起因
在我工作过程中，始终会碰到这样一个问题，把程序运行过程中的数据，保护在内存中，不便下次查找。最近才发现自己的写法始终有问题。
之前的写法
public static void main(String[] args) { List<String> names = Stream.of("aaa","bbb","ccc","bbb").collect(Collectors.toList()); Map<String, Integer> result = new HashMap<>(8); for (String name : names) { if(result.containsKey(name)){ result.put(name, result.get(name) + 1); }else{ result.put(name, 1); } } System.out.println(result); }下面这种写法，在对于简略数据处理上还能承受，然而对于简单数据，比方3层构造，写起来就比拟凌乱了。
public static void main(String[] args) { List<String> lunch1 = Stream.of("cjq","2010710","咖喱").collect(Collectors.toList()); List<String> lunch2 = Stream.of("cjq","2010711","牛肉").collect(Collectors.toList()); List<String> lunch3 = Stream.of("cbx","2010710","咖喱").collect(Collectors.toList()); List<String> lunch4 = Stream.of("cbx","2010710","鸡肉").collect(Collectors.toList()); List<List<String>> all = new LinkedList<>(); all.add(lunch1); all.add(lunch2); all.add(lunch3); all.add(lunch4); Map<String, Map<String, String>> result = new HashMap<>(8); for (List<String> lunch : all) { if(result.containsKey(lunch.get(0))){ Map<String, String> day = result.get(lunch.get(0)); if(day.containsKey(lunch.get(1))){ day.put(lunch.get(1), day.get(lunch.get(1)) + "," + lunch.get(2)); }else { day.put(lunch.get(1), lunch.get(2)); } }else{ Map<String, String> day = new HashMap<>(8); day.put(lunch.get(1), lunch.get(2)); result.put(lunch.get(0), day); } } System.out.println(result); }代码改良
public static void main(String[] args) { List<String> names = Stream.of("aaa","bbb","ccc","bbb").collect(Collectors.toList()); Map<String, Integer> result = new HashMap<>(8); for (String name : names) { result.put(name, result.getOrDefault(name, 0) + 1); } System.out.println(result); } public static void main(String[] args) { List<String> lunch1 = Stream.of("cjq", "2010710", "咖喱").collect(Collectors.toList()); List<String> lunch2 = Stream.of("cjq", "2010711", "牛肉").collect(Collectors.toList()); List<String> lunch3 = Stream.of("cbx", "2010710", "咖喱").collect(Collectors.toList()); List<String> lunch4 = Stream.of("cbx", "2010710", "鸡肉").collect(Collectors.toList()); List<List<String>> all = new LinkedList<>(); all.add(lunch1); all.add(lunch2); all.add(lunch3); all.add(lunch4); Map<String, Map<String, String>> result = new HashMap<>(8); for (List<String> lunch : all) { Map<String, String> day = result.getOrDefault(lunch.get(0), new HashMap<>()); String food = day.getOrDefault(lunch.get(1), null); if (food != null) { day.put(lunch.get(1), food + "," + lunch.get(2)); }else{ day.put(lunch.get(1), lunch.get(2)); } result.put(lunch.get(0), day); } System.out.println(result); }应用到了map提供的接口getOrDefault。
源码解析
588行：判断了这个key是否为空，如果没有就返回咱们传入的默认值，简化了代码逻辑。