How to Use Java Generics to Enforce a Class and Interface Constraint Together?

How can I create a Java Generics constraint for a class that extends ClassA and implements InterfaceB?

// Example of how to use generics with a class and interface together
public class GenericClass<T extends ClassA & InterfaceB> {
    private T instance;

    public GenericClass(T instance) {
        this.instance = instance;
    }
    
    public void doSomething() {
        // Use instance here
    }
}

In Java, generics allow you to define methods, classes, or interfaces with a type parameter. To enforce that a certain class extends a specific base class and implements a specific interface, you can combine both constraints using the ampersand (&) operator in the type parameter declaration.

public class GenericExample<T extends ClassA & InterfaceB> {
    public void performAction(T obj) {
        // Implementation here
    }
}

Causes

• Understanding of generics in Java is essential.
• Knowledge about extending classes and implementing interfaces.

Solutions

• Use the syntax <T extends ClassA & InterfaceB> to define a type parameter that must meet both constraints.
• Implement a generic class or method that utilizes this syntax.

Mistake: Assuming you can use multiple extends keywords, such as <T extends ClassA extends ClassB> instead of using the '&' operator.

Solution: Use a single 'extends' followed by '&' to combine interfaces.

Mistake: Not realizing that the class datatype can incorporate interfaces along with the class type.

Solution: Always combine class types and interfaces using the '&' operator.