The principles and application scenarios of Java generic type erasure

Java generic type erasure principle: During compilation, generic type information will be eliminated, leaving only the type-erased bytecode to improve efficiency and compatibility. Application scenarios include reusable code, type safety, and common data structures. However, please note the limitations of type conversion, the inability of reflection to obtain type information, and the limitation that it can only be erased to primitive or generic types.

The principles and application scenarios of Java generic type erasure

The principle of generic type erasure

In Java, generics are used to provide type-safe collections and classes. However, when compiling Java generic code, the type information is erased (eliminated) and only the type-erased bytecode is executed.

The reasons for type erasure are:

• Improve efficiency: Type erasure simplifies bytecode and reduces memory usage.
• Compatibility: Type erasure ensures that generic code is compatible with older versions of Java code.

Code after type erasure

// 泛型类
class MyClass<T> {
    private T value;

    public MyClass(T value) {
        this.value = value;
    }

    public T getValue() {
        return value;
    }
}

After compilation:

// 泛型类 (擦除后)
class MyClass {
    private Object value;

    public MyClass(Object value) {
        this.value = value;
    }

    public Object getValue() {
        return value;
    }
}

Application scenario

Type erasure has many application scenarios in actual project development, such as:

• Reusable code: Generics allow writing reusable code without knowing specific types.
• Type safety: Generics provide compile-time type checking to ensure the type safety of collections or classes.
• General data structures: You can use generics to create general data structures, such as lists, maps, and stacks, which can store elements of different types.

Practical case: Precautions for type erasure

Although generic type erasure provides convenience, you need to pay attention to the following matters when using it:

• Type conversion: Type erasure will eliminate type information, so type conversion is required when obtaining elements from the erased generic class.
• Reflection: Cannot obtain generic type information through reflection because the type information is erased at compile time.
• Restrictions: Generic types can only be erased to primitive types (such as Object) or other generic types.

Extended reading:

• [Official Document: Generics](https://docs.oracle.com/javase/tutorial/java /generics/index.html)
• [Stack Overflow: Pros and cons of generic type erasure](https://stackoverflow.com/questions/4343202/pros-and-cons-of-java-generics- erasure)

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