Java8, The Map<K, V>interface adds a new method putIfAbsent(K key, V value), function is: If the current Mapkey is not present keyor the keyassociated value null, then the execution put(key, value); otherwise, they will not perform put operations. This method is equivalent to the following code:
(Digression: putIfAbsentmethods and putmethods as returns before the method invocation parameters keyassociated value)
Using putIfAbsentmodified getElementPositionsmethods:
public static Map<String, List<Integer>> getElementPositions(List<String> list) {
Map<String, List<Integer>> positionsMap = new HashMap<>();
for (int i = 0; i < list.size(); i++) {
String str = list.get(i);
positionsMap.putIfAbsent(str, new ArrayList<>(1)); // 如果 positionsMap 不存在键 str 或者 str 关联的 List<Integer> 为 null，那么就会进行 put；否则不执行 put
positionsMap.get(str).add(i);
}
return positionsMap;
}
public static void main(String[] args) throws Exception {
List<String> list = Arrays.asList("a", "b", "b", "c", "c", "c", "d", "d", "d", "f", "f", "g");
System.out.println("使用 putIfAbsent：");
Map<String, List<Integer>> elementPositions = getElementPositions(list);
System.out.println(elementPositions);
}
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operation result:
It can be seen using the putIfAbsentfollowing getElementPositionssimple point, that it could be more concise?
Check Mapthe interface method can be found in JDK1.8time, we add the following two methods:
Check computemethod of APIdocuments, you can find computea method with the following code is equivalent to
V oldValue = map.get(key);
V newValue = remappingFunction.apply(key, oldValue);
if (oldValue != null ) {
if (newValue != null)
map.put(key, newValue);
else
map.remove(key);
} else { // 即 原来的 key 不存在 Map 中或该 key 关联的 value 为 null
if (newValue != null)
map.put(key, newValue);
elsereturn null;
}
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computeThe original method and putthe difference method is:
put(K key, V value)Method, if keythe Mapdoes not exist, then added directly; if it already exists, then the use of the new valuereplacing the old value;
The compute(K key, BiFunction remappingFunction)method can be calculated by a new BiFunction value, BiFunctionthe old parameters keyand valuereturn to calculate a new value- and putdifferent methods, computethe method returns would be the latest and keyassociated value, instead of the old value. It is possible to use computea method rewritten getElementPositionsas follows:
public static Map<String, List<Integer>> getElementPositions(List<String> list) {
Map<String, List<Integer>> positionsMap = new HashMap<>();
for (int i = 0; i < list.size(); i++) {
positionsMap.compute(list.get(i), (k, v) -> v == null ? new ArrayList<>(1) : v).add(i);
}
return positionsMap;
}
public static void main(String[] args) throws Exception {
List<String> list = Arrays.asList("a", "b", "b", "c", "c", "c", "d", "d", "d", "f", "f", "g");
System.out.println("使用 compute：");
Map<String, List<Integer>> elementPositions = getElementPositions(list);
System.out.println(elementPositions);
}
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(k, v) -> v == null ? new ArrayList<>(1) : vThat is, if the current value null, then the BiFunctionreturn value new ArrayList<>(1); if not null, then the return value is itself. And because the computemethod returns a new value- and at this time is the list.get(i)(key) associated with ArrayList- so we can call it a direct addapproach.
operation result:
Great - but also more concise yet? Let us look at computeIfAbsentmethods: computeIfAbsentand computerelationship, similar to putIfAbsentand putrelationships: computeIfAbsentin keyabsence Mapor with keyassociated valueis nullonly when the function is performed by computing a new valueoperation, and otherwise does; computeIfAbsentthe return value is the keylatest associated a value. Its default implementation is as follows:
And computedifferent, computeIfAbsentacceptable operation is a function Functionand not BiFunction- this is well understood, computeIfAbsentonly key is not Map, or with the keyassociated valueto nullonly perform the function operation, then clearly this time and keyrelated valueto null, so computeIfAbsentonly accept Functioncan be used as parameters - which Functioncan be used keyas a new parameter calculation value. Use computeIfAbsentrewrite getElementPositions:
public static Map<String, List<Integer>> getElementPositions(List<String> list) {
Map<String, List<Integer>> positionsMap = new HashMap<>();
for (int i = 0; i < list.size(); i++) {
positionsMap.computeIfAbsent(list.get(i), k -> new ArrayList<>(1)).add(i);
}
return positionsMap;
}
public static void main(String[] args) throws Exception {
List<String> list = Arrays.asList("a", "b", "b", "c", "c", "c", "d", "d", "d", "f", "f", "g");
System.out.println("使用 computeIfAbsent：");
Map<String, List<Integer>> elementPositions = getElementPositions(list);
System.out.println(elementPositions);
}
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operation result:
In fact, as used herein, putIfAbsentwas present when the issue positionsMap.putIfAbsent(str, new ArrayList<>(1)); this code when each call will have a temporary ArrayList- when traversed List<String>when large, this may bring some negative effects; contrast computeand computeIfAbsentbenefits that they accept the parameters for the function, it will only be calculated when necessary to use a new function value. In the case of this paper, similar needs, you applicability and simplicity, it computeIfAbsentis superior compute. In JDK1.8the API documentation, also said in the need to generate a similar Map<K, Collection<V>>time of construction, computeIfAbsentit is suitable for this situation:
That computemethod is suitable for what happens then? Seen from the preceding description, computethe method is more suitable for updating keythe associated valuetime, the new value of the old value depends on the situation - such as a count List<String>the number of times each element appears:
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