Class ArrayList
Type Parameters: E — the type of elements in this list All Implemented Interfaces: Serializable , Cloneable , Iterable , Collection , List , RandomAccess Direct Known Subclasses: AttributeList , RoleList , RoleUnresolvedList
Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null . In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector , except that it is unsynchronized.)
The size , isEmpty , get , set , iterator , and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.
Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.
An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.
Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be «wrapped» using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:
List list = Collections.synchronizedList(new ArrayList(. ));
The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
This class is a member of the Java Collections Framework.
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Java Урок 60: КОЛЛЕКЦИИ, ArrayList
До появления JDK 5 в коллекциях хранились только объекты типа Object. Это было неудобно, т.к. во-первых, программисту самому приходилось отслеживать, какого типа объекты помещаются в коллекции, теперь этим занимается компилятор, и, во-вторых, теперь не надо приводить объекты типа Object обратно в необходимый тип данных.
Скачать исходники для статьи можно ниже
Однако, и сегодня коллекции содержащие тип Object используются довольно часто.
Наиболее часто используются коллекции ArrayList.
Пример использования ArrayList :
import java.util.ArrayList; public class Test < public static void main(String[] args) < Double numer1 = 5.666666666; System.out.println(numer1); Integer numer2 = 100; System.out.println(numer2); Car car1 = new Car("KIA"); System.out.println(car1); // объявляется ArrayList состоящий из объектов Object ArrayListarray1 = new ArrayList(); array1.add(numer1);// в массив добавляется объект типа Double array1.add(numer2);// в массив добавляется объект типа Integer array1.add(car1);// в массив добавляется объект типа Car System.out.println(array1.toString()); // объект типа Object приводится к объекту типа Car String name = ((Car)array1.get(2)).name; System.out.println(name); > > public class Car < String name; Car(String name) < this.name = name; >public String toString() < String s = "Автомобиль " + name + "."; return s; >> Вывод программы:
5.666666666
100
Автомобиль KIA.
[5.666666666, 100, Автомобиль KIA.] KIA
Добавить объект в Arraylist в Java
- Добавить объекты одного типа в ArrayList
- Добавление объектов разных типов в список ArrayList
В Java ArrayList представляет собой массив изменяемого размера, который также может быть определен как упорядоченная последовательность элементов. В отличие от простых массивов, Java ArrayList более гибкий и может содержать несколько типов данных. В этой статье будет показано, как можно использовать эту функцию.
Добавить объекты одного типа в ArrayList
Здесь мы добавим определяемые пользователем или настраиваемые объекты класса в ArrayList . В ArrayList мы можем обращаться к элементам по целочисленному индексу. Мы укажем или объявим тип объекта, который мы будем хранить в ArrayList внутри <> (угловые скобки).
В приведенном ниже коде у нас есть объект класса Book с конструктором и тремя переменными экземпляра bookName , author и rating типа данных соответственно. Создаем объект класса Book с помощью ключевого слова new , за которым следует вызов конструктора с параметрами; это присваивает переданное значение переменным экземпляра.
Метод add() вставляет объекты в arrayofBooks . Таким образом, мы добавили 4 объекта класса Book в наш arrayOfBooks . Затем мы запускаем цикл foreach для повторения и отображения всех данных Book .
Мы можем получить доступ к атрибутам класса Book с помощью символа . оператор точки. Ниже мы получили доступ к атрибуту bookName , вызвав функцию book.bookName .
import java.util.ArrayList; public class ArrayObject public static void main (String args[]) ArrayListBook> arrayOfBooks = new ArrayList<>(); arrayOfBooks.add(new Book("To Kill a Mockingbird", "Harper Lee", 3)); arrayOfBooks.add(new Book("1984", "George Orwell", 4)); arrayOfBooks.add(new Book("Harry Potter and the Philosopher's Stone", "J.K. Rowling", 4)); arrayOfBooks.add(new Book("The Lord of the Rings", "J.R.R. Tolkien", 4.5)); for (Book book: arrayOfBooks) System.out.println("BookTitle: "+book.bookName+", by"+book.author+ "with a rating of "+book.rating); > > > class Book String bookName; String author; double rating; Book(String bookName, String author, double rating) this.bookName = bookName; this.author = author; this.rating = rating; > > BookTitle: To Kill a Mockingbird, byHarper Leewith a rating of 3.0 BookTitle: 1984, byGeorge Orwellwith a rating of 4.0 BookTitle: Harry Potter and the Philosopher's Stone, byJ.K. Rowlingwith a rating of 4.0 BookTitle: The Lord of the Rings, byJ.R.R. Tolkienwith a rating of 4.5 Добавление объектов разных типов в список ArrayList
Как уже упоминалось, функция ArrayList также может содержать несколько типов объектов. Здесь arrayOfDifferentObject — это ArrayList , который может содержать объекты разных типов. Мы объявили наш ArrayList с помощью класса в синтаксисе, приведенном ниже в коде.
