I want to know if there are more efficient way to do this.
I am trying to call List from the List
public class GetL{
public List getList(){
List mainList = new ArrayList();
List subList = new ArrayList();
List subList2 = new ArrayList();
subList.add("one");
subList.add("two");
mainList.add(subList);
subList2.add("three");
subList2.add("four");
mainList.add(subList2);
return mainList;
}
}
and I would call it like this.
GetL getL = new GetL();
List tempList = getL.getList();
tempList.get(0);
tempList.get(1);
Is there a more efficient way to achieve the same result?
One of the biggest reason that I don't like this way is that I have to create so many different "List"
edit:
The biggest problem for me is that I am trying to query data from DB (ie id, age, name)and it can be well over 100 sets. And I cannot dynamically declare many List and store into "mainList"
My original design was using while loop and clear the list.
while( something ){
subList.add(something);
subList.add(something2);
mainList.add(subList);
subList.clear();
}
I didn't know clear() list also clear the stored value in mainList...
You could do something like this:
/**
* Returns a list of lists containing Foo objects.
*/
public List<List<Foo>> getListOfLists() {
ResultSet rs = // get data from database...
List<List<Foo>> mainList = new ArrayList<List<Foo>>();
for (Row r : rs) {
List<Foo> subList = new ArrayList<Foo>();
for (Foo f : r.getFooList()) {
subList.add(f);
}
mainList.add(subList);
}
return mainList;
}
The above code assumes that your database query returns a ResultSet.
It also assumes that each row from the ResultSet has a method that returns a list of Foo objects.
Each sub-list contains objects of type Foo (which you can replace with whatever class you're using).
For a more helpful answer, please try to share some example code that shows what you're trying to do.
Right now we can only guess at what you're after...
Related
I need to use an array list as I don't know how many rows I will need but I know I'll need 2 columns. I'm unsure of how to create such an array list, add to both columns and read data from both columns. Both columns will contain integers.
I have seen some suggest:
ArrayList<Arraylist<Integer>> name = new ArrayList<ArrayList<Integer>>();
but I can find an explanation of how to add to both columns.
I've also seen:
ArrayList<Integer[][]> name = new ArrayList<Integer[][]>();
and different variations of where and the number of square brackets.
Thanks.
Java is Object Oriented language, so why not create ArrayList<Column> ?
You can create a class Column which will cover your requirements: it can have setters and getters, and if you need to support other types other than Integer you can generify it. For example:
class Column<T> {
private T value;
public Column(T value) {
this.value = value;
}
public getValue() {
return this.value;
}
}
Then you declare:
List<Column<Integer>> list = new LinkedList<>();
list.add(new Column<Integer>(5));
System.out.println(list.get(0).getValue())
Example how create two dimension structure use lists like you to do:
List<List<Integer>> names = new ArrayList<>();
List<Integer> row = new ArrayList<>();
row.add(1); // first column
row.add(2); // second column
names.add(row); // add row with column
System.out.println(names.get(0).get(0)); // get first column from first row
System.out.println(names.get(0).get(1)); // get second column form first row
But best way is use Custom object like this:
class CustomRow {
private int col1;
private int col2;
// getters and setters
}
List<CustomRow> tables;
CustomRow cr = new CustomRow();
cr.setCol1(1);
cr.setCol2(2);
tables.add(cr);
Something like this:
public MyObject {
Integer integer1;
Integer integer2;
}
List<MyObject> myObjList = new ArrayList<>();
MyObject mo = new MyObject(){
...
myObjList.add(mo);
You could try
a Map and use the key and the value to hold values
a List of tuples
a List of Lists as you suggested
You didn't give enough information as to what you actually want to do, but if you have to use a List as a base, I'd typically go with a List of custom Tuple objects, each holding two values.
You can try creating a simple POJO class called Row and have two variables as column1 and column2. Then add this Row object to your list.
You basically need to create an ArrayList that holds an ArrayList of type Integer. You can then add two of these ArrayLists into the main array list.
List<List<Integer>> myList = new ArrayList<>();
List<Integer> x = new ArrayList<>();
x.add(5);
x.add(6);
List<Integer> y = new ArrayList<>();
y.add(5);
y.add(6);
myList.add(x);
myList.add(y);
Based off this answer:
How do I declare a 2D String arraylist?
I have the following objects in an ArrayList and a value in this object is illustrated as the numbers at the beginning, which are Id of something.
I need to create a Map object, whose key should be the id of objects and whose values should be the objects with id. At the end of the day, I would like to have a Map something like that.
I have already solved this problem with two for loops and lots of if statements but it seems very ugly to me.
Any cleaner solution would be appreciated.
Map<Integer, List<Foo>> result = list.stream().collect(Collectors.groupingBy(Foo::getId));
Edited to fit the question edit, it is as simple as this:
ArrayList<Element> list = ...;
HashMap<Integer, List<Element>> map = new HashMap<Integer, List<Element>>();
for(Element e : list) {
ArrayList<Element> auxList;
if(map.contains(e.getId()) {
auxList = map.get(e.getId());
} else {
auxList = new ArrayList<Element>();
map.put(e.getId(), auxList);
}
auxList.add(e);
}
Just iterate over the starting list and add the elements to the map. If the map already contains the id, add it to the list. If not, create a new list.
