Compatibility rules for Java function generics

The compatibility rules of Java function generics ensure type safety. Rules include: same type parameter lists, same type parameter bounds, and contravariant and covariant type parameters. For example, 9e828ca004921c559f1a0411055a0190> is compatible with 7e3f95175fc72649f7afad09c40191bd> (contravariant), while f7e83be87db5cd2d9a8a0b8117b38cd4 is compatible with a87fdacec66f0909fc0757c19f2d2b1d (covariant).

Compatibility rules for Java function generics

Java generic functions allow us to write code in a type-safe manner, But not following the correct compatibility rules can lead to compile-time errors. Let’s sort out the rules to avoid such problems.

Rule 1: The type parameter lists are the same

Only function types with the same parameter list are compatible. So the following example will result in an error:

public <T> void func1(T v) {}
public <U> void func2(U v) {}

Rule 2: Type parameters have the same bounds

Bounds define the allowed values ​​of a generic type. Functions are incompatible if they have different bounds for parameters of the same type. For example:

public <T extends Comparable<T>> void func1(T v) {}
public <T extends Number> void func2(T v) {}

Rule 3: Contravariant and covariant type parameters

• Contravariance: If subclass type T can replace superclass type S, then 8742468051c85b06f0a0af9e3e506b5c is type compatible with 1eefd63bbe027a2807ccada294a3372c. For example, 9e828ca004921c559f1a0411055a0190> is compatible with 7e3f95175fc72649f7afad09c40191bd>.
• Covariance: If the superclass type T can replace the subclass type S, then the 1eefd63bbe027a2807ccada294a3372c type is compatible with 8742468051c85b06f0a0af9e3e506b5c. For example, f7e83be87db5cd2d9a8a0b8117b38cd4 is compatible with a87fdacec66f0909fc0757c19f2d2b1d.

Practical case

Consider the following code:

public <T extends Animal> void func1(T t) {
    // 代码...
}

public void func2(Cat c) {
    // 代码...
}

func1 Expects an Animal or an instance of its subclass. func2 Expects a Cat instance. Since Cat extends Animal, func1 is compatible with func2 and can receive Cat type parameters.

Conclusion

It is crucial to follow the compatibility rules for function generics to avoid compile-time errors and ensure type safety.

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