Java教程
  • Introduction
  • Getting Started
    • The Java Technology Phenomenon
      • About the Java Technology
      • What Can Java Technology Do?
      • How Will Java Technology Change My Life?
    • The Hello World Application
    • A Closer Look at the Hello World Application
  • Learning the Java Language
    • Object-Oriented Programming Concepts
      • What Is an Object?
      • What Is a Class?
      • What Is Inheritance?
      • What Is an Interface?
      • What Is a package?
    • Language Basics
      • Java Language Keywords
    • Annotations
      • Annotations Basics
      • Declaring an Annotation Type
      • Predefined Annotation Types
      • Repeating Annotations
      • Type Annotations and Pluggable Type Systems
    • Generics
      • Why Use Generics?
      • Generic Types
        • Raw Types
      • Generic Methods
      • Bounded Type Parameters
        • Generic Methods and Bounded Type Parameters
      • Generics, Inheritance, and Subtypes
      • Type Inference
      • Wildcards
        • Upper Bounded Wildcards
        • Unbounded Wildcards
        • Lower Bounded Wildcards
        • Wildcards and Subtyping
        • Wildcard Capture and Helper Methods
        • Guidelines for Wildcard Use
      • Type Erasure
        • Erasure of Generic Types
        • Erasure of Generic Methods
        • Effects of Type Erasure and Bridge Methods
        • Non-Reifiable Types
      • Restrictions on Generics
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  1. Learning the Java Language
  2. Generics
  3. Type Erasure

Erasure of Generic Methods

Java编译器也会擦除泛型函数传参中的类型参数,以下面的泛型函数为例:

// Counts the number of occurrences of elem in anArray.
//
public static <T> int count(T[] anArray, T elem) {
    int cnt = 0;
    for (T e : anArray)
        if (e.equals(elem))
            ++cnt;
        return cnt;
}

T是无边界的,所以Java编译器将它替换为Object:

public static int count(Object[] anArray, Object elem) {
    int cnt = 0;
    for (Object e : anArray)
        if (e.equals(elem))
            ++cnt;
        return cnt;
}

假设有如下几个类的定义:

class Shape { /* ... */ }
class Circle extends Shape { /* ... */ }
class Rectangle extends Shape { /* ... */ }

我们可以编译一个通用的函数来画不同的形状:

public static <T extends Shape> void draw(T shape) { /* ... */ }

Java编译器会将T替换为Shape:

public static void draw(Shape shape) { /* ... */ }
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Last updated 5 years ago

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