Generic Types

Sometimes, we want things to support any type, including user defined types that we don't know about! For example, it would make sense that we don't care what type we make a List out of, since it's just a whole bunch of objects put together.
The Java solution is generics! Generic types are denoted by a <> and can be appended to methods and classes. Here's an example with classes:
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/**
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  * Creates a type SomeClass that takes * in a generic SomeType. SomeType can * be named anything.
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*/
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public class SomeClass<SomeType> {
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    private SomeType someThing;
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​
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    public void someMethod(SomeType stuff) {
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        doStuff(stuff);
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    }
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}
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​
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...
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/** Creates a new instance of SomeClass, setting SomeType to String.
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    We don't need to put the type on the right since it's already
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    defined on the left. */
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SomeClass<String> aClass = new SomeClass<>();
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In this example, SomeType is a Generic Type Variable that is not a real type, but can still be used inside the class as normal.
On the other hand, String is an Actual Type Argument that replaces SomeType during runtime. Now, every time SomeType is used in SomeClass Java treats it exactly like a String.
Generic Subtypes
Like in **Dynamic Method Selection, adding inheritance makes things tricky! Let's look at an example:
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List<String> LS = new ArrayList<String>();
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List<Object> LO = LS; // Line 3
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LO.add(42); // Line 4
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String s = LS.get(0); // Line 5
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Question 1
Q1 Answer
Will line 3 error?
No, line 3 is valid and will not error! This is because Object is a superclass of String. Generics work in a very similar way to the inheritance rules.
Question 2
Q2 Answer
Will line 4 error?
No, line 4 is valid and will not error! This is because LO is a list of Objects and integers are a subtype of Object, as all things are.
Question 3
Q3 Answer
Will line 5 error?
Yes, line 5 will error! This is because we put 42 into LO, which is an integer. Since LO is pointing to the same object as LS, 42 is also in LS! That means we are trying to assign a String equal to an integer.
Arrays have slightly different behavior than this and will throw an ArrayStoreException if types are mismatched in any way.
Type Bounds
Sometimes, we want to put constraints on what kinds of types can be passed into a generic type.
One way of doing is is to specify that a generic type must fit within a type bound: here, T must be some subtype of a specified type Number.
We can also do it the other way and specify that a type can be a supertype of a specified type. Both of these examples are shown below:
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class SomeClass<T extends Number> {
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    // A method that takes a type parameter T and takes any SUPERCLASS
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    // of T as a list generic type.
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    static <T> void doSomething(List<? super T> L) { ... }
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}
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Limitations of Generic Types
The biggest limitation is that primitive types cannot be used as generic types. For example, List<int> is invalid and will not work!
One workaround to this is to use the reference-type counterparts to primitives, such as Integer, Boolean, Character and so on. However, converting between these types and primitive types, which is called autoboxing, has significant performance penalties that must be taken into consideration.
Another limitation is that instanceof does not work properly with generic types. For instance, new List<X>() instanceof List<Y> will always be true regardless of what types X and Y are.

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Contents

Generic Subtypes

Type Bounds

Limitations of Generic Types