How to Use Parameterized Generic Types in an Inner Class

How can I implement parameterized generic types in an inner class?

class OuterClass<T> {
    class InnerClass<U> {
        private T outerField;
        private U innerField;
        
        InnerClass(T outerField, U innerField) {
            this.outerField = outerField;
            this.innerField = innerField;
        }
        
        public void display() {
            System.out.println("Outer Field: " + outerField);
            System.out.println("Inner Field: " + innerField);
        }
    }
}

Parameterized generic types allow you to create classes that can be used with different data types. Inner classes can also leverage these generics, enabling type-safe interactions between the outer and inner classes.

class OuterClass<T> {
    class InnerClass<U> {
        private T data;
        private U value;
        
        public InnerClass(T data, U value) {
            this.data = data;
            this.value = value;
        }
        
        public void showData() {
            System.out.println("Data: " + data + ", Value: " + value);
        }
    }
    
    public InnerClass<T> getInnerInstance(T data, T value) {
        return new InnerClass<>(data, value);
    }
}

public class Main {
    public static void main(String[] args) {
        OuterClass<String> outer = new OuterClass<>();
        OuterClass<String>.InnerClass<Integer> inner = outer.new InnerClass<>("Test", 100);
        inner.showData();
    }
}

Causes

• Understanding how generics work in Java is crucial.
• In Java, inner classes have access to the members of the outer class, including generic type parameters.

Solutions

• Define the outer class with a generic type parameter (e.g., `<T>`).
• Define the inner class with its own generic parameter(s) (e.g., `<U>`).
• Use the outer class's generic parameter within the inner class, enhancing flexibility.

Mistake: Not specifying generic types correctly when creating instances.

Solution: Ensure that the types match the generic parameters of both the outer and inner classes.

Mistake: Assuming inner classes don't have access to outer class generics.

Solution: Remember that inner classes can use outer class generics directly.