Java Set removeAll()用法及代碼示例

java.util.Set接口的removeAll()方法用於從該集合中刪除所有包含在指定集合中的元素。

用法:

public boolean removeAll(Collection c)

參數：此方法將集合c作為包含要從此集合中刪除的元素的參數。

返回值：如果此集合因調用而更改，則此方法返回true。

異常：如果此集合包含null元素並且指定的集合不允許null元素(可選)，或者指定的collection為null，則此方法引發NullPointerException。

以下示例說明了removeAll()方法。

示例1:

// Java program to demonstrate 
// removeAll() method for Integer value 
  
import java.util.*; 
  
public class GFG1 { 
    public static void main(String[] argv) throws Exception 
    { 
  
        try { 
            // Creating object of Set 
            Set<Integer> set1 = new HashSet<Integer>(); 
  
            // Populating set1 
            set1.add(1); 
            set1.add(2); 
            set1.add(3); 
            set1.add(4); 
            set1.add(5); 
  
            // print set1 
            System.out.println("Set before removeAll() operation : "
                               + set1); 
  
            // Creating another object of Set 
            Set<Integer> set2 = new HashSet<Integer>(); 
            set2.add(1); 
            set2.add(2); 
            set2.add(3); 
  
            // print set2 
            System.out.println("Collection Elements to be removed : "
                               + set2); 
  
            // Removing elements from set 
            // specified in set2 
            // using removeAll() method 
            set1.removeAll(set2); 
  
            // print set1 
            System.out.println("Set after removeAll() operation : "
                               + set1); 
        } 
  
        catch (NullPointerException e) { 
            System.out.println("Exception thrown : " + e); 
        } 
    } 
}

輸出：

Set before removeAll() operation : [1, 2, 3, 4, 5]
Collection Elements to be removed : [1, 2, 3]
Set after removeAll() operation : [4, 5]

示例2:對於NullPointerException。

// Java program to demonstrate 
// removeAll() method for Integer value 
  
import java.util.*; 
  
public class GFG1 { 
    public static void main(String[] argv) throws Exception 
    { 
        try { 
  
            // Creating object of Set 
            Set<Integer> set1 = new HashSet<Integer>(); 
  
            // Populating set1 
            set1.add(1); 
            set1.add(2); 
            set1.add(3); 
            set1.add(4); 
            set1.add(5); 
  
            // print set1 
            System.out.println("Set before removeAll() operation : "
                               + set1); 
  
            // Creating another object of Set<Integer> 
            Set<Integer> set2 = null; 
  
            // print set2 
            System.out.println("Collection Elements to be removed : "
                               + set2); 
  
            System.out.println("\nTrying to pass "
                               + "null as a specified element\n"); 
  
            // Removing elements from set 
            // specified in set2 
            // using removeAll() method 
            set1.removeAll(set2); 
  
            // print set1 
            System.out.println("Set after removeAll() operation : "
                               + set1); 
        } 
  
        catch (NullPointerException e) { 
            System.out.println("Exception thrown : " + e); 
        } 
    } 
}

輸出：

Set before removeAll() operation : [1, 2, 3, 4, 5]
Collection Elements to be removed : null

Trying to pass null as a specified element

Exception thrown : java.lang.NullPointerException

參考： https://docs.oracle.com/javase/7/docs/api/java/util/Set.html#removeAll(java.util.Collection)

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