想知道如何使用這個enum構造函數來存儲Class<T>類型的成員變量。

編輯：添加完整代碼以闡明意圖

Answer 1

public enum Algorithm {
    RECURSIVE(FibonacciGenerator.RecursiveFibonacciGenerator.class),
    ITERATIVE(FibonacciGenerator.IterativeFibonacciGenerator.class),
    MEMOIZED(FibonacciGenerator.MemoizedFibonacciGenerator.class);

    private final Class<? extends FibonacciGenerator> algorithmClass;

    private <T extends FibonacciGenerator> Algorithm(Class<T> algorithmClass) {
        this.algorithmClass = algorithmClass;
    }
}

這樣做是你想要的嗎？

另一種選擇是使用 class 的實例，但經過進一步思考，我認為這是一個壞主意。 如果您要使用 class 的實例，那么為什么首先需要枚舉？

Answer 2

嘗試提供一個真實實例而不是 class：

public enum Algorithm {
    RECURSIVE (new RecursiveFibonacciGenerator()),
    ITERATIVE (new IterativeFibonacciGenerator()),
    MEMOIZED (new MemoizedFibonacciGenerator());

    private <T extends FibonacciGenerator> Algorithm(T algorithm) {}
}

Answer 3

普勞是對的。 你可以聲明和初始化一個成員變量，就好像它是一個 class 而不是一個枚舉：

private final FibonacciGenerator _instance;

private Algorithm(FibonacciGenerator instance) {
    _instance = instance;
}

然后通過公共的 getFibonacciGenerator() 方法返回它。 請注意，如果您存儲實例而不是 Class，則無需使用 generics。 並記住將字段聲明為“final”

Answer 4

我不確定我是否很好地理解了這個問題，但是如果您只是想將策略保留為成員變量：

public enum Algorithm {
  RECURSIVE (RecursiveFibonacciGenerator.class),
  ITERATIVE (IterativeFibonacciGenerator.class),
  MEMOIZED (MemoizedFibonacciGenerator.class);

  private Class<? extends FibonacciGenerator> strategy;

  private Algorithm(Class<? extends FibonacciGenerator> algorithmClass) {
    this.strategy = algorithmClass;
  }

  Class<? extends FibonacciGenerator> getStrategy() {
    return strategy;
  }
}

Answer 5

我最終得到以下代碼：

package com.example.strategy;

public class Fibonacci {
    private enum Algorithm {
        UNDEFINED (null),
        RECURSIVE (RecursiveFibonacciGenerator.class),
        ITERATIVE (IterativeFibonacciGenerator.class),
        MEMOIZED (MemoizedFibonacciGenerator.class);

        private final Class<? extends FibonacciGenerator> algorithmClass;
        private FibonacciGenerator instance;
        private <T extends FibonacciGenerator> Algorithm(Class<T> algorithmClass) {
            this.algorithmClass = algorithmClass;
        }

        public FibonacciGenerator getInstance() {
            if (this.instance == null) {
                try {
                    this.instance = this.algorithmClass.newInstance();
                } catch (InstantiationException e) {
                    e.printStackTrace();
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
            return this.instance;
        }
    }

    public Integer getTerm(Integer termNumber) {
        return profileGenerator(termNumber, Algorithm.MEMOIZED);
    }

    private Integer profileGenerator(Integer termNumber, Algorithm algorithm) {
        System.out.print("Computing term using " + algorithm.toString() + " algorithm... ");
        Long startTimeMilliseconds = System.currentTimeMillis();
        Integer term = algorithm.getInstance().generateTerm(termNumber);
        Long endTimeMilliseconds = System.currentTimeMillis();
        Long computationTimeMilliseconds = endTimeMilliseconds - startTimeMilliseconds;
        System.out.println("term computed in " + computationTimeMilliseconds + " milliseconds");
        return term;
    }
}