如何按姓名（重名按姓氏）及薪资对Employee列表排序？

给定的Employee类和员工列表，要实现先按firstName排序，firstName相同则按lastName排序，最后按salary排序的逻辑，有以下几种常见实现方式：

方式一：自定义Comparator（兼容Java 8之前版本） #

通过匿名内部类实现Comparator<Employee>，重写compare方法依次比较三个字段：

Collections.sort(employeeList, new Comparator<Employee>() {
    @Override
    public int compare(Employee e1, Employee e2) {
        // 优先比较firstName
        int firstNameCompare = e1.getFirstName().compareTo(e2.getFirstName());
        if (firstNameCompare != 0) {
            return firstNameCompare;
        }
        // firstName相同时比较lastName
        int lastNameCompare = e1.getLastName().compareTo(e2.getLastName());
        if (lastNameCompare != 0) {
            return lastNameCompare;
        }
        // 前两个字段都相同时比较salary
        return e1.getSalary().compareTo(e2.getSalary());
    }
});

方式二：Java 8 Lambda表达式简化写法 #

用Lambda替代匿名内部类，逻辑和上述一致，代码更简洁：

Collections.sort(employeeList, (e1, e2) -> {
    int firstNameCompare = e1.getFirstName().compareTo(e2.getFirstName());
    if (firstNameCompare != 0) return firstNameCompare;
    int lastNameCompare = e1.getLastName().compareTo(e2.getLastName());
    if (lastNameCompare != 0) return lastNameCompare;
    return e1.getSalary().compareTo(e2.getSalary());
});

方式三：Java 8 Comparator链式调用（最简洁写法） #

利用Comparator.comparing方法链式指定排序字段，自动处理优先级：

employeeList.sort(Comparator.comparing(Employee::getFirstName)
        .thenComparing(Employee::getLastName)
        .thenComparing(Employee::getSalary));

如果需要对某个字段降序，可在对应字段后追加.reversed()，例如让salary按降序排列：

employeeList.sort(Comparator.comparing(Employee::getFirstName)
        .thenComparing(Employee::getLastName)
        .thenComparing(Employee::getSalary, Comparator.reverseOrder()));

方式四：让Employee实现Comparable接口 #

如果该排序逻辑是Employee的默认排序规则，可以让类实现Comparable<Employee>接口，重写compareTo方法：

class Employee implements Comparable<Employee> {
    String firstName;
    String lastName;
    Double salary;

    Employee(Double salary, String firstName, String lastName) {
        this.salary = salary;
        this.firstName = firstName;
        this.lastName = lastName;
    }

    // getter and setters

    @Override
    public int compareTo(Employee other) {
        int firstNameCompare = this.firstName.compareTo(other.firstName);
        if (firstNameCompare != 0) return firstNameCompare;
        int lastNameCompare = this.lastName.compareTo(other.lastName);
        if (lastNameCompare != 0) return lastNameCompare;
        return this.salary.compareTo(other.salary);
    }
}

之后直接调用列表的sort方法即可：

employeeList.sort(null);
// 或使用Collections.sort
Collections.sort(employeeList);

内容的提问来源于stack exchange，提问作者Abhijeet Ranade