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   • Collection in Java

     Top Answer

     Answered May 23, 2017 · 7 votes

     Simple answer

     Yes.

     More detailed answer

     You can either use generic collection, without <T> value, for example:

     ArrayList a = new ArrayList();a.add(2);a.add("String");

     Using collections without <T> is a bad habit and most IDEs / compilers give a warning here. You can circumvent it by using a collection of Object, i.e.:

     ArrayList<Object> a = new ArrayList<Object>();-

     Or you can find some common interface or supertype that these element must have in, for example ArrayList<Number> - and you can store various objects that have common Number superclass, i.e. BigDecimal, BigInteger, Byte, Double, Float, Integer, Long, Short:

     ArrayList<Number> a = new ArrayList<Number>();a.add(2); // integera.add(42L); // longa.add(123.45d); // doubleSystem.out.println(a.toString()); // => [2, 42, 123.45]

     Note that it essentially means that a elements are of Number class — i.e. you can't ask to execute subclass-specific methods (for example, Double#isInfinite(), which doesn't exist in Number superclass), although you can typecast in run-time if you somehow know it's safe to typecast:

     a.get(2).isInfinite() // compile-time error((Double) a.get(2)).isInfinite() // => false((Double) a.get(1)).isInfinite() // run-time error (ClassCastException)

     Run-time typecasting is also generally frowned upon, as it effectively circumvents proper compile-time type safety.

     Also note that it's impossible to assign (or use) ArrayList<Number> in place of ArrayList<Integer> and vice-versa, i.e. this will fail to compile:

     public void printNumbers(ArrayList<Number> list) { list.forEach(System.out::println);}ArrayList<Integer> a = new ArrayList<Integer>();printNumbers(a); // "incompatible types"

     as well as this:

     public void printIntegers(ArrayList<Integer> list) { list.forEach(System.out::println);}ArrayList<Number> a = new ArrayList<Number>();printIntegers(a); // "incompatible types"

     To declare a variable to be able to accept both ArrayList<Number> or any of its subclasses, one can use ArrayList<? extends Number> or ArrayList<? super Number> syntax. extends is generally used when you're going to consume (i.e. read) from the object in your method, super is used when you're going to produce (i.e. write). Given that printout is consuming, it's safe to use extends:

     public void printNumbers(ArrayList<? extends Number> list) { list.forEach(System.out::println);}ArrayList<Integer> listInt = new ArrayList<Integer>();printNumbers(listInt); // worksArrayList<Double> listDbl = new ArrayList<Double>();printNumbers(listDbl); // also works

     There is a good answer in Difference between <? super T> and <? extends T> in Java for more in-depth explanation.

     Other Answers

     Answered Nov 23, 2010 · 2 votes

     If you want them to hold any more than one type, use Collection<Object>. However, you won't know what you're getting without doing some if (x instanceof MyType) calls, which are rather inefficient.

     Other Answers

     Answered Nov 23, 2010 · 2 votes

     They have to be of the same Supertype. So if you have objects of type A, then a Collection<A> can store objects of type A and of every subtype of A.

     If you want to allow arbitrary types, then use Collection<Object>, otherwise take the most general appropriate super-class.

     However, you will then have to manually cast from the most general type (Object) to the specific type you have in mind. You can use the typeof operator to find out what the type is.

     Check all answers on Stack Overflow

     1/5

   • Collection Interface in Java with Examples

     The Collection interface is a member of the Java Collections Framework. It is a part of java.util package. It is one of the root interfaces of the Collection Hierarchy. The Collection interface is not directly implemented by any class. However, it is implemented indirectly via its subtypes or subinterfaces like List, Queue, and Set. 

     For Example, the HashSet class implements the Set interface which is a subinterface of the Collection interface. If a collection implementation doesn’t implement a particular operation, it should define the corresponding method to throw UnsupportedOperationException.

     The Hierarchy of Collection

     <img alt="Collection-Interface-in-Java-with-Examples">

     It implements the Iterable<E> interface. The sub-interfaces of Collection are BeanContext, BeanContextServices, BlockingDeque<E>, BlockingQueue<E>, Deque<E>, EventSet, List<E>, NavigableSet<E>, Queue<E>, Set<E>, SortedSet<E>, TransferQueue<E> .

     SubInterfaces of Collection Interface

     All the Classes of the Collection Framework implement the subInterfaces of the Collection Interface. All the methods of Collection interfaces are also contained in it’s subinterfaces. These subInterfaces are sometimes called as Collection Types or SubTypes of Collection. These include the following:

     List: This is a child interface of the collection interface. This interface is dedicated to the data of the list type in which we can...

     Check all answers on Geeksforgeeks

     2/5

   • Collection Interface in Java with Examples

     The Collection interface is a member of the Java Collections Framework. It is a part of java.util package. It is one of the root interfaces of the Collection Hierarchy. The Collection interface is not directly implemented by any class. However, it is implemented indirectly via its subtypes or subinterfaces like List, Queue, and Set. 

