Java中的memoization有哪些不同的技术？ [英] What are the different techniques for memoization in Java?

问题描述

我知道这个 http://onjava.com /pub/a/onjava/2003/08/20/memoization.html 但还有其他什么吗？

I know of this one http://onjava.com/pub/a/onjava/2003/08/20/memoization.html but is there anything else?

推荐答案

使用普通的简单类型安全Java，记忆也很容易。

Memoization is also easy with plain simple typesafe Java.

您可以从头开始使用以下可重用的类。

You can do it from scratch with the following reusable classes.

我将这些用作缓存，其生命周期是webapp上的请求。

I use these as caches whose lifespan are the request on a webapp.

当然使用Guava MapMaker 如果您需要驱逐策略或更多功能，如同步。

Of course use the Guava MapMaker if you need an eviction strategy or more features like synchronization.

如果你需要记忆一个包含许多参数的方法，只需将这些参数放在一个列表中，然后将该列表作为单个参数传递。

If you need to memoize a method with many parameters, just put the parameters in a list with both techniques, and pass that list as the single parameter.

abstract public class Memoize0<V> {
    //the memory
    private V value;
    public V get() {
        if (value == null) {
            value = calc();
        }
        return value;
    }
    /**
     * will implement the calculation that 
     * is to be remembered thanks to this class
     */
    public abstract V calc();
}

abstract public class Memoize1<P, V> {
    //The memory, it maps one calculation parameter to one calculation result
    private Map<P, V> values = new HashMap<P, V>();

    public V get(P p) {
        if (!values.containsKey(p)) {
            values.put(p, calc(p));
        }
        return values.get(p);
    }

    /**
     * Will implement the calculations that are
     * to be remembered thanks to this class
     * (one calculation per distinct parameter)
     */
    public abstract V calc(P p);
 }

这就像这样使用

    Memoize0<String> configProvider = new Memoize0<String>() {
        @Override
        public String calc() {
            return fetchConfigFromVerySlowDatabase();
        }
    };
    final String config = configProvider.get();

    Memoize1<Long, String> usernameProvider = new Memoize1<Long, String>() {
        @Override
        public String calc(Long id) {
            return fetchUsernameFromVerySlowDatabase(id);
        }
    };
    final String username = usernameProvider.get(123L);

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