Java ArrayList clear() 方法

描述

Java ArrayList clear() 方法會移除此列表中的所有元素。此呼叫返回後，列表將為空。

宣告

以下是 java.util.ArrayList.clear() 方法的宣告

public void clear()

引數

無

返回值

此方法不返回值。

異常

無

清空整數型 ArrayList 示例

以下示例演示了 Java ArrayList clear() 方法的用法。在此示例中，我們使用的是整數。第一步，我們填充 arrayList 物件並列印它。然後我們列印 arrayList 物件的大小並執行清空操作。清空後，我們列印 arrayList 物件的大小，現在為 0。

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {
     
      // create an empty array list
      ArrayList<Integer> arrayList = new ArrayList<>();

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

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

      // finding size of this arrayList
      int retval = arrayList.size();
      System.out.println("ArrayList consists of "+ retval +" elements");

      System.out.println("Performing clear operation !!");
      arrayList.clear();
      retval = arrayList.size();
      System.out.println("Now, ArrayList consists of "+ retval +" elements");
   }
}   

輸出

讓我們編譯並執行上述程式，這將產生以下結果：

ArrayList = [20, 30, 10]
ArrayList consists of 3 elements
Performing clear operation !!
Now, ArrayList consists of 0 elements

清空字串型 ArrayList 示例

以下示例演示了 Java ArrayList clear() 方法的用法。在此示例中，我們使用的是字串。第一步，我們填充 arrayList 物件並列印它。然後我們列印 arrayList 物件的大小並執行清空操作。清空後，我們列印 arrayList 物件的大小，現在為 0。

package com.tutorialspoint;

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {
     
      // create an empty array list
      ArrayList<String> arrayList = new ArrayList<>();

      // use add() method to add elements in the arrayList
      arrayList.add("A");
      arrayList.add("B");
      arrayList.add("C");

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

      // finding size of this arrayList
      int retval = arrayList.size();
      System.out.println("ArrayList consists of "+ retval +" elements");

      System.out.println("Performing clear operation !!");
      arrayList.clear();
      retval = arrayList.size();
      System.out.println("Now, ArrayList consists of "+ retval +" elements");
   }
}   

輸出

讓我們編譯並執行上述程式，這將產生以下結果：

ArrayList = [A, B, C]
ArrayList consists of 3 elements
Performing clear operation !!
Now, ArrayList consists of 0 elements

清空物件型 ArrayList 示例

以下示例演示了 Java ArrayList clear() 方法的用法。在此示例中，我們使用的是 Student 物件。第一步，我們填充 arrayList 物件並列印它。然後我們列印 arrayList 物件的大小並執行清空操作。清空後，我們列印 arrayList 物件的大小，現在為 0。

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);    

      // finding size of this arrayList
      int retval = arrayList.size();
      System.out.println("arrayList consists of "+ retval +" elements");

      System.out.println("Performing clear operation !!");
      arrayList.clear();
      retval = arrayList.size();
      System.out.println("Now, arrayList consists of "+ retval +" elements");
   } 		
} 

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 + " ]";
   }
}  

輸出

讓我們編譯並執行上述程式，這將產生以下結果：

ArrayList = [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ]]
arrayList consists of 3 elements
Performing clear operation !!
Now, arrayList consists of 0 elements