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

if (!map.containsKey("Java")) {
    map.put("Java", new ArrayList<>());
}
map.get("Java").add("Tom");

map.computeIfAbsent("Java", k -> new ArrayList<>()).add("Tom");

V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)

import java.util.*;

public class ComputeIfAbsentDemo {
    public static void main(String[] args) {
        Map<String, List<String>> courseMap = new HashMap<>();

        // 當(dāng)鍵不存在時(shí)，創(chuàng)建新列表
        courseMap.computeIfAbsent("Java", k -> new ArrayList<>()).add("Tom");
        courseMap.computeIfAbsent("Java", k -> new ArrayList<>()).add("Alice");
        courseMap.computeIfAbsent("Python", k -> new ArrayList<>()).add("Bob");

        System.out.println(courseMap);
    }
}

{Java=[Tom, Alice], Python=[Bob]}

public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
    Objects.requireNonNull(mappingFunction);
    Node<K,V> e;
    V v;
    if ((e = getNode(hash(key), key)) == null) {
        V newValue;
        if ((newValue = mappingFunction.apply(key)) != null) {
            putVal(hash(key), key, newValue, false, true);
            return newValue;
        }
    } else if ((v = e.value) == null) {
        V newValue;
        if ((newValue = mappingFunction.apply(key)) != null) {
            e.value = newValue;
            return newValue;
        }
    } else {
        return v;
    }
    return null;
}

map.computeIfAbsent("x", k -> {
    // ? 不要在這里修改 map 自身！
    map.put("y", "test");
    return "value";
});

map.computeIfAbsent("key", k -> null);
// 不會(huì)添加任何條目