Java HashMap putIfAbsent() 方法

示例

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import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    HashMap<String, String> capitalCities = new HashMap<String, String>();

    capitalCities.put("England", "London");
    capitalCities.put("Germany", "Berlin");
    capitalCities.put("Norway", "Oslo");
    capitalCities.put("USA", "Washington DC");
    
    // Add new entries only if they don't exist
    capitalCities.putIfAbsent("Canada", "Ottawa");
    capitalCities.putIfAbsent("England", "Cambridge");
    
    System.out.println(capitalCities);
  }
}

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