     For Example, the HashSet class implements the Set interface which is a subinterface of the Collection interface. If a collection implementation doesn’t implement a particular operation, it should define the corresponding method to throw UnsupportedOperationException.

     The Hierarchy of Collection

     <img alt="Collection-Interface-in-Java-with-Examples">

     It implements the Iterable<E> interface. The sub-interfaces of Collection are BeanContext, BeanContextServices, BlockingDeque<E>, BlockingQueue<E>, Deque<E>, EventSet, List<E>, NavigableSet<E>, Queue<E>, Set<E>, SortedSet<E>, TransferQueue<E> .

     SubInterfaces of Collection Interface

     All the Classes of the Collection Framework implement the subInterfaces of the Collection Interface. All the methods of Collection interfaces are also contained in it’s subinterfaces. These subInterfaces are sometimes called as Collection Types or SubTypes of Collection. These include the following:

     List: This is a child interface of the collection interface. This interface is dedicated to the data of the list type in which we can...

     Check all answers on Geeksforgeeks

     3/5

   • Collection Map in Java

     Top Answer

     Answered Apr 03, 2016 · 0 votes

     Best would be if you checked out some of the concepts behind HashMap, TreeMap, HeapMap etc. Once you understand those concepts, you're far better prepared for writing your own map when it comes to speed.

     In other words: unless you know the concepts of all available implementations, it is very unlikely your wheel-re-invention will be a better solution. Also be sure to test your implementations very thoroughly, as Collection are the backbone and heart of any good application.

     Two very very simple (but slow) solutions are these:

     1) As suggested above, you can use an ArrayList<Pair> and add your custom getItemByKey() (in Java commonly named 'get') method.

     2) You can use two arrays, both keeping the same size, and keeping keys and values matched by their respective indices.

     Check all answers on Stack Overflow

     4/5

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     5/5

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2. www.programiz.com › java-programming › collectionsJava Collections Framework - Programiz

   www.programiz.com › java-programming › collections

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   // The Collections framework is defined in the java.util package import java.util.ArrayList; class Main { public static void main(String[] args){ ArrayList<String> animals = new ArrayList<>(); // Add elements animals.add("Dog"); animals.add("Cat"); animals.add("Horse"); System.out.println("ArrayList: " + animals); } }

3. docs.oracle.com › javase › tutorialLesson: Introduction to Collections (The Java™ Tutorials - Oracle

   docs.oracle.com › javase › tutorial

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   Lesson: Introduction to Collections. A collection — sometimes called a container — is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data. Typically, they represent data items that form a natural group, such as a poker hand (a collection of ...

4. www.geeksforgeeks.org › java-collection-tutorialJava Collection Tutorial - GeeksforGeeks

   www.geeksforgeeks.org › java-collection-tutorial

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   Sep 26, 2023 · So in JDK 1.2, a separate framework was defined known as “Collection Framework” which holds all the collection classes and interfaces in it. The Collection interface ( java.util.Collection) and Map interface ( java.util.Map) are the two main “root” interfaces of Java collection classes.

5. docs.oracle.com › javase › tutorialTrail: Collections (The Java™ Tutorials) - Oracle Help Center

   docs.oracle.com › javase › tutorial

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   Here, you will learn what collections are and how they can make your job easier and programs better. You'll learn about the core elements — interfaces, implementations, aggregate operations, and algorithms — that comprise the Java Collections Framework.

6. www.baeldung.com › java-collectionsJava Collections | Baeldung

   www.baeldung.com › java-collections

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   Jan 18, 2024 · Join and Split Arrays and Collections in Java; Java - Combine Multiple Collections; Finding Max/Min of a List or Collection (popular) Collect a Java Stream to an Immutable Collection; Java Convenience Factory Methods for Collections; Java Collections Interview Questions; A Guide to Iterator in Java; Getting the Size of an Iterable in Java ...

7. www.geeksforgeeks.org › how-to-learn-java-collections-a-complete-guideHow to Learn Java Collections - A Complete Guide - GeeksforGeeks

   www.geeksforgeeks.org › how-to-learn-java-collections-a-complete-guide

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   Nov 29, 2021 · Since Java is an Object-Oriented Language it mimics the real world. In Java, a Collection is a group of multiple objects put together into a single unit. Java Collections is a very vast topic and as a beginner can be difficult to navigate your way while learning it. Here we have everything you need to know while starting off with Java Collections.

8. www.digitalocean.com › community › tutorialsCollections in Java - Everything You MUST Know | DigitalOcean

   www.digitalocean.com › community › tutorials

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   Aug 3, 2022 · Java Collections framework provides implementation classes for core collection interfaces. We can use them to create different types of collections in the Java program. Some important collection classes are ArrayList, LinkedList, HashMap, TreeMap, HashSet, and TreeSet.