Java ArrayList contains() 方法

描述

Java ArrayList contains(Object) 方法用于判断此列表是否包含指定的元素。为了使此操作成功，对象应该已实现 equals() 方法。

声明

以下是 java.util.ArrayList.contains() 方法的声明

public boolean contains(Object o)

参数

o − 要在此列表中测试其是否存在 的元素。

返回值

如果此列表包含指定的元素，则此方法返回 true。

异常

无

在整数 ArrayList 中检查元素是否存在示例

以下示例演示了 Java ArrayList contains() 方法的使用。我们使用的是整数。我们将添加一些元素，然后检查特定元素是否存在。

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {

      // create an empty arrayList
      ArrayList<Integer> arrayList = new ArrayList<>();

      // use add() method to add elements in the arrayList
      arrayList.add(20);
      arrayList.add(30);
      arrayList.add(10);
      arrayList.add(18);        

      // let us print all the elements available in arrayList
      System.out.println("ArrayList = " + arrayList);

      // arrayList contains element 10
      if (arrayList.contains(10)) {
         System.out.println("element 10 is present in the arrayList");
      } else {
         System.out.println("element 10 is not present in the arrayList");
      }

      // arrayList does not contain element 25
      if (arrayList.contains(25)) {
         System.out.println("element 25 is present in the arrayList");
      } else {
         System.out.println("element 25 is not present in the arrayList");    
      }
   }
}           

输出

让我们编译并运行上述程序，这将产生以下结果：

ArrayList = [20, 30, 10, 18]
element 10 is present in the arrayList
element 25 is not present in the arrayList

在字符串 ArrayList 中检查元素是否存在示例

以下示例演示了 Java ArrayList contains() 方法与字符串一起使用。我们将添加一些元素，然后检查特定元素是否存在。

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {

      // create an empty arrayList
      ArrayList<String> arrayList = new ArrayList<>();

      // use add() method to add elements in the arrayList
      arrayList.add("Welcome");
      arrayList.add("to");
      arrayList.add("tutorialspoint");
      arrayList.add(".com");        

      // let us print all the elements available in arrayList
      System.out.println("ArrayList = " + arrayList);

      // arrayList contains element tutorialspoint
      if (arrayList.contains("tutorialspoint")) {
         System.out.println("element tutorialspoint is present in the arrayList");
      } else {
         System.out.println("element tutorialspoint is not present in the arrayList");
      }

      // arrayList does not contain element html
      if (arrayList.contains("html")) {
         System.out.println("element html is present in the arrayList");
      } else {
         System.out.println("element html is not present in the arrayList");    
      }
   }
}           

输出

让我们编译并运行上述程序，这将产生以下结果：

ArrayList = [Welcome, to, tutorialspoint, .com]
element tutorialspoint is present in the arrayList
element html is not present in the arrayList

在对象 ArrayList 中检查元素是否存在示例

以下示例演示了 Java ArrayList contains() 方法与 Student 对象一起使用。我们将添加一些元素，然后检查特定元素是否存在。

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {

      // create an empty arrayList
      ArrayList<Student> arrayList = new ArrayList<>();

      // use add() method to add elements in the arrayList
      arrayList.add(new Student(1, "Julie"));
      arrayList.add(new Student(2, "Robert"));
      arrayList.add(new Student(3, "Adam"));      

      // let us print all the elements available in arrayList
      System.out.println("ArrayList = " + arrayList);

      // arrayList contains element Robert
      if (arrayList.contains(new Student(2, "Robert"))) {
         System.out.println("Student Robert is present in the arrayList");
      } else {
         System.out.println("Student Robert is not present in the arrayList");
      }

      // arrayList does not contain element Jane
      if (arrayList.contains(new Student(4, "Jane"))) {
         System.out.println("Student Jane is present in the arrayList");
      } else {
         System.out.println("Student Jane is not present in the arrayList");    
      }
   }
} 
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   
   @Override
   public boolean equals(Object obj) {
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }
}

输出

让我们编译并运行上述程序，这将产生以下结果：

ArrayList = [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ]]
Student Robert is present in the arrayList
Student Jane is not present in the arrayList