How can I write a generic method?

October 9, 2023

I need to write a generic method in the non-generic class. My code looks like the following:

public record Test<T>(T value);

public class MyClass {
    public <T> Test<T> getRecord() {
        // some method logic
        ...
        switch(type) {
            case 1: 
               return new Test<String>("my string"); // not compile
            ...
            default:
               return new Test<Integer>(5);          //not compile
        }
    }
}

The Intellij Idea suggests that I cast it, like this (Test<T>) new Test<Integer>(5), but it is not suitable for me.
Can anyone help to fix that issue?

>Solution :

You are trying to return different types from a single method. Java is a statically typed language, this means that the type of a variable is checked at compile time. Therefore, you can’t return different types from a single method.

To fix your code, you can use wildcard ? to represent an unknown type if you accept to relax type constraints a bit. Below is an example:

public Test<?> getRecord(int type) {
    switch(type) {
        case 1:
            return new Test<String>("my string");
        default:
            return new Test<Integer>(5);
    }
}

// ...


Test<?> record1 = myClass.getRecord(1);

// Test type of record1 and cast it to Test<String> variable
if (record1.value() instanceof String) {
     Test<String> tt = (Test<String>) record1;
     // ...
}

This way you still maintain some level of typesafety.