Java中T和?的区别

T和？的区别

@Test

public void testOne(){

// ?做对象的泛型来用

List<?> list = findAllList(1);

list.forEach(temp ->{

System.out.print(temp);

});

System.out.println();

List<?> lists = findAllList(2);

lists.forEach(temp ->{

System.out.print(temp);

});

//T做类的泛型，当输入的类型是One的时候，只能用One接

One one = new Class<One>().newInstance();

//而不能用Two接

One one = new Class<Two>().newInstance();

}

class Two{

private Two(){

System.out.println("Two");

}

}

static class Class<T>{

T newInstance() {

return (T) new One();

}

}

private static class One{

private One(){

System.out.println("One");

}

}

private List<?> findAllList(Integer type){

if (type == 1){

List<Integer> list = new ArrayList<>();

list.add(1);

list.add(2);

return list;

}else {

List<String> list = new ArrayList<>();

list.add("AA");

list.add("BB");

return list;

}

}