Java中的 Collection contains() 方法和示例

Java.util.Collection 接口的contains(Object element)用于检查该集合中是否存在元素“元素”。此方法返回一个布尔值，描述元素的存在。如果元素存在，则返回 true，否则返回 false。

句法：

Collection.contains(Object element)

参数：此方法接受要在此集合中检查的 Object 类型的强制参数元素。

返回值：此方法返回一个布尔值，描述元素的存在。如果添加了元素，则返回 true，否则返回 false。

异常：此方法引发以下异常：

• ClassCastException：如果指定元素的类阻止它被添加到此集合中
• NullPointerException：如果指定元素为空且此集合不允许空元素

下面的示例说明了 Collection contains() 方法：

示例 1：使用 LinkedList 类

// Java code to illustrate boolean contains() method
  
import java.io.*;
import java.util.*;
  
public class GFG {
    public static void main(String args[])
    {
  
        // creating an empty LinkedList
        Collection list = new LinkedList();
  
        // use add() method to add elements in the list
        list.add("Geeks");
        list.add("for");
        list.add("Geeks");
  
        // Output the present list
        System.out.println("The list is: " + list);
  
        // Checking the presence of Geeks
        // using contains() method
        boolean result = list.contains("Geeks");
  
        // printing the result
        System.out.println("Is Geeks present in the List: "
                           + result);
    }
}

输出：

The list is: [Geeks, for, Geeks]
Is Geeks present in the List: true

示例 2：使用 ArrayDeque 类

// Java code to illustrate contains() method
  
import java.util.*;
  
public class ArrayDequeDemo {
    public static void main(String args[])
    {
        // Creating an empty ArrayDeque
        Collection de_que = new ArrayDeque();
  
        // Use add() method to add elements into the Deque
        de_que.add("Welcome");
        de_que.add("To");
        de_que.add("Geeks");
        de_que.add("4");
        de_que.add("Geeks");
  
        // Displaying the ArrayDeque
        System.out.println("ArrayDeque: " + de_que);
  
        // Checking the presence of Geeks
        // using contains() method
        boolean result = de_que.contains("Geeks");
  
        // printing the result
        System.out.println("Is Geeks present in the ArrayDeque: "
                           + result);
    }
}

输出：

ArrayDeque: [Welcome, To, Geeks, 4, Geeks]
Is Geeks present in the ArrayDeque: true

示例 3：使用 ArrayList 类

// Java code to illustrate contains() method
  
import java.io.*;
import java.util.*;
  
public class ArrayListDemo {
    public static void main(String[] args)
    {
  
        // create an empty array list with an initial capacity
        Collection arrlist = new ArrayList(5);
  
        // use add() method to add elements in the list
        arrlist.add(15);
        arrlist.add(20);
        arrlist.add(25);
  
        // Output the present list
        System.out.println("ArrayList: " + arrlist);
  
        // Checking the presence of 20
        // using contains() method
        boolean result = arrlist.contains(20);
  
        // printing the result
        System.out.println("Is 20 present in the ArrayList: "
                           + result);
    }
}

输出：

ArrayList: [15, 20, 25]
Is 20 present in the ArrayList: true

示例 4：演示 NullPointer 异常

// Java code to illustrate boolean contains()
  
import java.util.*;
  
public class LinkedListDemo {
    public static void main(String args[])
    {
  
        // Creating an empty ArrayList
        Collection
            list = new ArrayList();
  
        // Displaying the list
        System.out.println("The ArrayList is: " + list);
  
        try {
            // Checking presence of null
            list.contains(null);
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

输出：

The ArrayList is: []

参考： https://docs.oracle.com/javase/9/docs/api/ Java/util/Collection.html#contains-java.lang.Object-