java泛型的一些常见用法

来源：互联网发布：苏州软件测试编辑：程序博客网时间：2024/05/19 13:28

本文主要参考下面几篇文章：

http://blog.csdn.net/seu_calvin/article/details/52230032

http://blog.csdn.net/orzlzro/article/details/7017435

http://sharewind.iteye.com/blog/1622164

https://www.zhihu.com/question/20400700

1、泛型类(接口)的写法以及继承

// 类似java.util.function.Functionpublic interface GenericInterface<T, R> {    public R apply(T t);}// 类似java.util.stream.IntStreampublic interface InheritGeneric0 extends GenericInterface<String, Integer> {}// 类似java.util.function.UnaryOperatorpublic interface InheritGeneric1<T> extends GenericInterface<T, T> {}// 类似java.util.HashMappublic interface InheritGeneric2<T, R> extends GenericInterface<T, R> {}// JDK库尚未找到类似的public interface InheritGeneric3<V, T, R> extends GenericInterface<T, R> {    public V apply2(T t, R r);}

2、泛型方法的写法

public class Retryer<T> {    private T value;    private int id = 0;    public Retryer() {    }    public Retryer(int id) {        this.id = id;    }    public T getValue() {        return value;    }    public void setValue(T value) {        this.value = value;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }}// 泛型的声明：方法的修饰符(public,static,final,abstract等)之后, 返回值(void,int等)之前public class SpecificClass {    // 入参泛型,返回值非泛型    public <L, R> int sum(L[] left, R[] right) {        return left.length + right.length;    }    // 入参非泛型,返回值泛型    public static <T> Retryer<T> build() {        return new Retryer<T>();    }    // 入参泛型,返回值泛型    public <L, R, V> Retryer<V> buildLength(L[] left, R[] right) {        return new Retryer<V>(left.length + right.length);    }    // 限制泛型: 只能是某个类的子类或者只能是实现了某些接口的类    // <T extends Comparable>    // <T extends Number&Serializable>    // <T extends Comparable&Serializable>    public <L extends Number, R extends Number> double add(L left, R right) {        return left.doubleValue() + right.doubleValue();    }}

3、泛型的一些基本特性：不能实例化对象和数组

import java.lang.reflect.Array;import java.lang.reflect.Method;import java.util.ArrayList;import java.util.List;public class GenericBasicFeature {    public static void main(String[] args) {        test1();        String[] stringArray = (String[]) Array.newInstance(String.class, 5);        System.out.println("stringArray.length=" + stringArray.length);        String[] stringArray2 = GenericBasicFeature.test2(5, String.class);        System.out.println("stringArray2.length=" + stringArray2.length);    }    // 泛型只在编译期有效    // http://blog.csdn.net/seu_calvin/article/details/52230032    public static void test1() {        List<String> stringList = new ArrayList<String>();        stringList.add("aaa");        // 编译报错        // stringList.add(new Object());        // 运行时通过反射添加元素        try {            Class clazz = stringList.getClass();            Method method = clazz.getMethod("add", Object.class);            method.invoke(stringList, 100);        } catch (Exception e) {            e.printStackTrace();        }        // [aaa, 100]        System.out.println(stringList);    }    // java不支持创建泛型数组    // http://blog.csdn.net/orzlzro/article/details/7017435    public static <T> T[] test2(int length, Class<T> newType) {        // T t = new T(); 编译错误        // T[] wrong = new T[length]; 编译错误        // T[] newArray = (T[]) new Object[length]; 运行时ClassCastException        // 参考Arrays.copyOf        T[] newArray = (T[]) Array.newInstance(newType, length);        return newArray;    }}

4、泛型的super和extend的PECS原则

// PECS（Producer Extends Consumer Super）public class Demo {    static class Food {    }    static class Fruit extends Food {    }    static class Apple extends Fruit {    }    public void testExtend() {        List<? extends Food> flist = new ArrayList<Food>();        flist.add(new Apple());  //compile error        flist.add(new Fruit()); //compile error        flist.add(new Object()); //compile error        Food fruit = flist.get(0); //ok    }    public void testSuper() {        List<? super Food> flist = new ArrayList<Food>();        flist.add(new Food()); //ok        flist.add(new Fruit()); //ok        flist.add(new Apple()); //ok        Food f = flist.get(0);// compile error    }}

5、一种使用例子

public class ExtendsContainer<T> {    private T data;    public ExtendsContainer(T data) {        this.data = data;    }    public T getData() {        return data;    }    public void setData(T data) {        this.data = data;    }}public class Demo {    static class Food {    }    static class Fruit extends Food {    }    static class Apple extends Fruit {    }    public static void main(String[] args) {        ExtendsContainer<Food> food = new ExtendsContainer<Food>(new Food());        ExtendsContainer<Fruit> fruit = new ExtendsContainer<Fruit>(new Fruit());        ExtendsContainer<Apple> apple = new ExtendsContainer<Apple>(new Apple());        ExtendsContainer<String> string = new ExtendsContainer<String>("abc");        getDataNoLimit(food);        getDataNoLimit(fruit);        getDataNoLimit(apple);        getDataNoLimit(string);        getDataOnlyFood(food);        getDataOnlyFood(fruit);        getDataOnlyFood(apple);        getDataOnlyFood(string);        getDataAnyFood(food);        getDataAnyFood(fruit);        getDataAnyFood(apple);        getDataAnyFood(string);    }    public static void getDataNoLimit(ExtendsContainer<?> container) {        System.out.println("data:" + container.getData());    }    public static void getDataOnlyFood(ExtendsContainer<Food> container) {        System.out.println("data:" + container.getData());    }    public static void getDataAnyFood(ExtendsContainer<? extends Food> data) {        System.out.println("data :" + data.getData());    }}

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