Using generic types with String.format in Java

Question

I have a generic class like this:

public class MyClass<T> {
    private T var;

    public MyClass(T init_value) {
        var=init_value;
    }

    ...
}

Now I would like to add toString() method to it, which returns the concrete type and value of var. I try something like this:

public String toString() {
    return String.format("MyClass object initialized with type %s and value %s", T, var);
}

This, however, does not work, as I have a problem that type variable T seems not to be visible as normal variable. Namely, I get an error:

T cannot be resolved to a variable

How can I overcome this problem and make a type variable printable?

Think about <T> like it's alias. So what should be <T> to make your code compile? — Aliaksei Yatsau
– Aliaksei Yatsau, Commented Aug 22, 2015 at 9:26

Community · Accepted Answer · 2017-05-23 11:52:11Z

2

As you already have object of class T you can use getClass on it:

String.format("MyClass object initialized with type %s and value %s", var.getClass().getSimpleName(), var);

If you don't have var, you option would be to provide argument of type Class<T> explicitly:

public class MyClass<T> {
    private Class<T> clazz;

    public MyClass(Class<T> clazz) {
        this.clazz = clazz;
    }

    public String toString() {
        return String.format("Type of T: %s", clazz.getSimpleName());
    }
}

You have to do this, because type T is usually not accessible in runtime due to type erasure. The only exception is when you define class that extends explicitly parameterized class: class MyIntegerClass extends MyClass<Integer>. In this case type parameters could be accessed via reflection.

edited May 23, 2017 at 11:52

CommunityBot

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answered Aug 22, 2015 at 10:40

Aivean

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