I'm having a config entry, from which I'm loading into an String array like
String s = "abc$#def$#ghi";
String[] scbHLNewArray = s.split("\\$\\#");
Here I'm comparing a string with the array values after splitting it like ,
for(String arrNewErrorInfo : scbHLNewArray) {
LOG.info("SCB HL New Error Value :"+arrNewErrorInfo+"\n");
if(errorInfo.equals(arrNewErrorInfo)) {
LOG.info("SCB HL Matched New value is :"+arrNewErrorInfo);
newState = ApplicationState.NEW;
addApplicationEvent(application.getId(),comment, ApplicationEventType.COMMENT,BBConstants.AUTOBOT);
scbHLNewStatus = "Matched";
break;
}
}
I want to use some util classes like List.. Any idea on append to list and compare the string with the list objecT?
Thanks,
Nizam
you can do this with List contains method.
ArrayList<Integer> arrlist = new ArrayList<Integer<(8);
// use add() method to add elements in the list
arrlist.add(20);
arrlist.add(25);
arrlist.add(10);
arrlist.add(15);
// list contains element 10
boolean retval = arrlist.contains(10); // It will return true.
Ok, let's try... First of all, you can create a List Object, wrapping your array very easily:
List<String> myList = Arrays.asList( scbHLNewArray );
Be carefull, because you can NOT add to this list, as it only wraps your array. If you want a list you can add to, you would have to create a new one, for example:
List<String> myModifiableList = new ArrayList<String>( myList );
This will create a new List that contains all the Strings from the first one but is also modifiable (you can add Strings, if you want).
In any case, you can use "contains", as Pratik has already shown, to test if a String is inside your list:
if (myList.contains("someString")) { ... }
This works because the String class already has well implemented equals(...) and hashCode() methods. If you want to put other Object than Strings into your list, you would have to make sure that these methods are implemented well, otherwise contains might not work as expected.
Yes you can use a list of course, you need to :
1. Take the result of split as an array.
2. Then convert this array to a list.
String s = "abc$#def$#ghi";
String[] scbHLNewArray = s.split("\\$\\#");
List<String> list=Arrays.asList(scbHLNewArray); //convert the array to a list
Take a look at Arrays.asList(Array a) and this Tutorial for further information about it.
And then to search the wanted String object you can use indexOf(Object o) or contains(Object o) List methods
I have some data structures, and I would like to use one as a temporary, and another as not temporary.
ArrayList<Object> myObject = new ArrayList<Object>();
ArrayList<Object> myTempObject = new ArrayList<Object>();
//fill myTempObject here
....
//make myObject contain the same values as myTempObject
myObject = myTempObject;
//free up memory by clearing myTempObject
myTempObject.clear();
now the problem with this of course is that myObject is really just pointing to myTempObject, and so once myTempObject is cleared, so is myObject.
How do I retain the values from myTempObject in myObject using java?
You can use such trick:
myObject = new ArrayList<Object>(myTempObject);
or use
myObject = (ArrayList<Object>)myTempObject.clone();
You can get some information about clone() method here
But you should remember, that all these ways will give you a copy of your List, not all of its elements. So if you change one of the elements in your copied List, it will also be changed in your original List.
originalArrayList.addAll(copyArrayList);
Please Note: When using the addAll() method to copy, the contents of both the array lists (originalArrayList and copyArrayList) refer to the same objects or contents. So if you modify any one of them the other will also reflect the same change.
If you don't wan't this then you need to copy each element from the originalArrayList to the copyArrayList, like using a for or while loop.
There are no implicit copies made in java via the assignment operator. Variables contain a reference value (pointer) and when you use = you're only coping that value.
In order to preserve the contents of myTempObject you would need to make a copy of it.
This can be done by creating a new ArrayList using the constructor that takes another ArrayList:
ArrayList<Object> myObject = new ArrayList<Object>(myTempObject);
Edit: As Bohemian points out in the comments below, is this what you're asking? By doing the above, both ArrayLists (myTempObject and myObject) would contain references to the same objects. If you actually want a new list that contains new copies of the objects contained in myTempObject then you would need to make a copy of each individual object in the original ArrayList
Came across this while facing the same issue myself.
Saying arraylist1 = arraylist2 sets them both to point at the same place so if you alter either the data alters and thus both lists always stay the same.
To copy values into an independent list I just used foreach to copy the contents:
ArrayList list1 = new ArrayList();
ArrayList list2 = new ArrayList();
fill list1 in whatever way you currently are.
foreach(<type> obj in list1)
{
list2.Add(obj);
}
Supopose you want to copy oldList into a new ArrayList object called newList
ArrayList<Object> newList = new ArrayList<>() ;
for (int i = 0 ; i<oldList.size();i++){
newList.add(oldList.get(i)) ;
}
These two lists are indepedant, changes to one are not reflected to the other one.
