Java Genrics is one of the most important features introduced in Java 5. If you have been working on Java Collections and with version 5 or higher, I am sure that you have used it. Generics in Java with collection classes is very easy but it provides a lot more features than just creating the type of collection. We will try to learn the features of generics in this article. Understanding generics can become confusing sometimes if we go with jargon words, so I would try to keep it simple and easy to understand.
We will look into below topics of generics in java.
Generics in Java
Java Generic Class
Java Generic Interface
Java Generic Type
Java Generic Method
Java Generics Bounded Type Parameters
Java Generics and Inheritance
Java Generic Classes and Subtyping
Java Generics Wildcards
Java Generics Upper Bounded Wildcard
Java Generics Unbounded Wildcard
Java Generics Lower bounded Wildcard
Subtyping using Generics Wildcard
Java Generics Type Erasure
Generics FAQs
1. Generics in Java
Generics was added in Java 5 to provide compile-time type checking and removing risk of ClassCastException that was common while working with collection classes. The whole collection framework was re-written to use generics for type-safety. Let’s see how generics help us using collection classes safely.
List list = new ArrayList[]; list.add["abc"]; list.add[new Integer[5]]; //OK for[Object obj : list]{ //type casting leading to ClassCastException at runtime String str=[String] obj; }Above code compiles fine but throws ClassCastException at runtime because we are trying to cast Object in the list to String whereas one of the element is of type Integer. After Java 5, we use collection classes like below.
List list1 = new ArrayList[]; // java 7 ? List list1 = new ArrayList[]; list1.add["abc"]; //list1.add[new Integer[5]]; //compiler error for[String str : list1]{ //no type casting needed, avoids ClassCastException }Notice that at the time of list creation, we have specified that the type of elements in the list will be String. So if we try to add any other type of object in the list, the program will throw compile-time error. Also notice that in for loop, we don’t need typecasting of the element in the list, hence removing the ClassCastException at runtime.
2. Java Generic Class
We can define our own classes with generics type. A generic type is a class or interface that is parameterized over types. We use angle brackets [] to specify the type parameter. To understand the benefit, let’s say we have a simple class as:
package com.journaldev.generics; public class GenericsTypeOld { private Object t; public Object get[] { return t; } public void set[Object t] { this.t = t; } public static void main[String args[]]{ GenericsTypeOld type = new GenericsTypeOld[]; type.set["Pankaj"]; String str = [String] type.get[]; //type casting, error prone and can cause ClassCastException } }Notice that while using this class, we have to use type casting and it can produce ClassCastException at runtime. Now we will use java generic class to rewrite the same class as shown below.
package com.journaldev.generics; public class GenericsType { private T t; public T get[]{ return this.t; } public void set[T t1]{ this.t=t1; } public static void main[String args[]]{ GenericsType type = new GenericsType[]; type.set["Pankaj"]; //valid GenericsType type1 = new GenericsType[]; //raw type type1.set["Pankaj"]; //valid type1.set[10]; //valid and autoboxing support } }Notice the use of GenericsType class in the main method. We don’t need to do type-casting and we can remove ClassCastException at runtime. If we don’t provide the type at the time of creation, the compiler will produce a warning that “GenericsType is a raw type. References to generic type GenericsType should be parameterized”. When we don’t provide the type, the type becomes Object and hence it’s allowing both String and Integer objects. But, we should always try to avoid this because we will have to use type casting while working on raw type that can produce runtime errors.
Tip: We can use @SuppressWarnings["rawtypes"] annotation to suppress the compiler warning, check out java annotations tutorial.
Also notice that it supports java autoboxing.
3. Java Generic Interface
Comparable interface is a great example of Generics in interfaces and it’s written as:
package java.lang; import java.util.*; public interface Comparable { public int compareTo[T o]; }In similar way, we can create generic interfaces in java. We can also have multiple type parameters as in Map interface. Again we can provide parameterized value to a parameterized type also, for example new HashMap[]; is valid.
4. Java Generic Type
Java Generic Type Naming convention helps us understanding code easily and having a naming convention is one of the best practices of Java programming language. So generics also comes with its own naming conventions. Usually, type parameter names are single, uppercase letters to make it easily distinguishable from java variables. The most commonly used type parameter names are:
- E - Element [used extensively by the Java Collections Framework, for example ArrayList, Set etc.]
