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Bounded-types in generics in Java?
Whenever you want to restrict the type parameter to subtypes of a particular class you can use the bounded type parameter. If you just specify a type (class) as bounded parameter, only sub types of that particular class are accepted by the current generic class. These are known as bounded-types in generics in Java.
Defining bounded-types for class
You can declare a bound parameter just by extending the required class with the type-parameter, within the angular braces as −
class Sample <T extends Number>
Example
In the following Java example the generic class Sample restricts the type parameter to the sub classes of the Number classes using the bounded parameter.
class Sample <T extends Number>{
T data;
Sample(T data){
this.data = data;
}
public void display() {
System.out.println("Data value is: "+this.data);
}
}
public class BoundsExample {
public static void main(String args[]) {
Sample<Integer> obj1 = new Sample<Integer>(20);
obj1.display();
Sample<Double> obj2 = new Sample<Double>(20.22d);
obj2.display();
Sample<Float> obj3 = new Sample<Float>(125.332f);
obj3.display();
}
}Output
Data value is: 20 Data value is: 20.22 Data value is: 125.332
Now, if you pass other types as parameters to this class (say, String for example) a compile time error will be generated.
Example
public class BoundsExample {
public static void main(String args[]) {
Sample<Integer> obj1 = new Sample<Integer>(20);
obj1.display();
Sample<Double> obj2 = new Sample<Double>(20.22d);
obj2.display();
Sample<String> obj3 = new Sample<String>("Krishna");
obj3.display();
}
}Compile time error
BoundsExample.java:16: error: type argument String is not within bounds of type-variable T
Sample<String> obj3 = new Sample<String>("Krishna");
^
where T is a type-variable:
T extends Number declared in class Sample
BoundsExample.java:16: error: type argument String is not within bounds of type-variable T
Sample<String> obj3 = new Sample<String>("Krishna"); ^
where T is a type-variable:
T extends Number declared in class Sample
2 errorsDefining bounded-types for methods
Just like with classes to define bounded-type parameters for generic methods, specify them after the extend keyword. If you pass a type which is not sub class of the specified bounded-type an error will be generated.
Example
In the following example we are setting the Collection<Integer> type as upper bound to the typed-parameter i.e. this method accepts all the collection objects (of Integer type).
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
public class GenericMethod {
public static <T extends Collection<Integer>> void sampleMethod(T ele){
Iterator<Integer> it = ele.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}
}
public static void main(String args[]) {
ArrayList<Integer> list = new ArrayList<Integer>();
list.add(24);
list.add(56);
list.add(89);
list.add(75);
list.add(36);
sampleMethod(list);
}
}Output
24 56 89 75 36
Now, if you pass types other than collection as typed-parameter to this method, it generates a compile time error.
Example
public class GenericMethod {
public static <T extends Collection<Integer>> void sampleMethod(T ele){
Iterator<Integer> it = ele.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}
}
public static void main(String args[]) {
ArrayList<Integer> list = new ArrayList<Integer>();
list.add(24);
list.add(56);
list.add(89);
list.add(75);
list.add(36);
sampleMethod(list);
Integer [] intArray = {24, 56, 89, 75, 36};
sampleMethod(intArray);
}
}Compile time error
GenericMethod.java:23: error: method sampleMethod in class GenericMethod cannot be applied to given types; sampleMethod(intArray); ^ required: T found: Integer[] reason: inferred type does not conform to upper bound(s) inferred: Integer[] upper bound(s): Collection<Integer> where T is a type-variable: T extends Collection<Integer> declared in method <T>sampleMethod(T) 1 error
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