JavaGenerics
Java Generics
Generics allow you to write classes, interfaces, and methods that work with different data types, without having to specify the exact type in advance.
This makes your code more flexible, reusable, and type-safe.
Why Use Generics?
- Code Reusability: Write one class or method that works with different data types.
- Type Safety: Catch type errors at compile time instead of runtime.
- Cleaner Code: No need for casting when retrieving objects.
Generic Class Example
You can create a class that works with different data types using generics:
T value; // T is a placeholder for any data type
void set(T value) {
this.value = value;
}
T get() {
return value;
}
}
public class Main {
public static void main(String[] args) {
// Create a Box to hold a String
Box<String> stringBox = new Box<>();
stringBox.set("Hello");
System.out.println("Value: " + stringBox.get());
// Create a Box to hold an Integer
Box<Integer> intBox = new Box<>();
intBox.set(50);
System.out.println("Value: " + intBox.get());
}
}
T is a generic type parameter. It's like a placeholder for a data type.
- When you create a
Box<String>,TbecomesString. - When you create a
Box<Integer>,TbecomesInteger.
This way, the same class can be reused with different data types without rewriting the code.
Generic Method Example
You can also create methods that work with any data type using generics:
// Generic method: works with any type T
public static <T> void printArray(T[] array) {
for (T item : array) {
System.out.println(item);
}
}
public static void main(String[] args) {
// Array of Strings
String[] names = {"Jenny", "Liam"};
// Array of Integers
Integer[] numbers = {1, 2, 3};
// Call the generic method with both arrays
printArray(names);
printArray(numbers);
}
}
Example Explained
<T>is a generic type parameter - it means the method can work with any type:String,Integer,Double, etc.- The method
printArray()takes an array of typeTand prints every element. - When you call the method, Java figures out what
Tshould be based on the argument you pass in.
This is useful when you want to write one method that works with multiple types, instead of repeating code for each one.
Bounded Types
You can use theextends keyword to limit the types a generic class or method can accept.
For example, you can require that the type must be a subclass ofNumber:
T[] nums;
// Constructor
Stats(T[] nums) {
this.nums = nums;
}
// Calculate average
double average() {
double sum = 0;
for (T num : nums) {
sum += num.doubleValue();
}
return sum / nums.length;
}
}
public class Main {
public static void main(String[] args) {
// Use with Integer
Integer[] intNums = {10, 20, 30, 40};
Stats<Integer> intStats = new Stats<>(intNums);
System.out.println("Integer average: " + intStats.average());
// Use with Double
Double[] doubleNums = {1.5, 2.5, 3.5};
Stats<Double> doubleStats = new Stats<>(doubleNums);
System.out.println("Double average: " + doubleStats.average());
}
}
Even thoughint values are used in the first case, the.doubleValue() method converts them todouble, so the result is shown with a decimal point.
Example Explained
<T extends Number>: RestrictsTto only work with numeric types likeInteger,Double, orFloat..doubleValue(): Converts any number to adoublefor calculation.- Works for any array of numbers as long as the elements are subclasses of
Number.
Generic Collections
Java Collections likeArrayList andHashMap use generics internally:
list.add("Apple");
String fruit = list.get(0); // No need to cast
Summary
- Generics make your code flexible and type-safe.
- Use
Tor another letter to define a type placeholder. - Generics can be applied to classes, methods, and interfaces.
- Use bounds to limit what types are allowed.
