Computer Science
Java Arraylist
Java ArrayList is a dynamic array that can grow or shrink in size as per the requirement. It is a part of the Java Collection Framework and provides various methods to perform operations on elements such as add, remove, search, etc. It is widely used in Java programming for storing and manipulating data.
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8 Key excerpts on "Java Arraylist"
• The ArrayList class supplies methods for common tasks, such as inserting and removing elements. In the following sections, you will learn how to work with array lists. An array list expands to hold as many elements as needed. Special Topic 6.5 An array list stores a sequence of values whose size can change. Michael Brake/iStockphoto. 302 Chapter 6 Arrays and Array Lists 6.8.1 Syntax 6.4 Array Lists ArrayList friends = new ArrayList(); The index must be ≥ 0 and < friends.size() . An array list object of size 0 Use the get and set methods to access an element. friends.add(Cindy); String name = friends.get(i); friends.set(i, Harry); Variable type Variable name The add method appends an element to the array list, increasing its size. To construct an array list: new ArrayList< typeName >() To access an element: arraylistReference .get(index) arraylistReference .set(index, value) Syntax Declaring and Using Array Lists The following statement declares an array list of strings: ArrayList names = new ArrayList(); The ArrayList class is contained in the java.util package. In order to use array lists in your program, you need to use the statement import java.util.ArrayList . The type ArrayList denotes an array list of String elements. The angle brackets around the String type tell you that String is a type parameter . You can replace String with any other class and get a different array list type. For that reason, ArrayList is called a generic class . However, you cannot use primitive types as type parameters—there is no ArrayList or ArrayList . Section 6.8.5 shows how you can collect numbers in an array list. It is a common error to forget the initialization: ArrayList names; names.add(Harry); // Error— names not initialized Here is the proper initialization: ArrayList names = new ArrayList() ; Note the () after new ArrayList on the right-hand side of the initialization.
eBook - PDF- Doug Lowe(Author)
- 2023(Publication Date)
- For Dummies(Publisher)
In other words, with generics you can spec- ify the type of data that can be stored in a collection and ensure that data of the wrong type can’t accidentally be put into the collection. Because generics are an integral part of how collections work, I incorporate the generics feature into this chapter from the very start. I point out the differences for using ArrayList without generics along the way, just in case you’re using an older version of Java or are working with programs that were written before Java 1.5 became available. (For a complete explanation of how the generics feature works, you can move ahead to Book 5, Chapter 5.) Understanding the ArrayList Class An array list is the most basic type of Java collection. You can think of an array list as being an array on steroids. It’s similar to an array but averts many of the most common problems of working with arrays, specifically the following: » An array list automatically resizes itself whenever necessary. If you create an array with 100 elements, and then fill it up and need to add a 101st element, you’re out of luck. The best you can do is create a new array with 101 elements, copy the 100 elements from the old array to the new one, and then put the new data in the 101st element. With an array list, there’s never a limit to how many elements you can create. You can keep adding elements as long as you want. » An array list lets you insert elements into the middle of the collection. With an array, inserting elements is pretty hard to do. Suppose that you have an array that can hold 100 elements, but only the first 50 have data. If you need to insert a new element after the 25th item, you first must make a copy of elements 26 through 50 to make room for the new element. With an array list, you just tell the array list you want to insert the new element after the 25th item; the array list takes care of shuffling things around.
eBook - PDFData Structures
Abstraction and Design Using Java
- Elliot B. Koffman, Paul A. T. Wolfgang(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
2.2 The List Interface and ArrayList Class 65 We briefly discuss the RandomAccess interface and the two abstract classes AbstractList and AbstractSequentialList in Section 2.10. The ArrayList Class The simplest class that implements the List interface is the ArrayList class. An ArrayList object is an improvement over an array object in that it supports all of the operations just listed. ArrayList objects are used most often when a programmer wants to be able to grow a list by adding new elements to the end but still needs the capability to access the elements stored in the list in arbitrary order. These are the features we would need for an e-mail address book application: New entries should be added at the end, and we would also need to find e-mail addresses for entries already in the address book. The size of an ArrayList automatically increases as new elements are added to it, and the size decreases as elements are removed. An ArrayList object has an instance method size that returns its current size. Each ArrayList object has a capacity, which is the number of elements it can store. If you add a new element to an ArrayList whose current size is equal to its capacity, the capacity is automatically increased. EXAMPLE 2.8 The statements List yourList; yourList = new ArrayList<>(); List myList = new ArrayList<>(); ‹‹interface›› RandomAccess ‹‹interface›› List ArrayList Vector AbstractList AbstractSequentialList Stack LinkedList F I G U R E 2 . 4 The java.util.List Interface and Its Imple- menters F O R P Y T H O N P R O G R A M M E R S The Python list class is similar to the Java Arraylist class. Both can store a collection of objects and both automatically expand when extra space is needed. Both have methods to add elements, insert elements, and get the list length. However, you cannot use array index notation (e.g., scores[3]) with an ArrayList but you can with a Python list.