В Java ArrayList может содержать объекты классов-оболочек, такие как double, integer и string.
Затем мы добавляем элементы в ArrayList с помощью метода add() . Сначала мы добавили строковое значение в наш ArrayList , затем значение типа double, integer и float соответственно. Мы также можем заменить элемент новым значением по выбранному нами индексу, используя метод set() .
Мы заменили arrayOfDifferentObject.set(1,»David Wells») и двойное значение в индексе 1 строковым значением. После этого мы видим, что вывод изменен.
import java.util.ArrayList; public class ArrayObject public static void main (String args[]) ArrayListObject> arrayOfDifferentObject = new ArrayListObject>(); arrayOfDifferentObject.add("John Doe"); arrayOfDifferentObject.add(10.00D); arrayOfDifferentObject.add(10); arrayOfDifferentObject.add(10.11F); System.out.println("ArrayList after all insertion:-"); for (int i=0; iarrayOfDifferentObject.size(); i++) System.out.println(arrayOfDifferentObject.get(i)); > arrayOfDifferentObject.set(1,"David Wells"); System.out.println("ArrayList after modification:-"); for (int i=0; iarrayOfDifferentObject.size(); i++) System.out.println(arrayOfDifferentObject.get(i)); > > > ArrayList after all insertion:- John Doe 10.0 10 10.11 ArrayList after modification:- John Doe David Wells 10 10.11 Rupam Saini is an android developer, who also works sometimes as a web developer., He likes to read books and write about various things.
Store User-defined Class Objects in ArrayList
In this tutorial, we will learn how to store user-defined (custom) class objects in Java ArrayList in an easy way and step by step.
In the previous ArrayList tutorial, we have learned that ArrayList class uses generic from Java 1.5 or later. Using generic, ArrayList class can be used to store any type of object.
In other words, we can store multiple types of objects in an ArrayList using generic feature.
For example, we could have an ArrayList of Book objects, an ArrayList of Employee objects, or an ArrayList of Strings. The specified type must be a class, not a primitive type.
If you want to store primitive data types in ArrayList, you will have to use one of java wrapper classes such as Integer, Double, or Character.
Declaration of ArrayList Objects
The syntax for declaring an ArrayList of objects is as follows:
Inside angle brackets, we declare the class types of objects that will be stored in ArrayList. Let’s understand with the help of some examples.
1. ArrayList emp = new ArrayList(); // Here, Employee is the name of class. // ArrayList indicates that only Employee objects can be stored in the array list. 2. ArrayList b = new ArrayList();
How to store User-defined (Custom) Class Objects in Java ArrayList?
Let’s take an example program where we will store user-defined (custom) class objects in ArrayList.
Suppose there are three students whose name, id, rollNo have to add to the custom ArrayList. First, we will create a class Student.
In this class, we will define a constructor and three instance variables name, id, and rollNo of data type String, int, and int respectively. Look at the below program source code.
Program code 1:
package customArrayList; public class Student < // Create instance variables name, id, and rollNo of data type String, int, and int respectively. String name; int id; int rollNo; // Create three parameters constructor with parameters name, id, and rollNo. Student(String name, int id, int rollNo) < this.name = name; this.rollNo = rollNo; this.id = id; >>
Now create another class addingData in which we will store three Student class objects in the ArrayList.
package customArrayList; import java.util.ArrayList; public class AddingData < // Create a ArrayList method of generic type 'Student'. ArrayListstudentData() < // Create three objects of the class Student and pass arguments to the constructor. Student s1 = new Student("Deep", 1234, 04); Student s2 = new Student("Shubh", 4321, 20 ); Student s3 = new Student("Riddhi", 1212, 02); // Create the object of ArrayList of generic type 'Student'. ArrayListstudentlist = new ArrayList(); // Now add Student objects in the ArrayList using reference variable studentlist. studentlist.add(s1); studentlist.add(s2); studentlist.add(s3); // Return object reference variable 'studentlist' of the array list to the method 'studentValue'. return studentlist; > > Now create one more class to retrieve students data from the above class’s studentData() method and iterate over them to get the student detail.
package customArrayList; import java.util.ArrayList; public class RetriveStudents < public static void main(String[] args) < // Call AddingData class by creating object of that class. AddingData data = new AddingData(); // Call studentData() method using reference variable data. ArrayListlistst = data.studentData(); // Now iterate and display all the Student data. for(Student st:listst) < System.out.println("Student's name: " +st.name); System.out.println("Student ID " +st.id); System.out.println("Roll number: " +st.rollNo); >> > Output: Student's name: Deep Student ID 1234 Roll number: 4 Student's name: Shubh Student ID 4321 Roll number: 20 Student's name: Riddhi Student ID 1212 Roll number: 2
The same program can also be done without using any constructor. Look at the below source code and follow all steps.
