有没有更快的方法从对象集合中提取唯一值？

Question

我有一个方法从作为员工信息的对象集合中提取值:

public class Employee
{
    public String AREA;
    public String EMPLOYEE_ID;
    public String EMPLOYEE_NAME;
}

我想获得我所做的所有不同区域我认为更容易,只需检查ArrayList是否包含值,如果不是添加它,则需要187ms才能完成,:

    long startTime = System.currentTimeMillis();
    ArrayList<String> distinct_areas = new ArrayList<String>();
    for (int i = 0; i < this.employeeTress.length; i++)
    {
        if (!distinct_areas.contains(this.employeeTress[i].AREA))
            distinct_areas.add(this.employeeTress[i].AREA);
    }
    String[] unique = new String[distinct_areas.size()];
    distinct_areas.toArray(unique);
    long endTime = System.currentTimeMillis();
    System.out.println("Total execution time: " + (endTime - startTime) + "ms");

然后我想以不同的方式做它,看它是否变得更快,排序数组然后只检查最后一项如果它不同然后添加它,它更快一点,它需要121ms才能完成:

    startTime = System.currentTimeMillis();
    String[] vs = new String[this.employeeTress.length];
    for (int i = 0; i < this.employeeTress.length; i++)
    {
        vs[i] = this.employeeTress[i].AREA;
    }
    Arrays.sort(vs);
    ArrayList<String> vsunique = new ArrayList<String>();
    vsunique.add(vs[0]);
    for (int i = 0; i < vs.length; i++)
    {
        if (!vsunique.get(vsunique.size()-1).equals(vs[i]))
        {
            vsunique.add(vs[i]);
        }
    }
    String[] uni = new String[vsunique.size()];
    vsunique.toArray(uni);
    endTime = System.currentTimeMillis();
    System.out.println("Total execution time: " + (endTime - startTime) + "ms");

我是Java新手我想知道更好的方法.*注意,这段代码应该可以在android姜饼API LVL 10中使用.

Answer 1

如果要获取或计算员工列表中的不同区域,可以使用一组字符串.我正在更改变量名称以匹配Java标准.你可以事后得到计数.理想情况下,这些都是懒惰的方法.

命令代码

public Set<String> areas(final List<Employee> employees) {
    Set<String> areas = new HashSet<>();
    for(final Employee employee: employees) {
        areas.add(employee.getArea());
    }
    return areas;
}

功能代码(Google Guava)

public Set<String> areas(final List<Employee> employees) {
    return Sets.newHashSet(
        Lists.transform(employees, new Function<Employee, String>() {
            public String apply(Employee e) {
                return e.getArea();
            }
        }));
}

Lambdas(Java 8)

public Set<String> areas(final List<Employee> employees) {
    return new HashSet<String>(employees.map(e => e.getArea()));
}