Java 資料結構 - 集合的差集

假設我們透過新增兩個（或多個）集合的內容來建立了一個集合物件，當您需要完全從該集合中刪除特定集合的內容時，可以使用 removeAll() 方法來實現。

此方法屬於 Set 介面，並且繼承自 Collection 介面。它接受一個集合物件，並立即從當前 Set（物件）中完全刪除其內容。

示例

import java.util.HashSet;
import java.util.Set;

public class SubtractingTwoSets {
   public static void main(String args[]) {      
      Set set1 = new HashSet();      
      set1.add(100);
      set1.add(501);
      set1.add(302);
      set1.add(420);
      System.out.println("Contents of set1 are: ");
      System.out.println(set1); 

      Set set2 = new HashSet();      
      set2.add(200);
      set2.add(630);
      set2.add(987);
      set2.add(665);
      System.out.println("Contents of set2 are: ");
      System.out.println(set2); 
      
      set1.addAll(set2);
      System.out.println("Contents of set1 after addition: ");
      System.out.println(set1);
      
      set1.removeAll(set2);
      System.out.println("Contents of set1 after removal");
      System.out.println(set1);
   }
}

輸出

Contents of set1 are: 
[100, 420, 501, 302]
Contents of set2 are: 
[630, 200, 665, 987]
Contents of set1 after addition: 
[100, 420, 501, 630, 200, 665, 987, 302]
Contents of set1 after removal
[100, 420, 501, 302]