問題描述

專案使用mysql，主鍵使用int自增，實體對映為Integer，而自增主鍵勢必會大於127，那麼問題來了 ，使用java8迴圈做parentId（類似選單，多層級）判斷時候怎麼辦呢？

示例

  //id 是傳遞的父id  用來過濾用 ，可以為空
        List<Channel> channels = allchannel.stream()
                .filter(channel -> {
                    if (id==null) {
                        return channel.getParentId()==null;
                    }else {
                        return channel.getParentId()!=null&&channel.getParentId().equals(id);
                    }
                })

                .collect(Collectors.toList());

解析

為什麼大於127就不行了呢？ 因為java8內部的變數一般修飾符是final，而Integer可快取的值是-128–127(詳情查詢原始碼729行)。為了解決此種問題可以使用equal取代==判斷

附integer快取原始碼

    /**
     * Cache to support the object identity semantics of autoboxing for values between
     * -128 and 127 (inclusive) as required by JLS.
     *
     * The cache is initialized on first usage.  The size of the cache
     * may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
     * During VM initialization, java.lang.Integer.IntegerCache.high property
     * may be set and saved in the private system properties in the
     * sun.misc.VM class.
     */

    private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];

        static {
            // high value may be configured by property
            int h = 127;
            String integerCacheHighPropValue =
                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                try {
                    int i = parseInt(integerCacheHighPropValue);
                    i = Math.max(i, 127);
                    // Maximum array size is Integer.MAX_VALUE
                    h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            }
            high = h;

            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);

            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }

        private IntegerCache() {}
    }