public static void main(String[] args) {
 Map<String, Integer> map = new ConcurrentHashMap<>();
 // 初始时 map 为空
 // 计算键 "key1" 对应的值,如果不存在则根据计算逻辑生成值
 Integer value1 = map.putIfAbsent("key1", 1);
 System.out.println("map = " + map); // 输出: map = {key1=1}
 System.out.println("value1 = " + value1); // 输出: value1 = 1
 // 再次计算键 "key1" 对应的值,由于键已存在,直接返回已有的值
 Integer value2 = map.putIfAbsent("key1", 2);
 System.out.println("map = " + map); // 输出: map = {key1=1}
 System.out.println("value2 = " + value2); // 输出: value2 = 1
 }