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Map.md

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Java 8之Map新增方法

getOrDefault

map.getOrDefault("one", "s");

forEach

map.forEach(new BiConsumer<String, String>() {
    @Override
    public void accept(String key, String value) {
        System.out.println(key + ", " + value);
    }
});

replace(K key, V newValue)

类似

if (map.containsKey(key)) {
    return map.put(key, newValue);
} else {
    return null;
}

replace(K key, V oldValue, V newValue);

如果key和oldValue都匹配，则替换成newValue。类似：

if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) {
    map.put(key, newValue);
    return true;
} else {
    return false;
}

replaceAll

替换Map中所有Entry的value值，这个值由旧的key和value计算得出

map.replaceAll(new BiFunction<String, String, String>() {
    @Override
    public String apply(String key, String value) {
        if (key.equals("two")) {
            return value + ".old";
        }
        return value;
    }
});

类似如下代码：

for (Entry entry : map.entrySet()) {
    entry.setValue(function.apply(entry.getKey(), entry.getValue()));
}

putIfAbsent(K key, V value)

如果key不存在，则put，并返回null。如果key存在，则什么也不做，返回key对应的value

map.putIfAbsent("four", "charge");

remove(K key, V value)

map.remove(key, value);

类似：

if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
    map.remove(key);
    return true;
} else {
    return false;
}

computeIfAbsent(K key, Function f)

String s = map.computeIfAbsent("four", new Function<String, String>() {
    @Override
    public String apply(String key) {
        return key + "@";
    }
});

类似

if (map.get(key) == null) {
    V newValue = function.apply(key);
    if (newValue != null) {
        map.put(key, newValue);
        return newValue;
    }
} else {
    return map.get(key);
}

computeIfPresent(K key, BiFunction f)

map.computeIfPresent("one", new BiFunction<String, String, String>() {
    @Override
    public String apply(String key, String value) {
        return null;
    }
});

类似：

if (map.get(key) != null) {
    V oldValue = map.get(key);
    V newValue = function.apply(key, oldValue);
    if (newValue != null) {
        map.put(key, newValue);
    } else {
        map.remove(key);
    }
    return newValue;
} else {
    return null;
}

compute(K key, BiFunction f)

map.compute(key, new BiFunction<String, String, String>() {
    @Override
    public String apply(String key, String value) {
        return key + "@" + value;
    }
});

类似：

V oldValue = map.get(key);
V newValue = function.apply(key, value);

if (oldValue != null) {
    if (newValue != null) {
        map.put(key, newValue);
    } else {
        map.remove(key);
    }
} else {
    if (newValue != null) {
        map.put(key, newValue);
    }
}
return newValue;

merge(K key, V value, BiFunction f)

apply中第一个参数是oldValue，第二个参数是merge传入的第二个参数

map.merge(key, value, new BiFunction<String, String, String>() {
    @Override
    public String apply(String oldValue, String value) {
        return oldValue + "@" + value;
    }
});

类似：

if (value == null) {
    throw new NullPointerException();
}

V oldValue = map.get(key);
V newValue = (oldValue == null) ? value : function.apply(oldValue, value);

if (newValue == null) {
    map.remove(key);
} else {
    map.put(key, newValue);
    return newValue;
}
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