How to Update Items in a List With Java
One of the powerful features of the Java programming language is its implementation of many powerful and common data structures (besides the ordinary array), as part of its built-in package of classes. One of these data structures is the collection of classes that derive from the List interface. Unlike ordinary arrays, lists can be efficiently and easily resized to accommodate data sets that regularly change size. However, it is not without its disadvantages: one is that, once a list is built, it can be difficult to modify the items; simply retrieving the items from the list and changing them is ineffective. The retrieval of items creates a new instance of the item, so the original in the list does not reflect changes.
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Instructions
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1
Open your favorite text editor. Any will do, from Windows Notepad to the Netbeans Java development environment. Paste the following skeleton code to get started:
import java.util.List;
import java.util.ArrayList;
public class SimpleListTutorial {
public static void main(String[] args) {
}
}
All the code for this tutorial will go within the "main" brackets.
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Create a list of strings by using the following code:
List<String> l = new ArrayList<String>();
l.add("First item");
l.add("Second item");
l.add("Fourth item");
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Add an item to the middle of the list with the following code:
l.add(2, "Third item");
This adds the string "Third item" to the third place in the list. Remember, list indexes, like all counting in Java, starts at 0, rather than 1.
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4
Modify an item in a list into something else. Use the "set" method:
l.set(2, "A new third item");
This will completely replace the item in the third slot with the new item given. It is the same as the following code:
l.remove(2);
l.add(2, "A new third item.");
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Print the current list to ensure it worked:
for(String s : l) {
System.out.println(s);
}
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References
- Photo Credit Comstock Images/Comstock/Getty Images
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Comments
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Isn't the same code and has not the same effect... l.set(2, "A new third item") replace the item in third position without remove element of te list. The following will remove item and add another one. l.remove(2); l.add(2, "A new third item."); The big different is that the list size will be modify and so you cannot execute the substitution in a implict for. for(String s : list){ l.remove(2); l.add(2, "A new third item."); } will throw exception...
