How to restrict type parameters in Java generic methods?

In order to restrict type parameters in Java generic methods, you need to use the syntax, where Bound is a type or interface. As such, parameters only accept types that inherit from the Bound type or implement the Bound interface. For example, > restricts T to types that can be compared with itself.

How to restrict type parameters in Java generic methods

Preface

Java generics enable you to create reusable code regardless of the actual type passed to the code. Sometimes, it is necessary to restrict generic type parameters to ensure that the code behaves the expected way.

Bounds usage

Use the and <code>extends qualifiers to limit generic type parameters. By specifying a type or interface, you can restrict parameters to only accept types that inherit from that type or implement that interface.

Syntax:

public <T extends Bound> void someMethod(T arg) {
    // ...
}

Practical case

Consider a compare method that compares the size of two elements:

public <T extends Comparable<T>> void compare(T a, T b) {
    if (a.compareTo(b) > 0) {
        System.out.println("a is greater than b");
    } else if (a.compareTo(b) < 0) {
        System.out.println("a is less than b");
    } else {
        System.out.println("a is equal to b");
    }
}

Here, a parameter of type T is constrained to implement the Comparable interface, which means it can be compared with itself.

Other limits

• Multiple limits: You can specify multiple limits using the & operator. For example, & Serializable>.
• Native types: Native types can be restricted by using wrapper classes. For example, <t extends number></t>.
• Wildcard: You can use the ? wildcard to further restrict the type without specifying a specific type. For example, <t extends number></t>.

Notes

• Ensure that type parameters meet the specified boundaries, otherwise compilation errors may occur.
• Restricting type parameters helps ensure code robustness and maintainability.

Additional Resources

• [Official Java Documentation: Generics](https://docs.oracle.com/javase/tutorial/java /generics/)

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