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
1.8以来的新功能是一个List.sort()方法，而不是使用Collection.sort() 直接调用mylistcontainer。sort()
下面是一个演示List.sort()特性的代码片段:
List<Fruit> fruits = new ArrayList<Fruit>();
fruits.add(new Fruit("Kiwi","green",40));
fruits.add(new Fruit("Banana","yellow",100));
fruits.add(new Fruit("Apple","mixed green,red",120));
fruits.add(new Fruit("Cherry","red",10));
// a) using an existing compareto() method
fruits.sort((Fruit f1,Fruit f2) -> f1.getFruitName().compareTo(f2.getFruitName()));
System.out.println("Using String.compareTo(): " + fruits);
//Using String.compareTo(): [Apple is: mixed green,red, Banana is: yellow, Cherry is: red, Kiwi is: green]
// b) Using a comparable class
fruits.sort((Fruit f1,Fruit f2) -> f1.compareTo(f2));
System.out.println("Using a Comparable Fruit class (sort by color): " + fruits);
// Using a Comparable Fruit class (sort by color): [Kiwi is green, Apple is: mixed green,red, Cherry is: red, Banana is: yellow]
Fruit类是:
public class Fruit implements Comparable<Fruit>
{
private String name;
private String color;
private int quantity;
public Fruit(String name,String color,int quantity)
{ this.name = name; this.color = color; this.quantity = quantity; }
public String getFruitName() { return name; }
public String getColor() { return color; }
public int getQuantity() { return quantity; }
@Override public final int compareTo(Fruit f) // sorting the color
{
return this.color.compareTo(f.color);
}
@Override public String toString()
{
return (name + " is: " + color);
}
} // end of Fruit class
其他回答
你可以试试番石榴点菜:
Function<Item, Date> getStartDate = new Function<Item, Date>() {
public Date apply(Item item) {
return item.getStartDate();
}
};
List<Item> orderedItems = Ordering.natural().onResultOf(getStartDate).
sortedCopy(items);
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。
这些代码片段可能很有用。如果你想对一个对象进行排序 在我的例子中，我想按VolumeName排序:
public List<Volume> getSortedVolumes() throws SystemException {
List<Volume> volumes = VolumeLocalServiceUtil.getAllVolumes();
Collections.sort(volumes, new Comparator<Volume>() {
public int compare(Volume o1, Volume o2) {
Volume p1 = (Volume) o1;
Volume p2 = (Volume) o2;
return p1.getVolumeName().compareToIgnoreCase(
p2.getVolumeName());
}
});
return volumes;
}
这个作品。我在jsp中使用它。
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) {
...
}
});
是的，你可以。比较项有两个选项，Comparable接口和Comparator接口。
这两个接口都允许不同的行为。Comparable允许您使对象的行为与您刚刚描述的String类似(实际上，String实现了Comparable)。第二个，Comparator，允许你做你要求做的事情。你会这样做:
Collections.sort(myArrayList, new MyComparator());
这将导致收集。方法来使用比较器的排序机制。如果数组列表中的对象实现了可比性，你可以这样做:
Collections.sort(myArrayList);
Collections类包含许多这些有用的常用工具。
