Java ArrayList クラス

ArrayList は List インターフェースの実装であり、Java のコレクション フレームワークの下にあり、配列のサイズを動的に、つまり実行時に拡大することができます。このクラスは内部的に java.util.package で使用でき、配列データ構造を使用します。 ArrayList では Java のラッパー クラスのみが許可され、ユーザーがクラスを定義します。

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構文:

ArrayList<T> list = new ArrayList<>();

List<T> list = new ArrayList<>();

ArrayList のインスタンスを直接使用するか、List 参照に割り当てることができます。

コンストラクター:

配列リストには、次の 3 つのコンストラクターが利用可能です。

• ArrayList(intInitialCapacity): ここで、配列リストの初期長を指定できます。ただし、サイズが前述のInitialCapacityを超えた場合は、ArrayList クラスが処理します。
• ArrayList(): これにより、初期容量なしの空のリストを作成できるため、この場合、デフォルトの初期容量は 10 になります。
• ArrayList(Collection c): コレクションのリスト。

Java ArrayList クラスのメソッド

次に、Java ArrayList クラスのメソッドを示します。

• void trimToSize(): This method will trim the list to the current list size.
• Object[] toArray(): Return the array of object.
• boolean remove(Object o): This method is used to remove the object, but it will remove the first occurrence as the list contain duplicate.
• boolean removeAll(Collection c): This method is used to remove all the elements of the list.
• Boolean removeIf(Predicate filter): This method is used to remove the predicate passed. Used as a filter.
• Add (E e): This method s used to add elements to the list.
• Void add(int index, E element): It takes two-parameter and adds elements t the specific index we mentioned.
• Boolean addAll(Collection c): This takes a list as an argument and adds all the elements to the end of the current list.
• boolean addAll(int index, Collection c): This method adds all the elements to the current list at the specified index we pass.
• Get (int index): This method is used to get the element. It will return the element present at the specified position in the list.
• Int indexOf(Object o): This method is used to get the element’s index passed. It will always return the first occurrence of the element into the list because the list can contain duplicate elements.
• ListIterator listIterator(int index): This method returns an iterator with the specific index.
• Remove (int index): This method removes the element. It will remove the element with the corresponding index passed.
• Protected void removeRange(int fromIndex, int toIndex): This removes the elements from a specified range.
• Boolean retainAll(Collection c): This method will retain all elements contained in the specified collection.
• Set (int index, E element): This method will set the element to the specified index.
• Void sort(Comparator c): This method is used to sort the collection element.
• List subList(int fromIndex, int toIndex): This method will be used to return the sublist from the specified index.
• Void clear(): This mentioned is used to clear the elements in the list.
• Object clone(): These methods create a copy of the list.
• Boolean contains(Object o): This method is used to check whether the passing object is present in the list or not.
• Void ensureCapacity(int minCapacity): This method is used to increase the capacity of the array list.
• Boolean isEmpty(): This method is used to check whether the array list is empty or not.
• Iterator iterator(): This method returns iterator.
• int lastIndexOf(Object o): This method returns the last index of the object passed. If the object does not present in the list, it will return -1.
• ListIterator listIterator(): This methods return iterator.

Examples of Java ArrayList Class

Examples of Java ArrayList Class are given below:

1. add an element in ArrayList

The below example will show how to add an element to an array list.

Code:

package com.cont.article;
import java.util.ArrayList;
public class ArratListTest {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
list.add("abc");
list.add("xyz");
list.add("yyy");
list.add("some name");
list.add("other name");
System.out.println("list is :: " + list);
}
}

Output:

2. Copying elements of list one to another list

Code:

import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
list.add("abc");
list.add("xyz");
list.add("yyy");
list.add("some name");
list.add("other name");
ArrayList<String> list2 = new ArrayList<>();
list2.addAll(list);
System.out.println("Elements in list one : " + list);
System.out.println("Elements in list two :  " + list2);
}
}

Output:

3. Remove element from ArrayList

Code:

import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
list.add("abc");
list.add("xyz");
list.add("yyy");
list.add("some name");
list.add("other name");
System.out.println("Size of list before remove ::" + list.size());
System.out.println("Elements in list are before remove " + list);
list.remove(4);
System.out.println("Size of list after removinf element :: " +list.size());
System.out.println("Elements in list are after remove" + list);
}
}

Output:

4. Clear all the elements from ArrayList

Code:

import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
list.add("abc");
list.add("xyz");
list.add("yyy");
list.add("some name");
list.add("other name");
list.clear();
System.out.println("Clering all elements of list " +list.size());
}
}

Output:

5. Iterate all the elements of ArrayList

Code:

import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>();
list.add("abc");
list.add("xyz");
list.add("yyy");
list.add("some name");
list.add("other name");
System.out.println("Priniting out element.");
for (String string : list) {
System.out.println("Elemnt in the list is");
System.out.println(string);
}
}
}

Output:

The array list in java does not contain duplicate elements. Also, the insertion order is maintained in a list, which means we put over elements that will generate the output in the same sequence. Some detailed points which need to be remembered for the array list in java are as follows:

It implements various interfaces:

• Serializable,
• Iterable,
• Cloneable,
• Collection,
• List,
• RandomAccess

The class hierarchy is as follows:

java.lang.Object  >> java.util.AbstractCollection<E> >> java.util.AbstractList<E> >> java.util.ArrayList<E>

By default, the array list is not synchronized in nature and is not thread-safe, But we can make them synchronized using the collections class synchronized method. The syntax is described below for reference :

List arrList = Collections.synchronizedList (new ArrayList(...));

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