[英]How to use Collections.sort() in Java?
I got an object Recipe
that implements Comparable<Recipe>
: 我有一个实现Comparable<Recipe>
的对象Recipe
:
public int compareTo(Recipe otherRecipe) {
return this.inputRecipeName.compareTo(otherRecipe.inputRecipeName);
}
I've done that so I'm able to sort the List
alphabetically in the following method: 我已经这样做了所以我能够按以下方法按字母顺序对List
进行排序:
public static Collection<Recipe> getRecipes(){
List<Recipe> recipes = new ArrayList<Recipe>(RECIPE_MAP.values());
Collections.sort(recipes);
return recipes;
}
But now, in a different method, lets call it getRecipesSort()
, I want to sort the same list but numerically, comparing a variable that contains their ID. 但是现在,在另一种方法中,让我们将其getRecipesSort()
,我想对数据进行排序,但是数字,比较包含其ID的变量。 To make things worse, the ID field is of the type String
. 更糟糕的是,ID字段的类型为String
。
How do I use Collections.sort() to perform the sorts in Java? 如何使用Collections.sort()在Java中执行排序?
Use this method Collections.sort(List,Comparator) . 使用此方法Collections.sort(List,Comparator) 。 Implement a Comparator and pass it to Collections.sort().
实现Comparator并将其传递给Collections.sort().
class RecipeCompare implements Comparator<Recipe> {
@Override
public int compare(Recipe o1, Recipe o2) {
// write comparison logic here like below , it's just a sample
return o1.getID().compareTo(o2.getID());
}
}
Then use the Comparator
as 然后使用Comparator
作为
Collections.sort(recipes,new RecipeCompare());
The answer given by NINCOMPOOP can be made simpler using Lambda Expressions: 使用Lambda表达式可以使NINCOMPOOP给出的答案更简单:
Collections.sort(recipes, (Recipe r1, Recipe r2) ->
r1.getID().compareTo(r2.getID()));
Also introduced after Java 8 is the comparator construction methods in the Comparator interface. Java 8之后还介绍了Comparator接口中的比较器构造方法。 Using these, one can further reduce this to 1 : 使用这些,可以进一步将其减少到1 :
recipes.sort(comparingInt(Recipe::getId));
1 Bloch, J. Effective Java (3 rd Edition). 1布洛赫,J. 有效的Java( 第三版 )。 2018. Item 42, p. 2018.项目42,p。 194. 194。
Create a comparator which accepts the compare mode in its constructor and pass different modes for different scenarios based on your requirement 创建一个比较器,在其构造函数中接受比较模式,并根据您的要求为不同的场景传递不同的模式
public class RecipeComparator implements Comparator<Recipe> {
public static final int COMPARE_BY_ID = 0;
public static final int COMPARE_BY_NAME = 1;
private int compare_mode = COMPARE_BY_NAME;
public RecipeComparator() {
}
public RecipeComparator(int compare_mode) {
this.compare_mode = compare_mode;
}
@Override
public int compare(Recipe o1, Recipe o2) {
switch (compare_mode) {
case COMPARE_BY_ID:
return o1.getId().compareTo(o2.getId());
default:
return o1.getInputRecipeName().compareTo(o2.getInputRecipeName());
}
}
} }
Actually for numbers you need to handle them separately check below 实际上对于数字你需要单独处理它们检查下面
public static void main(String[] args) {
String string1 = "1";
String string2 = "2";
String string11 = "11";
System.out.println(string1.compareTo(string2));
System.out.println(string2.compareTo(string11));// expected -1 returns 1
// to compare numbers you actually need to do something like this
int number2 = Integer.valueOf(string1);
int number11 = Integer.valueOf(string11);
int compareTo = number2 > number11 ? 1 : (number2 < number11 ? -1 : 0) ;
System.out.println(compareTo);// prints -1
}
Use the method that accepts a Comparator
when you want to sort in something other than natural order. 如果要对自然顺序以外的其他内容进行排序,请使用接受Comparator
的方法。
Collections.sort(List, Comparator) Collections.sort(List,Comparator)
Sort the unsorted hashmap in ascending order. 按升序对未排序的hashmap进行排序。
// Sorting the list based on values
Collections.sort(list, new Comparator<Entry<String, Integer>>() {
public int compare(Entry<String, Integer> o1, Entry<String, Integer> o2)
{
return o2.getValue().compareTo(o1.getValue());
}
});
// Maintaining insertion order with the help of LinkedList
Map<String, Integer> sortedMap = new LinkedHashMap<String, Integer>();
for (Entry<String, Integer> entry : list) {
sortedMap.put(entry.getKey(), entry.getValue());
}
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