eBook - PDF- Cay S. Horstmann(Author)
- 2014(Publication Date)
- Wiley(Publisher)
290 In many programs, you need to collect large numbers of values. In Java, you use the array and array list constructs for this purpose. Arrays have a more concise syntax, whereas array lists can automatically grow to any desired size. In this chapter, you will learn about arrays, array lists, and common algorithms for processing them. 6.1 Arrays We start this chapter by introducing the array data type. Arrays are the fundamental mechanism in Java for collecting multiple values. In the following sections, you will learn how to declare arrays and how to access array elements. 6.1.1 Declaring and Using Arrays Suppose you write a program that reads a sequence of values and prints out the sequence, marking the largest value, like this: 32 54 67.5 29 35 80 115 <= largest value 44.5 100 65 You do not know which value to mark as the largest one until you have seen them all. After all, the last value might be the largest one. Therefore, the program must first store all values before it can print them. Could you simply store each value in a separate variable? If you know that there are ten values, then you could store the values in ten variables value1, value2, value3, …, value10. However, such a sequence of variables is not very practical to use. You would have to write quite a bit of code ten times, once for each of the variables. In Java, an array is a much better choice for storing a sequence of values of the same type. Here we create an array that can hold ten values of type double: new double[10] The number of elements (here, 10) is called the length of the array. The new operator constructs the array. You will want to store the array in a variable so that you can access it later. The type of an array variable is the type of the element to be stored, followed by []. In this example, the type is double[], because the element type is double.
eBook - PDF- Michael T. Goodrich, Roberto Tamassia, Michael H. Goldwasser(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
In reality, elements of an ArrayList are stored in a traditional array, and the precise size of that traditional array must be internally declared in order for the system to properly allocate a consecutive piece of memory for its storage. For example, Figure 7.2 displays an array with 12 cells that might be stored in memory locations 2146 through 2157 on a computer system. 2160 2145 2146 2147 2148 2149 2150 2151 2152 2153 2154 2155 2156 2157 2158 2144 2159 Figure 7.2: An array of 12 cells, allocated in memory locations 2146 through 2157. Because the system may allocate neighboring memory locations to store other data, the capacity of an array cannot be increased by expanding into subsequent cells. The first key to providing the semantics of an unbounded array is that an array list instance maintains an internal array that often has greater capacity than the current length of the list. For example, while a user may have created a list with five elements, the system may have reserved an underlying array capable of storing eight object references (rather than only five). This extra capacity makes it easy to add a new element to the end of the list by using the next available cell of the array. If a user continues to add elements to a list, all reserved capacity in the underly- ing array will eventually be exhausted. In that case, the class requests a new, larger array from the system, and copies all references from the smaller array into the beginning of the new array. At that point in time, the old array is no longer needed, so it can be reclaimed by the system. Intuitively, this strategy is much like that of the hermit crab, which moves into a larger shell when it outgrows its previous one. 244 Chapter 7. List Abstractions 7.2.2 Implementing a Dynamic Array We now demonstrate how our original version of the ArrayList, from Code Frag- ments 7.2 and 7.3, can be transformed to a dynamic-array implementation, having unbounded capacity.
