Question

我在测试方法（ displayEmployees ）时遇到问题，该方法返回对象的列表。

public class Task {
    static List<Employee> employeeList;

    public static void main(String[] args) {
        Employee employee1 = new Employee("Dobrobaba", "Irina", "Ivanovna",
                "Moskva", 1900, 6);
        Employee employee2 = new Employee("Shmal", "Anna", "Nikolaevna",
                "Krasnodar", 2017, 8);
        Employee employee3 = new Employee("Kerimova", "Niseimhalum", "Magomedmirzaevna",
                "New-York", 2017, 3);
        Employee employee4 = new Employee("Dobryden", "Yuri", "Viktorovich",
                "Auckland", 2014, 11);
        Employee employee5 = new Employee("Lopata", "Leonid", "Nikolaevich",
                "Beijing", 2014, 11);
        employeeList = new ArrayList<>(Arrays.asList(employee1, employee2, employee3, employee4, employee5));
    }

    /**
     * Prints employees' information, which have worked more than 'n' year(s) for now.
     *
     * @param n years quantity
     */
    public static List<Object> displayEmployees(int n) {
        List<Employee> requiredList = new ArrayList<>();
            employeeList.forEach(employee -> {
                if (Year.now().getValue() - employee.getEmploymentYear() >= n) {
                    requiredList.add(employee);
                }
            });
        return Collections.singletonList(requiredList);
    }
}

这就是我要进行测试的方法，将2个列表相互比较：

public class TaskTest {
    public static void main(String[] args) {
        displayEmployeesTest();
    }

    public static void assertEquals(String testName, List<Object> expected, List<Object> actual) {
        if (expected.equals(actual)) {
            System.out.println(testName + " passed");
        } else {
            System.out.println(testName + " failed: expected " + expected + ", actual " + actual);
        }
    }

    private static void displayEmployeesTest() {
        List<Object> expectedList;
        Employee employee1 = new Employee("Dobrobaba", "Irina", "Ivanovna",
                "Moskva", 1900, 6);
        Employee employee2 = new Employee("Shmal", "Anna", "Nikolaevna",
                "Krasnodar", 2017, 8);
        Employee employee3 = new Employee("Kerimova", "Niseimhalum", "Magomedmirzaevna",
                "New-York", 2017, 3);
        Employee employee4 = new Employee("Dobryden", "Yuri", "Viktorovich",
                "Auckland", 2014, 11);
        Employee employee5 = new Employee("Lopata", "Leonid", "Nikolaevich",
                "Beijing", 2014, 11);
        expectedList = new ArrayList<>(Arrays.asList(employee1, employee4, employee5));

        assertEquals("displayEmployeesTest", expectedList, displayEmployees(3));
    }
}

问题是-运行 TaskTest 时，出现错误“ java.lang.NullPointerException” 。

该错误发生的原因是什么？谢谢您的关注！

Answer 1

要比较两个列表，可以使用列表的equals方法。但是，为了使用此方法，列表的内容类必须覆盖equals。

employeeList1.equals(employeeList2)

并且您的Employee类必须重写等于：

public class Employee {
    private String name;
    private int id;
    ...

    //getters and setters

    ...

    public boolean equals(Object obj) {
        if(obj == null) return false;
        if(obj instanceof Employee) {
            Employee e = (Employee) obj;
            return this.name.equals(e.name) && this.id == e.id;
        } else return false;
    }
}

建议使用eclipse或Intellij这样的IDE来实现equals

由于static List<Employee> employeeList正在用您的测试方法从未调用过的main方法中初始化，因此您得到了一个空指针。

为了创建美观而整洁的测试，您应该使用Junit。检查以下内容：http://www.vogella.com/tutorials/JUnit/article.html

希望这会有所帮助

Answer 2

您调用Task.displayEmployees时不会调用Task.main，因此Task中的employeeList字段不会被初始化

Answer 3

如果要比较创建的对象，则可以让该类实现可比较的接口。该方法只有一个方法，即comparTo，并重写该方法以根据给定的属性比较员工。使用预定义的comparTo可能无法像您想要比较员工那样

包含测试失败Java的列表

3 个答案: