Java pop from array

Java pop last item array java code example

Below programs illustrate the Java.util.ArrayDeque.pop() method: Program 1:Output: Program 2: Output: how to get last element of array java A list is an ordered collection that allows us to store and access elements sequentially.

How to pop items from a collection in Java?

If you’re looking for a stack-like structure I suggest accepting a Deque ( LinkedList is the most common implementation) instead of a Collection .

If you don’t actually need to treat it as a stack, just get an iterator from the Collection and use the remove() method:

for (Iterator it = elements.iterator(); it.hasNext(); )

Do note that remove is an optional operation, and some implementations may throw an UnsupportedOperationException (for example, the iterator returned by a Collection from Collections.unmodifiable. () will).

Edit : After looking more closely at your question, I think you just need this:

elements.removeAll(elementsToRemove); 

If your main point is you need to know exactly which elements were actually popped, I think you’re stuck with your original code.

There is no such method in the standard JDK-provided methods. Apache Commons provides the ListUtils.subtract() method.

Edit: As other answerers have noted, your use of the term pop is nonstandard. Usually,

The pop operation removes an item from the top of [a stack]

Wikipedia has a nice description of stacks.

I guess no, because you definition of ‘pop’ operation is highly non-standard. Usually it takes no arguments (except collection itself) and returns and removes the top-most one.

But once you noted apache commons, this would achieve the same effect as your code.

Collection result = CollectionUtils.intersection(a, b); a.removeAll(b); 

edit
http://commons.apache.org/collections/api-release/index.html

JavaScript Array pop() Method, If the array is empty, then this function returns undefined. Below examples illustrate the JavaScript Array pop () method: Example 1: In this example the pop () method removes the last element from the array, which is 4 and returns it. var arr = [34, 234, 567, 4]; var popped = arr.pop (); print (popped); print (arr);

Java Program to Get First and Last Elements from an Array List

In this article, we will understand how to get first and last elements from an array list. A list is an ordered collection that allows us to store and access elements sequentially. It contains the index-based methods to insert, update, delete and search the elements. It can also have the duplicate elements.

Below is a demonstration of the same −

Suppose our input is −

The desired output would be −

The first element is : 40 The last element is : 300

Algorithm

Step 1 - START Step 2 - Declare ArrayList namely input_list. Step 3 - Define the values. Step 4 - Check of the list is not an empty list. Use the .get(0) and .get(list.size - 1) to get the first and last elements. Step 5 - Display the result Step 6 - Stop

Example 1

Here, we bind all the operations together under the ‘main’ function.

import java.util.ArrayList; public class Demo < public static void main(String[] args)< ArrayListinput_list = new ArrayList(5); input_list.add(40); input_list.add(60); input_list.add(150); input_list.add(300); System.out.print("The list is defined as: " + input_list); int first_element = input_list.get(0); int last_element = input_list.get(input_list.size() - 1); System.out.println("\nThe first element is : " + first_element); System.out.println("The last element is : " + last_element); > >

Output

The list is defined as: [40, 60, 150, 300] The first element is : 40 The last element is : 300

Example 2

Here, we encapsulate the operations into functions exhibiting object oriented programming.

import java.util.ArrayList; public class Demo < static void first_last(ArrayListinput_list) < int first_element = input_list.get(0); int last_element = input_list.get(input_list.size() - 1); System.out.println("\nThe first element is : " + first_element); System.out.println("The last element is : " + last_element); >public static void main(String[] args) < ArrayListinput_list = new ArrayList(5); input_list.add(40); input_list.add(60); input_list.add(150); input_list.add(300); System.out.print("The list is defined as: " + input_list); first_last(input_list); > >

Output

The list is defined as: [40, 60, 150, 300] The first element is : 40 The last element is : 300

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ArrayDeque pop() Method in Java

The Java.util.ArrayDeque.pop() method in Java is used to pop an element from the deque. The element is popped from the top of the deque and is removed from the same.

Parameters: The method does not take any parameters.

Return Value: This method returns the element present at the front of the Deque.

Exceptions: The method throws NoSuchElementException is thrown if the deque is empty.

Below programs illustrate the Java.util.ArrayDeque.pop() method:
Program 1:

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Class ArrayList

Type Parameters: E — the type of elements in this list All Implemented Interfaces: Serializable , Cloneable , Iterable , Collection , List , RandomAccess Direct Known Subclasses: AttributeList , RoleList , RoleUnresolvedList

Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null . In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector , except that it is unsynchronized.)

The size , isEmpty , get , set , iterator , and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.

Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.

An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.

Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be «wrapped» using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

List list = Collections.synchronizedList(new ArrayList(. ));

The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

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How to Remove Array Elements in Java

While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

When we create an array in Java, we specify its data type and size. This is used by JVM to allocates the necessary memory for array elements. There are no specific methods to remove elements from the array.

1. Removing an element from Array using for loop

This method requires the creation of a new array. We can use for loop to populate the new array without the element we want to remove.

package com.journaldev.java; import java.util.Arrays; public class Main  public static void main(String[] args)  int[] arr = new int[]1,2,3,4,5>; int[] arr_new = new int[arr.length-1]; int j=3; for(int i=0, k=0;iarr.length;i++) if(i!=j) arr_new[k]=arr[i]; k++; > > System.out.println("Before deletion :" + Arrays.toString(arr)); System.out.println("After deletion :" + Arrays.toString(arr_new)); > > 

The code removes the element at index 3. This method simply copies all the elements except the one at index 3 to a new array.

2. Deleting an array element by its value

Unlike the previous case, this code will delete the element based on its value. This will not work with duplicates since the size of the array after deletion has to be known.

package com.journaldev.java; import java.util.Arrays; public class Main  public static void main(String[] args)  int[] arr = new int[]1,2,3,4,5>; int[] arr_new = new int[arr.length-1]; int j=3; for(int i=0, k=0;iarr.length;i++) if(arr[i]!=j) arr_new[k]=arr[i]; k++; > > System.out.println("Before deletion :" + Arrays.toString(arr)); System.out.println("After deletion :" + Arrays.toString(arr_new)); > > 

The only difference between this and the previous case is arr[i]!=j in the if condition in place of i!=j .

3. Deleting element by its value when the array contains duplicates

Performing deletion based on the value in case of duplicates requires using ArrayList. Since ArrayList doesn’t require the size to be now in advance, it allows us to expand dynamically.

package com.journaldev.java; import java.util.ArrayList; import java.util.Arrays; public class Main  public static void main(String[] args)  int[] arr = new int[]1,3,3,4,5>; ArrayListInteger> arr_new = new ArrayList>(); int j=3; for(int i=0;iarr.length;i++) if(arr[i]!=j) arr_new.add(arr[i]); > > System.out.println("Before deletion :" + Arrays.toString(arr)); System.out.println("After deletion :" +arr_new); > > 

4. Shifting elements in the same array

This method involves shifting the elements in the same array. Shifting of elements replaces the element to be deleted by the element at the next index.

package com.journaldev.java; import java.util.Arrays; public class Main  public static void main(String[] args)  int[] arr = new int[]1,3,3,4,5>; int j=3; System.out.println("Before deletion :" + Arrays.toString(arr)); int count =0; for(int i = 0; i  arr.length; i++) if(arr[i] == j) count++; // shifting elements for(int k = i; k  arr.length - 1; k++) arr[k] = arr[k+1]; > i--; // break; > > System.out.print("After Deletion :" ); for(int i = 0; i  arr.length-count; i++) System.out.print(" " + arr[i]); > System.out.println(); System.out.println("Whole array :" + Arrays.toString(arr)); > > 

Count variable indicates the number of elements deleted. This variable is essential to keep a track of index till which the array should be printed. This method takes care of duplicates as well.

5. Deleting elements from ArrayList

ArrayList is backed by arrays. The deletion of an element in the ArrayList is straight forward. It requires one simple call to an inbuilt function.

package com.journaldev.java; import java.util.ArrayList; import java.util.Arrays; public class Main  public static void main(String[] args)  int[] arr = new int[]1,3,3,4,5>; ArrayListInteger> arr_new = new ArrayListInteger>(); for (int i : arr)  arr_new.add(i); > arr_new.remove(3); System.out.println("Before deletion :" + Arrays.toString(arr)); System.out.println("After Deletion:" + arr_new); > > 

A call to the remove(i) function removes the element at index i. Deletion in ArrayLists is relatively easier as compared to Arrays.

Conclusion

We saw some examples of deleting elements in an array using different methods. The difference between the deletion of an element in an Array and an ArrayList is clearly evident. If deletion is to be performed again and again then ArrayList should be used to benefit from its inbuilt functions. Same is the case with the addition of elements to an array. ArrayList due to its dynamic nature provides easier and less cumbersome ways to alter an array.

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