【Java性能优化】Map.merge()方法：告别繁琐判空，3行代码搞定统计累加！

if(yearMap.containsKey(year)){
    yearMap.put(year, yearMap.get(year) + inputRate);
}else{
    yearMap.put(year, inputRate);
}

yearMap.put(year, yearMap.get(year) == null ? 0 : yearMap.get(year) + inputRate);

default V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)

graph TD
    A[开始] --> B{Key是否存在?}
    B -->|不存在| C[用默认值value作为新值]
    B -->|存在| D[用remappingFunction合并旧值和新值]
    C --> E[将结果存入Map]
    D --> E
    E --> F[结束]

Map<String, Double> yearMap = new HashMap<>();
Map<String, Integer> countMap = new HashMap<>();
for(String projectId : projectIdList){
    if(StringUtils.isEmpty(projectId)) continue;
    
    Double inputRate = famClient.calculateProjectInputRate(projectId).getData();
    Project project = projectMapper.selectById(projectId);
    
    if(project != null){
        String year = project.getReportedDate().substring(0,4);
        
        // 传统写法
        yearMap.put(year, yearMap.get(year) == null ? 0 : yearMap.get(year) + inputRate);
        countMap.put(year, countMap.get(year) == null ? 0 : countMap.get(year) + 1);
    }
}

Map<String, Double> yearMap = new HashMap<>();
Map<String, Integer> countMap = new HashMap<>();
projectIdList.stream()
    .filter(StringUtils::isNotEmpty)
    .forEach(projectId -> {
        Double inputRate = famClient.calculateProjectInputRate(projectId).getData();
        Project project = projectMapper.selectById(projectId);
        
        if(project != null){
            String year = project.getReportedDate().substring(0,4);
            yearMap.merge(year, inputRate, Double::sum);  // 累加投入率
            countMap.merge(year, 1, Integer::sum);       // 计数+1
        }
    });

Map<String, DoubleAdder> yearMap = new ConcurrentHashMap<>();
Map<String, AtomicInteger> countMap = new ConcurrentHashMap<>();
projectIdList.parallelStream()
    .filter(StringUtils::isNotEmpty)
    .forEach(projectId -> {
        Double inputRate = famClient.calculateProjectInputRate(projectId).getData();
        Project project = projectMapper.selectById(projectId);
        
        if(project != null){
            String year = project.getReportedDate().substring(0,4);
            yearMap.computeIfAbsent(year, k -> new DoubleAdder()).add(inputRate);
            countMap.computeIfAbsent(year, k -> new AtomicInteger()).incrementAndGet();
        }
    });

Map<String, DoubleSummaryStatistics> stats = projectIdList.stream()
    .filter(StringUtils::isNotEmpty)
    .map(projectId -> {
        Double inputRate = famClient.calculateProjectInputRate(projectId).getData();
        Project project = projectMapper.selectById(projectId);
        return project != null ? 
            new AbstractMap.SimpleEntry<>(
                project.getReportedDate().substring(0,4), 
                inputRate
            ) : null;
    })
    .filter(Objects::nonNull)
    .collect(Collectors.groupingBy(
        Map.Entry::getKey,
        Collectors.summarizingDouble(Map.Entry::getValue)
    ));
// 输出统计结果
stats.forEach((year, stat) -> {
    System.out.printf("%s年: 总和=%.2f, 项目数=%d, 平均值=%.2f%n",
        year, stat.getSum(), stat.getCount(), stat.getAverage());
});

@Override
public V merge(K key, V value,
               BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    if (value == null)
        throw new NullPointerException();
    if (remappingFunction == null)
        throw new NullPointerException();
    
    int hash = hash(key);
    Node<K,V>[] tab; Node<K,V> first; int n, i;
    int binCount = 0;
    
    // 这里省略了部分实现细节...
    
    V oldValue = (old == null) ? null : old.value;
    V newValue = (oldValue == null) ? value :
               remappingFunction.apply(oldValue, value);
    if (newValue == null) {
        // 如果新值为null，则删除该条目
        // 省略删除逻辑...
    } else {
        // 更新或插入新值
        // 省略更新逻辑...
    }
    return newValue;
}

Map<String, Double> → Map<String, DoubleAdder>
Map<String, Integer> → Map<String, AtomicInteger>

list.parallelStream().forEach(...)

new HashMap<>(expectedSize)

// compute()示例
countMap.compute(year, (k, v) -> v == null ? 1 : v + 1);

// 先确保键存在，再合并
countMap.putIfAbsent(year, 0);
countMap.merge(year, 1, Integer::sum);

Map<String, List<String>> categoryMap = new HashMap<>();
// 合并两个列表
categoryMap.merge("Java", Arrays.asList("Spring", "Hibernate"), 
    (oldList, newList) -> {
        List<String> merged = new ArrayList<>(oldList);
        merged.addAll(newList);
        return merged;
    });

// 只合并大于100的值
yearMap.merge(year, inputRate, 
    (oldVal, newVal) -> newVal > 100 ? oldVal + newVal : oldVal);