Java教程 - 什么是Java中的泛型类型

术语泛型意味着参数化类型。使用泛型，可以创建与不同类型的数据一起使用的单个类。在参数化类型上操作的类，接口或方法称为通用。

语法

以下是声明通用类的语法:

class className<type-param-list> {}

下面是声明对一个泛型类的引用的语法:

className<type-arg-list> varName = new className<type-arg-list>()

例子

简单泛型示例

// T is a type parameter that will be replaced by a real type 
// when an object of type Gen is created. 
class Gen<T> {
  T ob; // declare an object of type T
  Gen(T o) {
    ob = o;
  }
  // Return ob.
  T getob() {
    return ob;
  }
  // Show type of T.
  void showType() {
    System.out.println("Type of T is " + ob.getClass().getName());
  }
}

public class Main {
  public static void main(String args[]) {
    Gen<Integer> iOb = new Gen<Integer>(88);
    iOb.showType();
    int v = iOb.getob();
    System.out.println("value: " + v);
    Gen<String> strOb = new Gen<String>("Generics Test");
    strOb.showType();
    String str = strOb.getob();
    System.out.println("value: " + str);
  }
}

T 是类型参数的名称。 T 用于声明一个对象。泛型只与对象一起工作通用类型根据其类型参数而有所不同。

上面的代码生成以下结果。

Type of T is java.lang.Integer

value: 88

Type of is java.lang.String

value: Generics Test

例2

您可以在泛型类型中声明多个类型参数。

// A simple generic class with two type parameters: T and V. 
class TwoGen<T, V> {
  T ob1;
  V ob2;
  TwoGen(T o1, V o2) {
    ob1 = o1;
    ob2 = o2;
  }
  void showTypes() {
    System.out.println("Type of T is " + ob1.getClass().getName());
    System.out.println("Type of V is " + ob2.getClass().getName());
  }

  T getob1() {
    return ob1;
  }

  V getob2() {
    return ob2;
  }
}

public class Main {
  public static void main(String args[]) {
    TwoGen<Integer, String> tgObj = new TwoGen<Integer, String>(88, "Generics");
    tgObj.showTypes();
    int v = tgObj.getob1();
    System.out.println("value: " + v);
    String str = tgObj.getob2();
    System.out.println("value: " + str);
  }
}

上面的代码生成以下结果。

Type of T is java.lang.Integer

Type of V is java.lang.String

value: 88

value: Generics

例3

以下代码声明并使用队列<E> 通用类型。

class Queue<E> {
  private E[] elements;
  private int head=0, tail=0;

  Queue(int size) {
    elements = (E[]) new Object[size];
  }

  void insert(E element) throws QueueFullException {
    if (isFull())
      throw new QueueFullException();
    elements[tail] = element;
    tail = (tail + 1) % elements.length;
  }

  E remove() throws QueueEmptyException {
    if (isEmpty()){
      throw new QueueEmptyException();
    }
    E element = elements[head];
    head = (head + 1) % elements.length;
    return element;
  }

  boolean isEmpty() {
    return head == tail;
  }

  boolean isFull() {
    return (tail + 1) % elements.length == head;
  }
}
class QueueEmptyException extends Exception {
}

class QueueFullException extends Exception {
}
public class Main{
  public static void main(String[] args) 
      throws QueueFullException, QueueEmptyException {
    Queue<String> queue = new Queue<String>(6);
    System.out.println("Empty: " + queue.isEmpty());
    System.out.println("Full: " + queue.isFull());
    queue.insert("A");
    queue.insert("B");
    queue.insert("C");
    queue.insert("D");
    queue.insert("E");
    System.out.println("Empty: " + queue.isEmpty());
    System.out.println("Full: " + queue.isFull());
    System.out.println("Removing " + queue.remove());
    System.out.println("Empty: " + queue.isEmpty());
    System.out.println("Full: " + queue.isFull());
    System.out.println("Adding F");
    queue.insert("F");
    while (!queue.isEmpty()){
      System.out.println("Removing " + queue.remove());
    }
      
    System.out.println("Empty: " + queue.isEmpty());
    System.out.println("Full: " + queue.isFull());
  }
}

输出:

Empty: true

Full: false

Empty: false

Full: true

Removing A

Empty: false

Full: false

Adding F

Removing B

Removing C

处理旧代码

为了处理向泛型的转换，Java允许使用没有任何类的通用类类型参数。

下面是一个显示原始类型的示例:

class MyClass<T> {
  T ob;
  MyClass(T o) {
    ob = o;
  }
  T getob() {
    return ob;
  }
}
public class Main {
  public static void main(String args[]) {
    MyClass raw = new MyClass(new Double(98.6));
    double d = (Double) raw.getob();
    System.out.println("value: " + d);
  }
}

输出:

value: 98.6