Lets try the example
ArrayList<String> firstArrayList = new ArrayList<>();
firstArrayList.add("One");
firstArrayList.add("Two");
firstArrayList.add("Three");
firstArrayList.add("Four");
firstArrayList.add("Five");
firstArrayList.add("Six");
//copy array list content into another array list
ArrayList<String> secondArrayList=new ArrayList<>();
secondArrayList.addAll(firstArrayList);
//print all the content of array list
Iterator itr = secondArrayList.iterator();
while (itr.hasNext()) {
System.out.println(itr.next());
}
In print output as below
One
Two
Three
Four
Five
Six
We can also do by using clone() method for which is used to create exact copy
for that try you can try as like
**ArrayList<String>secondArrayList = (ArrayList<String>) firstArrayList.clone();**
And then print by using iterator
**Iterator itr = secondArrayList.iterator();
while (itr.hasNext()) {
System.out.println(itr.next());
}**
You need to clone() the individual object. Constructor and other methods perform shallow copy. You may try Collections.copy method.
Straightforward way to make deep copy of original list is to add all element from one list to another list.
ArrayList<Object> originalList = new ArrayList<Object>();
ArrayList<Object> duplicateList = new ArrayList<Object>();
for(Object o : originalList) {
duplicateList.add(o);
}
Now If you make any changes to originalList it will not impact duplicateList.
to copy one list into the other list, u can use the method called
Collection.copy(myObject myTempObject).now after executing these line of code u can see all the list values in the myObject.
Copy of one list into second is quite simple , you can do that as below:-
ArrayList<List1> list1= new ArrayList<>();
ArrayList<List1> list2= new ArrayList<>();
//this will your copy your list1 into list2
list2.addAll(list1);
Here is a workaround to copy all the objects from one arrayList to another:
ArrayList<Object> myObject = new ArrayList<Object>();
ArrayList<Object> myTempObject = new ArrayList<Object>();
myObject.addAll(myTempObject.subList(0, myTempObject.size()));
subList is intended to return a List with a range of data. so you can copy the whole arrayList or part of it.
Suppose you have two arraylist of String type .
Like
ArrayList<String> firstArrayList ;//This array list is not having any data.
ArrayList<String> secondArrayList = new ArrayList<>();//Having some data.
Now we have to copy the data of second array to first arraylist like this,
firstArrayList = new ArrayList<>(secondArrayList );
Done!!
The simplest way is:
ArrayList<Object> myObject = new ArrayList<Object>();
// fill up data here
ArrayList<Object> myTempObject = new ArrayList(myObject);
This is a simple question but if I do
List<Object> list = getObjectsFromDatabase();
This would not be the correct way to handle this?
But this would?
List<Object> firstList = getObjectsFromDatabase();
List<Object> list = new ArrayList<Object>(firstList);
Or if I had a class
public class ReportDisplayModel<T> {
public ReportDisplayModel(List<T> data) {
this.data = data;
}
public List<T> data;
}
And I wanted to set the data in this model I would use the constructor?
ReportDisplayModel<Object> model = new ReportDisplayModel<Object>(getData());
Instead of
ReportDisplayModel<Object> model = new ReportDisplayModel<Object>();
model.data = getData();
Just need a clarification. Thanks.
It depends entirely on what getData() returns.
usually it is made to return Collections.unmodifiableList(result) so that clients can't modify the result.
if this result is not used anywhere else, and modifications to it doesn't mess with anything, it is fine to use the result as-is
It is rarely needed to use the copy constructor - use it when you are sure that modifying the data will impact some other component.
Regarding
List<Object> list = getObjectsFromDatabase();
vs
List<Object> firstList = getObjectsFromDatabase();
List<Object> list = new ArrayList<Object>(firstList);
either approach is fine. Depends on if you want list to refer to the list returned by getObjectsFromDatabase() or if you want it to refer to a copy of it.
If simply want to, say, print the database objects, the first approach is fine.
If you want to, say, filter out half of the database objects (i.e., remove objects from the list), and you can't say for sure that getObjectsFromDatabase() returns a mutable list, then you'll have to go with the second approach.
Regarding
ReportDisplayModel<Object> model = new ReportDisplayModel<Object>(getData());
vs
ReportDisplayModel<Object> model = new ReportDisplayModel<Object>();
model.data = getData();
I'd prefer the first method. Simply because I wouldn't want to worry about null pointer exceptions etc if I accidentally do something like
ReportDisplayModel<Object> model = new ReportDisplayModel<Object>();
model.printData();
model.data = getData();
I don't quite get your question, but I'll give it a try.
The main difference is that using the copy constructor creates a new independent copy of the list, i.e.
List<Object> firstList = getObjectsFromDatabase(); // firstList is the list returned by the database
List<Object> list = new ArrayList<Object>(firstList); //list is an independent copy of firstList
Now if you change firstList the list returned by getObjectsFromDatabase() would be changed as well (or would throw an exception if changes are not supported). On the other hand list could freely be changed without the original list being affected.
Avoid using the equal sign, because it breaks encapsulation (bad practice). Go for the copy constructor (best practice).