我读过关于使用比较器排序数组列表的内容，但在所有的例子中，人们都使用了compareTo，根据一些研究，它是字符串的一种方法。
我想根据自定义对象的一个属性(Date对象)对其数组列表进行排序 (getStartDay())。通常我通过item1.getStartDate().before(item2.getStartDate())比较它们，所以我想知道我是否可以写一些像这样的东西:
public class CustomComparator {
public boolean compare(Object object1, Object object2) {
return object1.getStartDate().before(object2.getStartDate());
}
}
public class RandomName {
...
Collections.sort(Database.arrayList, new CustomComparator);
...
}
当前回答
好吧，如果你使用Java 8或更老的版本，这里是最好的解决方案。
Collections.sort(studentList, Comparator.comparing(Student::getCgpa).reversed().thenComparing(Student:: getFname).thenComparing(Student::getId));
在这种情况下，它将首先使用'getCgpa'进行排序，对于第二部分，它将使用getFname和getId进行排序。这是场进入pojo类。
其他回答
java8 lambda表达式
Collections.sort(studList, (Student s1, Student s2) ->{
return s1.getFirstName().compareToIgnoreCase(s2.getFirstName());
});
OR
Comparator<Student> c = (s1, s2) -> s1.firstName.compareTo(s2.firstName);
studList.sort(c)
我发现这些答案大部分(如果不是全部的话)依赖于底层类(Object)来实现类似的接口或具有类似的helper接口。
没有我的解决方案!下面的代码允许您通过知道对象的字符串名称来比较对象的字段。您可以很容易地修改它，不使用名称，但随后需要公开它或构造一个想要进行比较的对象。
Collections.sort(anArrayListOfSomeObjectPerhapsUsersOrSomething, new ReflectiveComparator(). new ListComparator("name"));
public class ReflectiveComparator {
public class FieldComparator implements Comparator<Object> {
private String fieldName;
public FieldComparator(String fieldName){
this.fieldName = fieldName;
}
@SuppressWarnings({ "unchecked", "rawtypes" })
@Override
public int compare(Object object1, Object object2) {
try {
Field field = object1.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
Comparable object1FieldValue = (Comparable) field.get(object1);
Comparable object2FieldValue = (Comparable) field.get(object2);
return object1FieldValue.compareTo(object2FieldValue);
}catch (Exception e){}
return 0;
}
}
public class ListComparator implements Comparator<Object> {
private String fieldName;
public ListComparator(String fieldName) {
this.fieldName = fieldName;
}
@SuppressWarnings({ "unchecked", "rawtypes" })
@Override
public int compare(Object object1, Object object2) {
try {
Field field = object1.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
Comparable o1FieldValue = (Comparable) field.get(object1);
Comparable o2FieldValue = (Comparable) field.get(object2);
if (o1FieldValue == null){ return -1;}
if (o2FieldValue == null){ return 1;}
return o1FieldValue.compareTo(o2FieldValue);
} catch (NoSuchFieldException e) {
throw new IllegalStateException("Field doesn't exist", e);
} catch (IllegalAccessException e) {
throw new IllegalStateException("Field inaccessible", e);
}
}
}
}
在Java 8中，你可以为你的比较器使用方法引用:
import static java.util.Comparator.comparing;
Collections.sort(list, comparing(MyObject::getStartDate));
具有自然排序顺序的类(例如类Number)应该实现Comparable接口，而没有自然排序顺序的类(例如类Chair)应该提供Comparator(或匿名Comparator类)。
两个例子:
public class Number implements Comparable<Number> {
private int value;
public Number(int value) { this.value = value; }
public int compareTo(Number anotherInstance) {
return this.value - anotherInstance.value;
}
}
public class Chair {
private int weight;
private int height;
public Chair(int weight, int height) {
this.weight = weight;
this.height = height;
}
/* Omitting getters and setters */
}
class ChairWeightComparator implements Comparator<Chair> {
public int compare(Chair chair1, Chair chair2) {
return chair1.getWeight() - chair2.getWeight();
}
}
class ChairHeightComparator implements Comparator<Chair> {
public int compare(Chair chair1, Chair chair2) {
return chair1.getHeight() - chair2.getHeight();
}
}
用法:
List<Number> numbers = new ArrayList<Number>();
...
Collections.sort(numbers);
List<Chair> chairs = new ArrayList<Chair>();
// Sort by weight:
Collections.sort(chairs, new ChairWeightComparator());
// Sort by height:
Collections.sort(chairs, new ChairHeightComparator());
// You can also create anonymous comparators;
// Sort by color:
Collections.sort(chairs, new Comparator<Chair>() {
public int compare(Chair chair1, Chair chair2) {
...
}
});
你的customComparator类必须实现java.util.Comparator才能被使用。 它还必须覆盖compare() AND equals()
compare()必须回答这个问题:对象1是否小于、等于或大于对象2?
完整文档:http://java.sun.com/j2se/1.5.0/docs/api/java/util/Comparator.html
