Transcript ArrayList names = new ArrayList

7.2 Array Lists
• ArrayList class manages a sequence of objects
• Can grow and shrink as needed
• ArrayList class supplies methods for many common tasks,
such as inserting and removing elements
• ArrayList is a generic class:
ArrayList<T>
collects objects of type parameter T:
ArrayList<String> names = new ArrayList<String>();
OR
ArrayList<String> names = new ArrayList<>();
names.add("Emily");
names.add("Bob");
names.add("Cindy");
• size method yields number of elements
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Adding Elements
To add an object to the end of the array list, use the add
method:
names.add("Emily");
names.add("Bob");
names.add("Cindy");
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Retrieving Array List Elements
• To obtain the value of an element at an index, use the get
method
• Index starts at 0
• String newName = names.get(2);
// gets the third element of the array list
• Bounds error if index is out of range
• Most common bounds error:
int i = names.size();
name = names.get(i); // Error
Big Java by Cay Horstmann
// legal index values are 0 ...
i-1
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Setting Elements
• To set an element to a new value, use the set method:
names.set(2, "Carolyn");
Removing Elements
• To remove an element at an index, use the remove method:
names.remove(1);
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Adding and Removing Elements
names.add("Emily");
names.add("Bob");
names.add("Cindy");
names.set(2, "Carolyn");
names.add(1, "Ann");
names.remove(1);
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Working with Array Lists
ArrayList<String> names =
new ArrayList<String>();
Constructs an empty array list that can hold
strings.
names.add("Ann");
names.add("Cindy");
Adds elements to the end.
System.out.println(names);
Prints [Ann, Cindy].
names.add(1, "Bob");
Inserts an element at index 1. names is now
[Ann, Bob, Cindy].
names.remove(0);
Removes the element at index 0. names is
now [Bob, Cindy].
names.set(0, "Bill");
Replaces an element with a different value.
names is now [Bill, Cindy].
ArrayList<String> names = new ArrayList< >([“Jimmy”, “Jason”]);
String firstName = names[ i ]; equivalent to String firstName = names.get(i);
names[ i ] = “Joey”; equivalent to names.set( i, “Joey”);
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Working with Array Lists (cont.)
String name = names.get(i);
Gets an element.
String last =
names.get(names.size() - 1);
Gets the last element.
ArrayList<Integer> squares =
new ArrayList<Integer>();
for (int i = 0; i < 10; i++) {
squares.add(i * i);
}
Constructs an array list holding the first ten
squares.
Note: ArrayList<Integer> squares = new ArrayList<Integer>();
creates a list of integer OBJECTS not primitive type.
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Syntax 7.2 Array Lists
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
ch07/arraylist/ArrayListTester.java
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import java.util.ArrayList;
/**
This program tests the ArrayList class.
*/
public class ArrayListTester
{
public static void main(String[] args)
{
ArrayList<BankAccount> accounts = new ArrayList<BankAccount>();
accounts.add(new BankAccount(1001));
accounts.add(new BankAccount(1015));
accounts.add(new BankAccount(1729));
accounts.add(1, new BankAccount(1008));
accounts.remove(0);
System.out.println("Size: " + accounts.size());
System.out.println("Expected: 3");
BankAccount first = accounts.get(0);
System.out.println("First account number: "
+ first.getAccountNumber());
System.out.println("Expected: 1008");
BankAccount last = accounts.get(accounts.size() - 1);
System.out.println("Last account number: "
+ last.getAccountNumber());
System.out.println("Expected: 1729");
}
}
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
ch07/arraylist/BankAccount.java
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/**
A bank account has a balance that can be changed by
deposits and withdrawals.
*/
public class BankAccount {
private int accountNumber;
private double balance;
/**
Constructs a bank account with a zero balance.
@param anAccountNumber the account number for this account
*/
public BankAccount(int anAccountNumber) {
accountNumber = anAccountNumber;
balance = 0;
}
Continued
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
ch07/arraylist/BankAccount.java (cont.)
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/**
Constructs a bank account with a given balance
@param anAccountNumber the account number for this account
@param initialBalance the initial balance
*/
public BankAccount(int anAccountNumber, double initialBalance) {
accountNumber = anAccountNumber;
balance = initialBalance;
}
/**
Gets the account number of this bank account.
@return the account number
*/
public int getAccountNumber() {
return accountNumber;
}
Continued
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
ch07/arraylist/BankAccount.java (cont.)
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/**
Deposits money into the bank account.
@param amount the amount to deposit
*/
public void deposit(double amount) {
double newBalance = balance + amount;
balance = newBalance;
}
/**
Withdraws money from the bank account.
@param amount the amount to withdraw
*/
public void withdraw(double amount) {
double newBalance = balance - amount;
balance = newBalance;
}
Continued
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
ch07/arraylist/BankAccount.java (cont.)
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/**
Gets the current balance of the bank account.
@return the current balance
*/
public double getBalance() {
return balance;
}
}
Program Run:
Size: 3
Expected: 3
First account number: 1008
Expected: 1008
Last account number: 1729
Expected: 1729
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Self Check 7.3
How do you construct an array of 10 strings? An array list of
strings?
Answer:
String[] arraystr = new String[10];
ArrayList<String> arrayliststr = new ArrayList<String>();
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Self Check 7.4
What is the content of names after the following statements?
ArrayList<String> names = new ArrayList<String>();
names.add("A");
names.add(0, "B");
names.add("C");
names.remove(1);
Answer: names contains the strings "B" and "C" at
positions 0 and 1
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
The Enhanced for Loop for Array Lists
• Works for ArrayLists too:
ArrayList<BankAccount> accounts = ...;
double sum = 0;
for (BankAccount account : accounts) {
sum = sum + account.getBalance();
}
• Equivalent to the following ordinary for loop:
double sum = 0;
for (int i = 0; i < accounts.size(); i++) {
BankAccount account = accounts.get(i);
sum = sum + account.getBalance();
}
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.