Program code 2:
package customArrayList2; public class Student < // Declare instance variables name, phyMarks, mathsMarks, chemMarks, total, and per. String name; int phyMarks; int mathsMarks; int chemMarks; int total; float per; >
package customArrayList2; import java.util.ArrayList; public class Studentdata < // Declare an ArrayList method of generic type Student. ArrayListaddData() < // Create two objects s1 and s2 of the student class and initialize the value of variables using reference variable s1 and s2. Student s1 = new Student(); s1.name = "Shubh"; s1.phyMarks = 95; s1.mathsMarks = 100; s1.chemMarks = 90; s1.total = 95 + 100 + 90; s1.per = ((s1.total)*100)/300; Student s2 = new Student(); s2.name = "Deep"; s2.phyMarks = 80; s2.mathsMarks = 85; s2.chemMarks = 90; s2.total = 80 + 85 + 90; s2.per = ((s2.total)*100)/300; // Create an ArrayList object of generic type Student. ArrayListal = new ArrayList(); // Call add() method to store student class objects in the array list using reference variable al. al.add(s1); al.add(s2); return al; > >
package customArrayList2; import java.util.ArrayList; public class RetriveStudentData < // Declare an instance method. void fetchStudentData() < // Create an object of the Studentdata class. Studentdata stdata = new Studentdata(); // Call addData() method using reference variable stdata. ArrayListlistst = stdata.addData(); // Now iterate and display all the student data. // enhance for loop - for each loop. for(Student student:listst) < System.out.println("Name: " +student.name); System.out.println("Physics Marks: " +student.phyMarks); System.out.println("Maths Marks: " +student.mathsMarks); System.out.println("Chemistry Marks: " +student.chemMarks); System.out.println("Total Marks: " +student.total); System.out.println("Percentage:" +student.per); >> > In this example program, we will create a client class to test our logic.
package customArrayList2; public class TestStudentOperation < public static void main(String[] args) < Studentdata st = new Studentdata(); st.addData(); RetriveStudentData rsd = new RetriveStudentData(); rsd.fetchStudentData(); >>
Output: Name: Shubh Physics Marks: 95 Maths Marks: 100 Chemistry Marks: 90 Total Marks: 285 Percentage: 95.0 Name: Deep Physics Marks: 80 Maths Marks: 85 Chemistry Marks: 90 Total Marks: 255 Percentage: 85.0
Let’s take another example program where we will create a class Employee and we will iterate and display all employee data from the Employee class. So let’s see the following source code.
Program code 3:
package customArrayList3; import java.util.ArrayList; public class Employee < int eNo; String name, address; Employee(int eNo, String name, String address) < this.eNo = eNo; this.name = name; this.address = address; >public Employee() < >// Display all employees data. void displayData(ArrayList list) < System.out.println("Employee Detail"); for(Employee emp: list ) < System.out.println("Employee number: " +emp.eNo); System.out.println("Employee Name: " +emp.name); System.out.println("Employee Address: " +emp.address); >> > package customArrayList3; import java.util.ArrayList; public class AddingEmployeeData < public static void main(String[] args) < Employee emp1 = new Employee(102, "Shubh", "Nagpur" ); Employee emp2 = new Employee(205, "Anjali", "Dhanbad"); Employee emp3 = new Employee(333, "Shanjna", "Mumbai"); ArrayListlist = new ArrayList(); list.add(emp1); list.add(emp2); list.add(emp3); Employee temp = new Employee(); // Call displayData method using temp reference variable object and pass list as a parameter. temp.displayData(list); > >
Output: Employee Detail Employee number: 102 Employee Name: Shubh Employee Address: Nagpur Employee number: 205 Employee Name: Anjali Employee Address: Dhanbad Employee number: 333 Employee Name: Shanna Employee Address: Mumbai
Hope that this tutorial has covered almost all important points related to store user-defined class objects in Java ArrayList. I hope that you will have understood and practiced all example programs.
Thanks for reading.