What does T angle brackets mean in Java

Question

I am currently studying Java and have recently been stumped by angle brackets  lt     What exactly do they mean   public class Pool lt T gt       public interface PoolFactory lt T gt           public T createObject              this freeObjects   new ArrayList lt T gt  maxsize      What does the  lt T gt  mean  Does it means that I can create an object of type T

User · Answer

is called a generic type  You can instantiate an object Pool like this   PoolFactory lt Integer gt  pool   new Pool lt Integer gt       The generic parameter can only be a reference type  So you can t use primitive types like int or double or char or other primitive types

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It s really simple  It s a new feature introduced in J2SE 5  Specifying angular brackets after the class name means you are creating a temporary data type which can hold any type of data   Example   class A lt T gt       T obj      void add T obj           this obj obj            T get            return obj          public class generics       static lt E gt  void print E   elements           for E element elements               System out println element                        public static void main String   args            A lt String gt  obj new A lt String gt             A lt Integer gt  obj1 new A lt Integer gt             obj add  hello            obj1 add 6           System out println obj get             System out println obj1 get              Integer   arr  1 3 5 7           print arr             Instead of  lt T gt   you can actually write anything and it will work the same way  Try writing  lt ABC gt  in place of  lt T gt    This is just for convenience     lt T gt  is referred to as any type  lt E gt  as element type  lt N gt  as number type  lt V gt  as value  lt K gt  as key   But you can name it anything you want  it doesn t really matter   Moreover  Integer  String  Boolean etc are wrapper classes of Java which help in checking of types during compilation  For example  in the above code  obj is of type String  so you can t add any other type to it  try obj add 1   it will cast an error   Similarly  obj1 is of the Integer type  you can t add any other type to it  try obj1 add  hello    error will be there

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lt T gt  is a generic and can usually be read as  of type T   It depends on the type to the left of the  lt   what it actually means   I don t know what a Pool or PoolFactory is  but you also mention ArrayList lt T gt   which is a standard Java class  so I ll talk to that   Usually  you won t see  T  in there  you ll see another type  So if you see ArrayList lt Integer gt  for example  that means  An ArrayList of Integers   Many classes use generics to constrain the type of the elements in a container  for example  Another example is HashMap lt String  Integer gt   which means  a map with String keys and Integer values    Your Pool example is a bit different  because there you are defining a class  So in that case  you are creating a class that somebody else could instantiate with a particular type in place of T  For example  I could create an object of type Pool lt String gt  using your class definition  That would mean two things    My Pool lt String gt  would have an interface PoolFactory lt String gt  with a createObject method that returns Strings  Internally  the Pool lt String gt  would contain an ArrayList of Strings    This is great news  because at another time  I could come along and create a Pool lt Integer gt  which would use the same code  but have Integer wherever you see T in the source

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lt  gt  is used to indicate generics in Java   T is a type parameter in this example  And no  instantiating is one of the few things that you can t do with T   Apart from the tutorial linked above Angelika Langers Generics FAQ is a great resource on the topic

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Generic classes are a type of class that takes in a data type as a parameter when it s created  This type parameter is specified using angle brackets and the type can change each time a new instance of the class is instantiated  For instance  let s create an ArrayList for Employee objects and another for Company objects  ArrayList lt Employee gt  employees   new ArrayList lt Employee gt     ArrayList lt Company gt  companies   new ArrayList lt Company gt       You ll notice that we re using the same ArrayList class to create both lists and we pass in the Employee or Company type using angle brackets  Having one generic class be able to handle multiple types of data cuts down on having a lot of classes that perform similar tasks  Generics also help to cut down on bugs by giving everything a strong type which helps the compiler point out errors  By specifying a type for ArrayList  the compiler will throw an error if you try to add an Employee to the Company list or vice versa

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It is related to generics in java  If I mentioned ArrayList lt String gt  that means I can add only String type object to that ArrayList    The two major benefits of generics in Java are    Reducing the number of casts in your program  thus reducing the number of potential bugs in your program  Improving code clarity

[java] What does <T> (angle brackets) mean in Java?

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