ArrayList初始化的4种方法

In the last post we discussed about class ArrayList in Javaand it’s important methods. Here we are sharing multiple ways to initialize an ArrayList with examples.

Method 1: Initialization using Arrays.asList

Syntax:

ArrayList<Type> obj = new ArrayList<Type>(
        Arrays.asList(Object o1, Object o2, Object o3, ....so on));
Example:

import java.util.*;
public class InitializationExample1 {
   public static void main(String args[]) {
	   ArrayList<String> obj = new ArrayList<String>(
		Arrays.asList("Pratap", "Peter", "Harsh"));
	  System.out.println("Elements are:"+obj);
   }
}

　　

Output:

Elements are:[Pratap, Peter, Harsh]

Method 2: Anonymous inner class method to initialize ArrayList

Syntax:

ArrayList<T> obj = new ArrayList<T>(){{
           add(Object o1);
           add(Object o2);
           add(Object o3);
                   ...
                   ...
           }};
Example:

import java.util.*;
public class InitializationExample2 {
   public static void main(String args[]) {
       ArrayList<String> cities = new ArrayList<String>(){{
           add("Delhi");
           add("Agra");
           add("Chennai");
           }};
      System.out.println("Content of Array list cities:"+cities);
   }
}

Output:

Content of Array list cities:[Delhi, Agra, Chennai]

Method3: Normal way of ArrayList initialization

Syntax:

ArrayList<T> obj = new ArrayList<T>();
       obj.add("Object o1");
       obj.add("Object o2");
       obj.add("Object o3");
                        ...
                        ...
Example:

import java.util.*;

public class Details {
   public static void main(String args[]) {
       ArrayList<String> books = new ArrayList<String>();
       books.add("Java Book1");
       books.add("Java Book2");
       books.add("Java Book3");
      System.out.println("Books stored in array list are: "+books);
   }
}

Output:

Books stored in array list are: [Java Book1, Java Book2, Java Book3]

Method 4: Use Collections.ncopies

Collections.ncopies method can be used when we need to initialize the ArrayList with the same value for all of its elements. Syntax: count is number of elements and element is the item value

ArrayList<T> obj = new ArrayList<T>(Collections.nCopies(count, element));
Example:

import java.util.*;

public class Details {
   public static void main(String args[]) {
       ArrayList<Integer> intlist = new ArrayList<Integer>(Collections.nCopies(10, 5));
      System.out.println("ArrayList items: "+intlist);
   }
}

Output:

ArrayList items: [5, 5, 5, 5, 5, 5, 5, 5, 5, 5]

参考原文链接：https://beginnersbook.com/2013/12/how-to-initialize-an-arraylist/

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