- K - Key [Used in Map]
- N - Number
- T - Type
- V - Value [Used in Map]
- S,U,V etc. - 2nd, 3rd, 4th types
5. Java Generic Method
Sometimes we don’t want the whole class to be parameterized, in that case, we can create java generics method. Since the constructor is a special kind of method, we can use generics type in constructors too. Here is a class showing an example of a java generic method.
package com.journaldev.generics; public class GenericsMethods { //Java Generic Method public static boolean isEqual[GenericsType g1, GenericsType g2]{ return g1.get[].equals[g2.get[]]; } public static void main[String args[]]{ GenericsType g1 = new GenericsType[]; g1.set["Pankaj"]; GenericsType g2 = new GenericsType[]; g2.set["Pankaj"]; boolean isEqual = GenericsMethods.isEqual[g1, g2]; //above statement can be written simply as isEqual = GenericsMethods.isEqual[g1, g2]; //This feature, known as type inference, allows you to invoke a generic method as an ordinary method, without specifying a type between angle brackets. //Compiler will infer the type that is needed } }Notice the isEqual method signature showing syntax to use generics type in methods. Also, notice how to use these methods in our java program. We can specify type while calling these methods or we can invoke them like a normal method. Java compiler is smart enough to determine the type of variable to be used, this facility is called type inference.
6. Java Generics Bounded Type Parameters
Suppose we want to restrict the type of objects that can be used in the parameterized type, for example in a method that compares two objects and we want to make sure that the accepted objects are Comparables. To declare a bounded type parameter, list the type parameter’s name, followed by the extends keyword, followed by its upper bound, similar like below method.
public static int compare[T t1, T t2]{ return t1.compareTo[t2]; }The invocation of these methods is similar to unbounded method except that if we will try to use any class that is not Comparable, it will throw compile-time error. Bounded type parameters can be used with methods as well as classes and interfaces. Java Generics supports multiple bounds also, i.e . In this case, A can be an interface or class. If A is class then B and C should be an interface. We can’t have more than one class in multiple bounds.
7. Java Generics and Inheritance
We know that Java inheritance allows us to assign a variable A to another variable B if A is subclass of B. So we might think that any generic type of A can be assigned to generic type of B, but it’s not the case. Let’s see this with a simple program.
package com.journaldev.generics; public class GenericsInheritance { public static void main[String[] args] { String str = "abc"; Object obj = new Object[]; obj=str; // works because String is-a Object, inheritance in java MyClass myClass1 = new MyClass[]; MyClass myClass2 = new MyClass[]; //myClass2=myClass1; // compilation error since MyClass is not a MyClass obj = myClass1; // MyClass parent is Object } public static class MyClass{} }We are not allowed to assign MyClass variable to MyClass variable because they are not related, in fact MyClass parent is Object.
8. Java Generic Classes and Subtyping
We can subtype a generic class or interface by extending or implementing it. The relationship between the type parameters of one class or interface and the type parameters of another are determined by the extends and implements clauses. For example, ArrayList implements List that extends Collection, so ArrayList is a subtype of List and List is subtype of Collection. The subtyping relationship is preserved as long as we don’t change the type argument, below shows an example of multiple type parameters.
interface MyList extends List{ }The subtypes of List can be MyList, MyList and so on.
9. Java Generics Wildcards
Question mark [?] is the wildcard in generics and represent an unknown type. The wildcard can be used as the type of a parameter, field, or local variable and sometimes as a return type. We can’t use wildcards while invoking a generic method or instantiating a generic class. In the following sections, we will learn about upper bounded wildcards, lower bounded wildcards, and wildcard capture.
9.1] Java Generics Upper Bounded Wildcard
Upper bounded wildcards are used to relax the restriction on the type of variable in a method. Suppose we want to write a method that will return the sum of numbers in the list, so our implementation will be something like this.
public static double sum[List list]{ double sum = 0; for[Number n : list]{ sum += n.doubleValue[]; } return sum; }Now the problem with above implementation is that it won’t work with List of Integers or Doubles because we know that List and List are not related, this is when an upper bounded wildcard is helpful. We use generics wildcard with extends keyword and the upper bound class or interface that will allow us to pass argument of upper bound or it’s subclasses types. The above implementation can be modified like the below program.
package com.journaldev.generics; import java.util.ArrayList; import java.util.List; public class GenericsWildcards { public static void main[String[] args] { List ints = new ArrayList[]; ints.add[3]; ints.add[5]; ints.add[10]; double sum = sum[ints]; System.out.println["Sum of ints="+sum]; } public static double sum[List