Lesson 4.5 -
The ArrayList Class
Purpose: To learn how to work with the ArrayList Class and its methods
Better Arrays So far, we have used what are called simple arrays. They are simple because they do not have built-in methods - they are not a class. Now we will look at a class that handles array structures in a much more elegant and powerful way - the ArrayList class. Because ArrayList is a class, it contains methods. You will find that these methods streamline many of the tasks involved with processing data in arrays. They are also intended to store objects, not just simple data types like ints and doubles. We will use them with String objects to begin with, then move on to Student Class objects and other pre-defined classes we have already worked with. Below is a list of the ArrayList methods we will use and their descriptions.
Method Summary |
void
|
add(int index, java.lang.Object obj)
Inserts the argument at the specified position in this list. |
boolean
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add(java.lang.Object obj)
Append (adds) the argument to the end of this ArrayList. |
boolean
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contains(java.lang.Object obj)
Returns true if this list contains an object equal to the object obj. |
java.lang.Object
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get(int index)
Returns the element at the specified position in this list. |
int
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indexOf(java.lang.Object obj)
Returns the first index that contains the element obj or -1 if obj is not in the ArrayList. |
java.lang.Object
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remove(int index)
Removes the element at the specified position from this list. |
java.lang.Object
|
set(int index, java.lang.Object obj)
Replaces the element at the specified position in this list with the specified object. |
int
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size()
Returns the number of elements in this list. |
Examples Here are some examples of using these methods
import java.util.ArrayList; // this goes at the top of the program
ArrayList<String> list = new ArrayList<String>(); // declares an empty ArrayList that can store String objects
list.add("Royal"); // adds the String "Royal" to end of list
// list = ["Royal"]
list.add("High"); // adds the String "High" to end of list
// list = ["Royal", "High"]
list.add("School"); // adds the String "School" to end of list
// list = ["Royal", "High", "School"]
list.add(1, "Valley"); // adds the String "Valley" to list at 1
// list = ["Royal", "Valley", "High", "School"]
int x = list.indexOf("School"); // returns index of "School"
// x = 3
list.remove(2); // removes the element at position 2
// list = ["Royal", "Valley", "School"]
list.set(2, "High"); // replaces element at pos 2 with "High"
// list = ["Royal", "Valley", "High"]
SOP(list.size()); // displays the size of list
// Outputs 3 to the screen
String word = list.get(1); //stores element 1 into word
// word = "Valley"
if(list.contains("Simi")) SOP("Yes!"); else SOP("No!");
// Displays "No!"
SOP(list); // displays the entire list
// Outputs ["Royal", "Valley", "High"] to the screen
In closing, ArrayLists store objects. So an item must be a class object to be stored in an ArrayList. |