Interface List
An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.
Unlike sets, lists typically allow duplicate elements. More formally, lists typically allow pairs of elements e1 and e2 such that e1.equals(e2) , and they typically allow multiple null elements if they allow null elements at all. It is not inconceivable that someone might wish to implement a list that prohibits duplicates, by throwing runtime exceptions when the user attempts to insert them, but we expect this usage to be rare.
The List interface places additional stipulations, beyond those specified in the Collection interface, on the contracts of the iterator , add , remove , equals , and hashCode methods. Declarations for other inherited methods are also included here for convenience.
The List interface provides four methods for positional (indexed) access to list elements. Lists (like Java arrays) are zero based. Note that these operations may execute in time proportional to the index value for some implementations (the LinkedList class, for example). Thus, iterating over the elements in a list is typically preferable to indexing through it if the caller does not know the implementation.
The List interface provides a special iterator, called a ListIterator , that allows element insertion and replacement, and bidirectional access in addition to the normal operations that the Iterator interface provides. A method is provided to obtain a list iterator that starts at a specified position in the list.
The List interface provides two methods to search for a specified object. From a performance standpoint, these methods should be used with caution. In many implementations they will perform costly linear searches.
The List interface provides two methods to efficiently insert and remove multiple elements at an arbitrary point in the list.
Note: While it is permissible for lists to contain themselves as elements, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a list.
Some list implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException . Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the list may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as «optional» in the specification for this interface.
Unmodifiable Lists
- They are unmodifiable. Elements cannot be added, removed, or replaced. Calling any mutator method on the List will always cause UnsupportedOperationException to be thrown. However, if the contained elements are themselves mutable, this may cause the List’s contents to appear to change.
- They disallow null elements. Attempts to create them with null elements result in NullPointerException .
- They are serializable if all elements are serializable.
- The order of elements in the list is the same as the order of the provided arguments, or of the elements in the provided array.
- The lists and their subList views implement the RandomAccess interface.
- They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
- They are serialized as specified on the Serialized Form page.
This interface is a member of the Java Collections Framework.
For (Object object : list) [java] and index element
This tutorial article will explain how we can create multiple objects inside another object in JavaScript. A JavaScript Object is a collection of Key-Value pairs, and nested objects are objects that have other objects inside them as their property. JavaScript Object TypeScript has a specific syntax for typing objects.
For (Object object : list) [java] and index element
Is there a way to get an element id of a list to get it later through list.get(index)
Only define a local var and manually incrementing it? Anything simpler?
No, for-each loop doesn’t keep track of index. You can either use a regular indexed loop, or do something like this:
int idx = 0; for (Object o : list)
This is risky since break / continue will make idx goes out of sync, so do use this infrequently, and only when the body is simple and only a few lines long.
If the elements are distinct, List.indexOf would also work, albeit at O(N) , and at that point you may want to consider a Set (unordered, but guaranteed distinct).
It should also be said that sometimes using a listIterator() also alleviates the need of an explicit index while iterating.
A ListIterator supports add , set and remove operations.
This is another clear advantage List has over arrays as far as iteration mechanism goes.
The best I can think of is to combine iteration and indexing as follows:
int idx = 0; for (Iterator it = list.iterator(); it.hasNext(); idx++) < Object o = it.next(); // . >This approach has the advantage that break and continue won’t mess up the idx calculations.
On the other hand, if the list is an ArrayList and you want to keep track of the index, you are probably better just using a variation of
This question is not worded very well, but from what I can tell you would like to find the position of a specific Object within a List , and then retrieve it later via that index?
First of all, if you know the Object you are looking for then you should not need to have to find it in the List , since you have it already.
That said, you could easily do something like this:
int index = -1; for (int i = 0; i < list.size(); i++) < if (list.get(i).equals(myObject)) < index = i; break; >> But I would take a second look at your application to determine whether this is really something that is necessary for you to do.
The documentation is here
@Eugene I see you mention in a comment to another answer that you plan to store the index of an object (with in the object in question?), that is probably not a good idea, be very careful since an objects index can change.
Array of Objects in C++ with Examples, Syntax: ClassName ObjectName [number of objects]; The Array of Objects stores objects. An array of a class type is also known as an array of objects. Example#1: Storing more than one Employee data. Let’s assume there is an array of objects for storing employee data emp [50]. Below is the C++ …
Object Inside an Object in JavaScript
This tutorial article will explain how we can create multiple objects inside another object in JavaScript.
A JavaScript Object is a collection of Key-Value pairs, and nested objects are objects that have other objects inside them as their property. Nesting is a widely used practice in programming as it provides the code with more enhanced details.
Each object has its own properties or keys, which are used to store the details of the object. A comma separates each property or key.
Let’s have a look at how we create an object and properties in Javascript.
Here, if you can see, const layer0 is the name of the object. This is how we create an object in Javascript. Now within the object, as you can see, layer1Item1 , layer1Item2 , layer1Item3 and layer1Item4 are the properties of the object. These properties must be unique and the differentiating factors that distinguish an object from another.
Now, if you want to create an object within another object, the inner object is created as a property of the outer one, and this inner object can only be accessed using the outer object .
Create Nested Objects in JavaScript
In this example, layer1Item2 is a new object inside of another object. However, layer1Item1 is a property of the object, and layer1Item2 is an object. Both look similar because the newly created object is also created as a property of the outer object layer0 .
Now, if you want to access the inner object, you will write the name of the outer object, and after a dot, you will write the name of the inner object.
And if you want to access some property within the inner object, the code will be:
layer0.layer1Item2.layer2Item1 Multiple Nesting in Javascript
There is no limit of nesting in Javascript. You can make n number of hierarchies. The method of accessing the objects would be the same for accessing the inner ones, as discussed in the previous example.
In this example, there are 3 objects created, layer0 , layer1 , and layer2 . The layer2 object is inside layer1 , and layer1 is inside layer0 . There are 3 layers or hierarchies in this example. Now, layer2 is written as a property of layer1 .
To access the properties of the layer2 , we will write the following code.
layer0.layer1Item2.layer2Item1.layer3Item1 Now, to add a new property to an object in the following code:
we will simply attach the object name with the object property with a dot and assign it a value like below:
layer0.layer1Item5 = "New Item created"; Now the resultant object will look like the following:
If you want to delete any property from the object, it is straightforward to understand. We use the delete keyword alongside the object name attached with the object property, separated with a dot you want to delete. Look at the following example:
Consider we have the following object.
And we want to delete the layer1Item4 property; we will write the following code.
Now the object will look like as below.
JavaScript Object
How to Destructure Objects in JavaScript, How to Destructure An Object in JavaScript. Instead of writing out const food = pet.food, we can utilize destructuring and simply write: const < food >= pet; Now if we print out food again, we can see we’ve created the food variable: console.log (food); // output: kibble. Using destructuring, our code is now more …
TypeScript Object Types
TypeScript has a specific syntax for typing objects.
Read more about objects in our JavaScript Objects chapter.
Example
Object types like this can also be written separately, and even be reused, look at interfaces for more details.
Type Inference
TypeScript can infer the types of properties based on their values.
Example
const car = <
type: «Toyota»,
>;
car.type = «Ford»; // no error
car.type = 2; // Error: Type ‘number’ is not assignable to type ‘string’.
Optional Properties
Optional Properties are properties that don’t have to be defined in the object definition.