Generic types in java - basic concept

[Zarah]

I am trying to understand the concept of Generics in java. In the introduction to Generic Types, this example is given:

public class Box {
    private Object object;

    public void set(Object object) { this.object = object; }
    public Object get() { return object; }
}

"Since Since its methods accept or return an Object, you are free to pass in whatever you want, provided that it is not one of the primitive types." - I understand this.

But then it has been changed to a generic class:

/**
 * Generic version of the Box class.
 * @param <T> the type of the value being boxed
 */
public class Box<T> {
    // T stands for "Type"
    private T t;

    public void set(T t) { this.t = t; }
    public T get() { return t; }
}

"As you can see, all occurrences of Object are replaced by T. A type variable can be any non-primitive type you specify: any class type, any interface type, any array type, or even another type variable."

We can use any type in place of an Object, because Object is a superclass of all classes. But T (or any other class) is not a superclass of all classes. So how do we justify any class being used in place of a random T or any other class?

Thank you in advance.

[JosAH]

... by filling in any 'real' type for 'T' (as in Box<String>, Box<Double> etc.)

kind regards,

Jos

[Zarah]

as in Box<String>

"One part of the code may place an Integer in the box and expect to get Integers out of it, while another part of the code may mistakenly pass in a String, resulting in a runtime error."

Reference: Generic Types (The Java™ Tutorials > Learning the Java Language > Generics (Updated))

[jim829]

That is the primary reason for generics. So your potential run time errors will be caught at compile time. In most cases it also frees you from having to use instanceof and then casting objects to the proper type. So the code becomes more readable.

Regards,
Jim

[Zarah]

@jim829, Thank you very much!