Java Collections addAll()方法及实例

java.util.Collections 类的 addAll() 方法是用来将所有指定的元素添加到指定的集合中。要添加的元素可以单独指定或作为一个数组。这个方便方法的行为与c.addAll(Arrays.asList(elements))相同，但在大多数实现中，这个方法可能运行得更快。

语法

public static  boolean 
    addAll(Collection c, T... elements)

参数： 该方法接受以下参数作为参数

• c- 要插入的元素的集合
• elements – 要插入到c中的元素

返回值: 如果集合因调用而改变，该方法返回true。

异常： 如果元素包含一个或多个空值，而c不允许空元素，或者c或元素是空的，这个方法会抛出 NullPointerException 。

下面是说明addAll()方法的例子

例1 :

// Java program to demonstrate
// addAll() method
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
        try {
  
            // creating object of List<String>
            List<String> arrlist = new ArrayList<String>();
  
            // Adding element to arrlist
            arrlist.add("A");
            arrlist.add("B");
            arrlist.add("C");
            arrlist.add("Tajmahal");
  
            // printing the arrlist before operation
            System.out.println("arrlist before operation : " + arrlist);
  
            // add the specified element to specified Collections
            // using addAll() method
            boolean b = Collections.addAll(arrlist, "1", "2", "3");
  
            // printing the arrlist after operation
            System.out.println("result : " + b);
  
            // printing the arrlist after operation
            System.out.println("arrlist after operation : " + arrlist);
        }
  
        catch (NullPointerException e) {
  
            System.out.println("Exception thrown : " + e);
        }
        catch (IllegalArgumentException e) {
  
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

arrlist before operation : [A, B, C, Tajmahal]
result : true
arrlist after operation : [A, B, C, Tajmahal, 1, 2, 3]

输出

arrlist before operation : [A, B, C, Tajmahal]
result : true
arrlist after operation : [A, B, C, Tajmahal, 1, 2, 3]

例2： 对于NullPointerException

// Java program to demonstrate
// addAll() method
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
        try {
  
            // creating object of List<String>
            List<String> arrlist = new ArrayList<String>();
  
            // Adding element to arrlist
            arrlist.add("A");
            arrlist.add("B");
            arrlist.add("C");
            arrlist.add("Tajmahal");
  
            // printing the arrlist before operation
            System.out.println("arrlist before operation : " + arrlist);
  
            // add the specified element to specified Collections
            // using addAll() method
            System.out.println("\nTrying to add the null value with arrlist");
            boolean b = Collections.addAll(null, arrlist);
  
            // printing the arrlist after operation
            System.out.println("result : " + b);
  
            // printing the arrlist after operation
            System.out.println("arrlist after operation : " + arrlist);
        }
  
        catch (NullPointerException e) {
  
            System.out.println("Exception thrown : " + e);
        }
        catch (IllegalArgumentException e) {
  
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

arrlist before operation : [A, B, C, Tajmahal]

Trying to add the null value with arrlist
Exception thrown : java.lang.NullPointerException