Java泛型方法定义及泛型类型推断

泛型的推断

    @Test
    public void test3(){
        //类型推断时使用两个类型的最小公倍数
        int x1 = add(3,4);
        Number x2 = add(3.5,4);
        Object obj = add(3,"abc");
    }
    //在返回类型前加上<T>
    public <T> T add(T x,T y){
        return x;
    }

传入一个数组，交换其中的两项：

    @Test
    public void test4(){
        String[] result = swap(new String[]{"abc","xyz","999"}, 1, 2);
        for(String s:result){
            System.out.println(s);
        }
        Book[] bs = swap(new Book[]{new Book("三国演义","罗贯中"),new Book("水浒传","施耐庵"),new Book("红楼梦","曹雪芹")}, 0, 2);
        for(Book b:bs){
            System.out.println(b.getName()+","+b.getAuthor());
        }
        //以下做法错误,T不可以为基本类型
        //int[] arr = swap(new int[]{1,2,3},1,2);
    }
    //在返回类型前加上<T>
    public <T> T[] swap(T[] a,int i,int j){
        T temp = a[i];
        a[i] = a[j];
        a[j] = temp;
        return a;
    }

定义泛型时限定类型：

    @Test
    public void test4(){
        String[] result = swap(new String[]{"abc","xyz","999"}, 1, 2);
        for(String s:result){
            System.out.println(s);
        }
        Book[] bs = swap(new Book[]{new Book("三国演义","罗贯中"),new Book("水浒传","施耐庵"),new Book("红楼梦","曹雪芹")}, 0, 2);
        for(Book b:bs){
            System.out.println(b.getName()+","+b.getAuthor());
        }
        //以下做法错误,T不可以为基本类型
        //int[] arr = swap(new int[]{1,2,3},1,2);
    }
    //在返回类型前加上<T>
    public <T extends Object> T[] swap(T[] a,int i,int j){
        T temp = a[i];
        a[i] = a[j];
        a[j] = temp;
        return a;
    }

上面代码要求T要是Object的子类，这其实是废话，如果把Object改为Number则编译时上面的两个方法都会报错，因为String和Book类都不是Number的子类。