How to use ArrayList and HashSet in Java Collection

##Collection

The Collection interface is inherited by the List interface and the Set interface

List

ArrayList is the implementation class of the List interface

ArrayList

The ArrayList class is an array that can be dynamically modified. The difference from ordinary arrays is that it has no fixed size. With the restrictions we can add or remove elements.

ArrayList inherits AbstractList and implements the List interface.

Create ArrayList object:

public class List {
    public static void main(String[] args) {
        var list = new ArrayList();
    }
}

Add data to the collection:

add()

public class List {
    public static void main(String[] args) {
        var list = new ArrayList();
        list.add("a");
        list.add(1);
        list.add(1);
        System.out.println(list);
    }
}

Clear the collection Data:

clear()

public class List {
    public static void main(String[] args) {
        var list = new ArrayList();
        list.add("a");
        list.add(1);
        list.add(1);
        System.out.println(list);
        list.clear();
        System.out.println(list);
    }
}

Modify the object at the specified index position

set()

public class List {
    public static void main(String[] args) {
        var list = new ArrayList();
        list.add("a");
        list.add(1);
        list.add(1);
        System.out.println(list);
        list.set(2, "a");
        System.out.println(list);
    }
}

Set

HashSet

##HashSet is implemented based on HashMap and is a set that does not allow duplicate elements.
HashSet allows null values.
HashSet is unordered, that is, the order of insertion is not recorded.
HashSet is not thread-safe. If multiple threads try to modify the HashSet at the same time, the final result is undefined. You must explicitly synchronize concurrent access to a HashSet when accessed by multiple threads.
HashSet implements the Set interface.
Create an ArrayList object:

public class Set {
    public static void main(String[] args) {
        var set = new HashSet<>();
      
    }
}

Add data to the collection:

add()

The elements in the collection cannot be repeated

public class Set {
    public static void main(String[] args) {
        var set = new HashSet<>();
        set.add(1);
        set.add("a");
        set.add("a");
        System.out.println(set);
    }
}

Clear the collection data:

clear()

public class Set {
    public static void main(String[] args) {
        var set = new HashSet<>();
        set.add(1);
        set.add("a");
        set.add("a");
        System.out.println(set);
        set.clear();
        System.out.println(set);
    }
}

Delete the specified element

public class Set {
    public static void main(String[] args) {
        var set = new HashSet<>();
        set.add(1);
        set.add("a");
        set.add("a");
        System.out.println(set);
        set.remove("a");
        System.out.println(set);
    }
}

The difference between ArrayList and HashSet

1.HashSet is non-duplicate and unordered! Uniqueness guaranteed. The equals method of duplicate objects returns true, The hashCode method of a repeated object returns the same integer. HashSet is actually a HashMap, but you can only operate the key part of this HashMap through the Set interface.

2. ArrayList is repeatable and ordered: high query efficiency, additions and deletions Inefficient and lightweight thread unsafe. Arraylist: Not fast at inserting and deleting data, but faster at random extraction

Generic

Generic programming (generic programming) is a style of programming language or paradigm. Generics allow programmers to write code in a strongly typed programming language using types that are specified later and specified as parameters at instantiation time. Various programming languages, their compilers, and runtime environments have different support for generics.

Generally, generics are used when using collections. Generics are to add a type constraint to the collection. It can be String or Object. Generics do not support the basic type int. Please use the wrapper type Integer

to create a generic:

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

In this way, only String types can be added to this collection.

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