How to use generics in Java?
Generics in Java enable type-safe code by allowing classes, interfaces, and methods to operate on type parameters. 1. Generic classes like Box
hold any type without casting. 2. Generic methods such as void printArray(T[] array) work across types. 3. Bounded type parameters (e.g., T extends Number) restrict types to subclasses of a given class. 4. Wildcards (?) handle unknown types: ? extends T for subtypes, ? super T for supertypes, and ? for any type. Generics prevent runtime errors by catching type mismatches at compile time and enhance reusability and safety in collections and custom structures.
Generics in Java allow you to define classes, interfaces, and methods with type parameters. This enables type-safe code and eliminates the need for casting when working with collections or custom data structures.
1. Generic Classes
Define a class with a type parameter by placing it within angle brackets after the class name.
// Example: A simple generic box that holds any type
public class Box
private T value;
public void set(T value) {
this.value = value;
}
public T get() {
return value;
}
}
Use it like this:
Box
stringBox.set("Hello");
String content = stringBox.get(); // No cast needed
2. Generic Methods
You can create methods that work with different types by declaring a type parameter before the return type.
public
for (T item : array) {
System.out.println(item);
}
}
Call it with any array:
String[] words = {"a", "b", "c"};
Integer[] numbers = {1, 2, 3};
printArray(words);
printArray(numbers);
3. Bounded Type Parameters
Restrict the types that can be used with generics using extends for upper bounds.
public
double total = 0;
for (T num : numbers) {
total = num.doubleValue();
}
return total;
}
This method only accepts types that are subclasses of Number, like Integer, Double, etc.
4. Wildcards
Use wildcards (?) when you want to accept unknown types.
- ? extends T: Unknown type that is a subclass of T (upper bounded wildcard)
- ? super T: Unknown type that is a superclass of T (lower bounded wildcard)
- ?: Unbounded wildcard — any type
Example:
public void processList(java.util.List extends Number> list) {
// Can read Number objects from the list
}
Generics help catch type errors at compile time and make your code more reusable and safer. Use them whenever you're building reusable components that work across multiple data types.
Basically just replace raw types with parameterized ones and let the compiler help you avoid runtime errors.
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