Java8集合filter过滤Integer数值为空问题解决方案

电玩女神 2022-06-11 02:42 103阅读 0赞

# 问题描述 #

项目使用mysql，主键使用int自增，实体映射为Integer，而自增主键势必会大于127，那么问题来了 ，使用java8循环做parentId（类似菜单，多层级）判断时候怎么办呢？

# 示例 #

//id 是传递的父id 用来过滤用 ，可以为空
            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 #

为什么大于127就不行了呢？ 因为java8内部的变量一般修饰符是final，而Integer可缓存的值是-128–127(详情查询源码729行)。为了解决此种问题可以使用equal取代==判断

# 附integer缓存源码 #

/** * 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() {}
        }