JDK1.8 series articles

  • New in JDK1.8 (1) : Lambda expressions
  • New in JDK1.8 (2) : Optional
  • JDK1.8 new feature (3) : Stream
  • New in JDK1.8 (4) : Maps
  • New JDK1.8 feature (5) : new date and time API

Maps does not support Streams. The Map interface itself does not have a stream() method. You can create specialized streams on mapped keys, values, or entries using map.keyset ().stream(), map.values().stream(), and map.entrySet ().stream(). In addition, MAPS supports a variety of new ways to perform common tasks.

Map<Integer, String> map = new HashMap<>();

for (int i = 0; i < 10; i++) {
    map.putIfAbsent(i, "val" + i);
}

map.forEach((id, val) -> System.out.println(val));
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The above code is explicitly checked by putIfAbsent to prevent us from writing null values. ForEach accepts a consumer to perform an action on each value of the map. This example shows how to calculate in a map:

map.computeIfPresent(3, (num, val) -> val + num);
map.get(3);             // val33

map.computeIfPresent(9, (num, val) -> null);
map.containsKey(9);     // false

map.computeIfAbsent(23, num -> "val" + num);
map.containsKey(23);    // true

map.computeIfAbsent(3, num -> "bam");
map.get(3);             // val33
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Next, let’s see how to delete a key-value pair with key 3 and value val3:

map.remove(3, "val3");
map.get(3);             // val33

map.remove(3, "val33");
map.get(3);             // null
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Another useful method:

map.getOrDefault(42, "not found");  // not found
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It is also easier to merge elements of a Map:

map.merge(9, "val9", (value, newValue) -> value.concat(newValue));
map.get(9);             // val9
map.merge(9, "concat", (value, newValue) -> value.concat(newValue));
map.get(9);             // val9concat
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What the Merge does is to insert the key name if it does not exist, and otherwise Merge the value corresponding to the original key and reinsert it into the map.

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