Java泛型 泛型类型的继承规则

泛型类型的继承规则可能不能凭直觉的来想当然。下面举个例子来看看泛型类型的继承规则。

首先有几个辅助类：

package generic;

public class Person extends Animal {
	private String name;

	public Person(String name) {
		super();
		this.name = name;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@Override
	public String toString() {
		return "Person [name=" + name + "]";
	}
}

package generic;

public class Student extends Person {
	private String studentNumber;

	public Student(String name, String registrationNumber) {
		super(name);
		this.studentNumber = registrationNumber;
	}

	public String getStudentNumber() {
		return studentNumber;
	}

	public void setStudentNumber(String studentNumber) {
		this.studentNumber = studentNumber;
	}

	@Override
	public String toString() {
		return "Student [studentNumber=" + studentNumber + ", name="
				+ getName() + "]";
	}

}

下面是测试类：

package generic;

import java.util.ArrayList;
import java.util.List;

public class Test2 {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		// test1
		/**
		 * test1说明，虽然Student是Person的子类，但是
		 * ArrayList<Student>并不是ArrayList<Person>
		 * 的子类
		 */
		ArrayList<Student> students= new ArrayList<Student>();
		// error
		/**
		 * Type mismatch: cannot convert
		 * from ArrayList<Student> to ArrayList<Person>
		 */
		ArrayList<Person> persons = students; // error

		// test2
		/**
		 * test2说明ArrayList<Person>是
		 * List<Student>的子类
		 */
		List<Student> students2 = students;

	}

}

test1和test2说明泛型类型只在类这个维度继承，类型变量必须一样，如ArrayList<E> implements List<E>。