6- Constraints

Jun 20, 2026 05:44 · 3:25 · English · Whisper Turbo · 2 Hangszórók

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There are times you want to add a constraint or a restriction on a

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type parameter.

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For example,

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let's say we only want to store numbers in this list.

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Perhaps this list is going to support some operations that only make sense

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for numbers.

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To do that here,

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we type extends number.

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Now,

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T can only be the number class or any of its child classes.

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But what is this number class?

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Well,

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if you search for Java number class,

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here you can find the documentation for this class.

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Let's look at the documentation for java platform standard edition

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version 8.

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So,

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here you can see that the number class is declared in java .lang

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package, and it's the base class for these classes,

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like byte,

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double,

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float,

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integer,

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long, and so on.

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So all these wrapper classes around numeric primitive types derive

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from the number class.

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So with this constraint,

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if we create a new list of string,

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we get a compilation error because this string is not

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a number.

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So here we can only pass the number class or any class that derives

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from the number, like integer or float or short.

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Also,

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this constraint doesn't have to be a class.

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It can also be an interface.

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For example,

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in Java we have a popular interface called comparable.

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We use this interface for implementing classes that can be compared with each other.

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For example,

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if we implement this interface in our user class,

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we can compare two user objects based on some criteria,

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like their last login date.

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Then we can sort all users based on their last login date.

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We'll look at the comparable interface later in this section.

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But let's see how applying this constraint works.

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So now we're saying that this list can only store objects that

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are comparable.

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So back in the main class,

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We can pass short here because short values are comparable.

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We can also pass integer or string,

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but we cannot pass user because our user class

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does not implement the comparable interface.

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Now,

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if you go to our user class and type implements comparable,

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now don't worry about this compilation error,

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we'll come back and fix this later.

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But we're saying that the user class implements the comparable interface.

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Now,

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you can see that the error is gone in the main method.

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Also,

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here we can add multiple interfaces as constraints.

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For example,

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we can type and clonable,

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this is another popular interface in Java.

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If you want to be able to clone or copy a class,

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we should implement this interface in that class.

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Now we are saying that this generic list can only store objects that are comparable

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and clonable.

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And by the way,

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note that here we have a single ampersand,

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so this is not the logical and operator.

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Now,

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back in the main class,

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we cannot pass the user class here anymore because this class only

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implements the comparable interface.

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We can only pass classes that implement both the comparable and clonable

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interfaces.

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So this is all about applying constraints.

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Now,

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with this constraint,

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we say that we have a bounded type parameter.

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This type parameter,

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this t, is bounded,

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it's restricted.

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