まず自作クラス

public class Employee {
	
	private String name;
    private int age;
    
    public Employee(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
}

listに要素を入れソートしてみる

public class Array {

	public static void main(String[] args) {
			
		List<Employee> list = new ArrayList<Employee>();
		list.add(new Employee("tanaka", 25));
		list.add(new Employee("yamada", 28));
		list.add(new Employee("suzuki", 20));
			
		Comparator<Employee> compare = Comparator.comparing(Employee::getAge);
		list.sort(compare);
			
		for (Employee e : list) {
			System.out.println(e.getName() + " : " + e.getAge());
		}
	}
}

出力結果は下記のようになる

suzuki : 20
tanaka : 25
yamada : 28

昇順の場合、Comparator.comparing()の第二引数は省略可能
降順の場合は第二引数にComparator.reverseOrder()を指定する必要がある

Comparator<Employee> compare = Comparator.comparing(Employee::getAge,Comparator.reverseOrder());