7- Type Erasure
Jun 20, 2026 06:03
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So now that you have a good understanding of what generics are,
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let's see how they work under the hood.
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So I've removed the constraint on this type parameter,
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we only have T.
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Now let's recompile our code.
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So on the top,
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from the build menu,
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let's go to build project.
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Next we open the project window,
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select the generic list class,
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and from the view menu we go to show bytecode.
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Here we can see the actual bytecode that is generated as
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a result of compiling our Java code.
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Remember bytecode?
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We talked about it in the first part of this series.
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Bytecode is a platform independent representation of our Java code.
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So when we compile our Java code,
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the Java compiler will produce this bytecode that can run on different platforms
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like Windows,
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Mac, and Linux.
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Now,
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when we run our program,
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the Java virtual machine will get this bytecode and convert it to the native
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code for the target platform.
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So if you run this code on Windows,
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we have an implementation of Java virtual machine that knows how to
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convert this bytecode into native code for Windows.
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We also have an implementation of Java virtual machine for Mac,
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for Linux and other operating systems.
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Now,
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why do we care about this?
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Well, if you scroll up,
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you can see different pieces of the generic list class.
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For example,
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over here we have our two private fields.
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Here we have the items field and the count field.
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Now look at the type of the items field.
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It's an object.
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Well, more accurately,
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it's an object array.
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Also,
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if you scroll down,
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you can see our add method.
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Now look at the type of the parameter of this method.
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It's the object class.
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So when we compile this code,
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the Java compiler will replace all these T's with the object class.
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And that means,
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internally,
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this class is implemented like the non -generic list that we created earlier.
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This class over here.
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So when we compile our code,
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our generic list will end up looking like this in bytecode.
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the difference between these two implementations is compile time type
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safety.
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So when we use the generic implementation,
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the java compiler will check for type errors at compile time.
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But internally,
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our integers,
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our strings,
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our user objects are stored in an array of objects.
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And this is not because we have set this field to a new object array.
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Even if we don't do this,
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the java compiler will still replace all these t's with the object class.
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Now let's see what happens when we apply a constraint.
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So here I'm going to type extends number.
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Now we recompile our code then
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from the project window we select generic list and go
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to show bytecode.
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Take a look.
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Now the type of this field is number.
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Similarly,
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if you look at the add method,
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the type of the parameter is changed to number.
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So when we apply a constraint,
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the Java compiler will replace all these T's based on
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the constraint that we have set here.
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Here our constraint is the number class,
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so all these T's will be replaced with the number class.
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If you have an interface,
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let's say,
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comparable,
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all these T's will be replaced with the comparable interface.
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Now what if we have two constraints,
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like comparable and clonable.
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Let's take a look.
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So,
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we recompile the code,
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then
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look at the bytecode,
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take a look.
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The type of the items field is comparable,
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and similarly,
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the type of the parameter of the add method is comparable.
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So when we have multiple interface constraints,
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the Java compiler will take the leftmost one and use that to
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replace all these t's.
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This is called type erasure,
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which means the Java compiler erases these type parameters and
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replaces them with a class or an interface depending on the constraints.
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If there are no constraints,
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all these t's are replaced with the object class.
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So this is how generics work under the hood.
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