eBook - PDFJava Concepts
Late Objects
- Cay S. Horstmann(Author)
- 2016(Publication Date)
- Wiley(Publisher)
In this chapter, you will learn about arrays, array lists, and common algorithms for processing them. 6.1 Arrays We start this chapter by introducing the array data type. Arrays are the fundamental mechanism in Java for collecting multiple values. In the following sections, you will learn how to declare arrays and how to access array elements. 6.1.1 Declaring and Using Arrays Suppose you write a program that reads a sequence of values and prints out the sequence, marking the largest value, like this: 32 54 67.5 29 35 80 115 <= largest value 44.5 100 65 You do not know which value to mark as the largest one until you have seen them all. After all, the last value might be the largest one. Therefore, the program must first store all values before it can print them. Could you simply store each value in a separate variable? If you know that there are ten values, then you could store the values in ten variables value1, value2, value3, …, value10. However, such a sequence of variables is not very practical to use. You would have to write quite a bit of code ten times, once for each of the variables. In Java, an array is a much better choice for storing a sequence of values of the same type. Here we create an array that can hold ten values of type double: new double[10] The number of elements (here, 10) is called the length of the array. The new operator constructs the array. You will want to store the array in a variable so that you can access it later. The type of an array variable is the type of the element to be stored, followed by []. In this example, the type is double[], because the element type is double. Here is the declaration of an array variable of type double[] (see Figure 1): double[] values; 1 When you declare an array variable, it is not yet initialized. You need to initialize the variable with the array: double[] values = new double[10]; 2 An array collects a sequence of values of the same type. © traveler1116/iStockphoto.
eBook - PDFBrief Java
Early Objects
- Cay S. Horstmann(Author)
- 2019(Publication Date)
- Wiley(Publisher)
In this chapter, you will learn about the Java collections framework, a hierarchy of interface types and classes for collecting objects. Each interface type is implemented by one or more classes (see Figure 1). At the root of the hierarchy is the Collection interface. That interface has methods for adding and removing elements, and so on. Table 1 on page 514 shows all the methods. Because all collections implement this interface, its methods are available for all collection classes. For example, the size method reports the number of ele- ments in any collection. The List interface describes an important category of collections. In Java, a list is a collection that remembers the order of its elements (see Figure 2). The ArrayList class implements the List interface. An ArrayList is simply a class containing an array that is expanded as needed. If you are not concerned about efficiency, you can use the Array- List class whenever you need to collect objects. However, several common opera- tions are inefficient with array lists. In particular, if an element is added or removed, the elements at larger positions must be moved. The Java library supplies another class, LinkedList, that also implements the List interface. Unlike an array list, a linked list allows efficient insertion and removal of elements in the middle of the list. We will discuss that class in the next section. A collection groups together elements and allows them to be retrieved later. ‹‹interface›› Map HashMap TreeMap ‹‹interface›› Collection HashSet TreeSet Stack LinkedList ‹‹interface›› List ‹‹interface›› Queue ‹‹interface›› Set ArrayList PriorityQueue Figure 1 Interfaces and Classes in the Java Collections Framework 15.1 An Overview of the Collections Framework 513 © Vladimir Trenin/iStockphoto. Figure 4 A Stack of Books You use a list whenever you want to retain the order that you established. For example, on your bookshelf, you may order books by topic.
No longer available |Learn moreThe Java Workshop
A Practical, No-Nonsense Introduction to Java Development
- David Cuartielles, Andreas Göransson, Eric Foster-Johnson(Authors)
- 2019(Publication Date)
- Packt Publishing(Publisher)
4. Collections, Lists and Java's Built-In APIs
OverviewThis chapter introduces you to the powerful Java collections framework, which is used to store, sort, and filter data. It will first take you through the structure of the built-in Collections Application Programming Interface (API ), the Java collections framework, which will simplify your dealings with complex data structures and allow you to use and create APIs with minimal effort. Through this framework, you will examine the relationship between lists and arrays, and learn to populate lists from arrays. Finally, in this chapter's final activity, you will create and complete a program in which you will be asked to perform standard operations on data stored in sets, lists, and maps in preparation for future chapters.Introduction
Java comes with a built-in Collections API, allowing you to manipulate data structures with very little effort. A collection is an object that contains multiple elements. Collections are used to store, share, process, and communicate aggregated data. We call this system the Java collections framework .As part of this framework, there are different components that are used to optimize our interaction with the actual data:- Interfaces : Abstract data types that represent collections
- Implementations : Specific implementations of the collection interfaces
- Algorithms : Polymorphic methods used to process the data within a collection for operations such as sorting and searchingNote Other programming languages have their own collection frameworks. For example, C++ has the Standard Template Library (STL
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
