极客教程Java 把一个列表分成两半
这里我们给了一个列表，任务是把它分成两个新闻列表，从下面的示例中我们可以更好地了解到。
示例
Input : list = {1, 2, 3, 4, 5, 6}
Output : first = {1, 2, 3}, second = {4, 5, 6}
Input : list = {1, 2, 3, 4, 5}
Output : first = {1, 2}, second = {3, 4, 5}
方法
- 使用循环（Naive Approach）。
- 使用List类的subList()方法
- 使用收集器类的partitioningBy()方法
- 使用Google guava库
让我们来讨论一下上述定义的方法的细节，并通过干净的java程序来实现，如下所示。
方法1： 使用循环
方法
- 创建两个新的空列表，并将原始列表的前一半元素。
- 重置到第二个空列表中。
例子
// Java Program to Split a List into Two Sublist
// Importing required classes
import java.util.ArrayList;
import java.util.List;
// Main class
public class GFG {
// Method 1
// To split a list into two sublists in Java
public static List[] split(List<String> list)
{
// Creating two empty lists
List<String> first = new ArrayList<String>();
List<String> second = new ArrayList<String>();
// Getting size of the list
// using size() method
int size = list.size();
// Step 1
// (First size)/2 element copy into list
// first and rest second list
for (int i = 0; i < size / 2; i++)
first.add(list.get(i));
// Step 2
// (Second size)/2 element copy into list first and
// rest second list
for (int i = size / 2; i < size; i++)
second.add(list.get(i));
// Returning a List of array
return new List[] { first, second };
}
// Method 2
// Main driver method
public static void main(String[] args)
{
// Creating an ArrayList of string type
List<String> list = new ArrayList<String>();
// Adding elements to list object
// using add() method
list.add("Geeks");
list.add("Practice");
list.add("Contribute");
list.add("IDE");
list.add("Courses");
// Calling split method which return List of array
List[] lists = split(list);
// Printing specific elements of list by
// passing arguments with in
System.out.println(lists[0]);
System.out.println(lists[1]);
}
}
输出
[Geeks, Practice]
[Contribute, IDE, Courses]
方法2： 使用List类的subList()方法
它返回该列表中从指定索引（include）到另一个索引（exclude）之间的部分的视图。例如，让我们任意地从2到5，这里的索引2将只包括。如果两个指定的索引相等，返回的列表是空的。List.subList()返回的是一个列表，所以返回的列表没有结构性变化。
例子
// Java Program to Split a List into Two SubList
// Using subList() method of List class
// Importing required classes
import java.util.ArrayList;
import java.util.List;
// Main class
public class GFG {
// Method 1
// To split a list into two sublists in Java
public static List[] split(List<String> list)
{
// Finding the size of the list using List.size()
// and putting in a variable
int size = list.size();
// Creating new list and inserting values which is
// returned by List.subList() method
List<String> first
= new ArrayList<>(list.subList(0, (size) / 2));
List<String> second = new ArrayList<>(
list.subList((size) / 2, size));
// Returning an List of array
return new List[] { first, second };
}
// Method 2
// Main driver method
public static void main(String[] args)
{
// Creatingan ArrayList of String type
List<String> list = new ArrayList<String>();
// Adding elements to List object
// Custom input elements
list.add("Geeks");
list.add("Practice");
list.add("Contribute");
list.add("IDE");
list.add("Courses");
// Calling split method which return List of array
List[] lists = split(list);
// Printing specific elements of list by
// passing arguments with in
System.out.println(lists[0]);
System.out.println(lists[1]);
}
}
输出
[Geeks, Practice]
[Contribute, IDE, Courses]
方法3： 使用收集器类的partitioningBy()方法
Java 8 Collectors.partitioningBy()是一个将流的元素总是分成两部分的方法，与Naive和List.subList()不同。它返回一个收集器，该收集器将值存储在一个Map中。该地图的键只能是布尔值。
语法： partitioningBy()方法
public static Collector<T, ?, Map<Boolean, List>>
partitioningBy(Predicate predicate)
例子
// Java Program to Split a List into Two Sub-List
/// Using partitioningBy() method of Collectors class
// Importing required classes
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
// Main class
public class GFG {
// Method 1
// To split a list into two sublists in Java
public static List[] split(List<String> list)
{
// Setting value of midIndex using comparators
int midIndex
= ((list.size() / 2)
- (((list.size() % 2) > 0) ? 0 : 1));
// Creating object of List with reference to
// ArrayList class Declaring object List<String>
// type
List<List<String> > lists = new ArrayList<>(
list.stream()
.collect(Collectors.partitioningBy(
s -> list.indexOf(s) > midIndex))
.values());
// Returning an array containing both lists
return new List[] { lists.get(0), lists.get(1) };
}
// Method 2
// Main driver method
public static void main(String[] args)
{
// Creating an ArrayList of String type
List<String> list = new ArrayList<String>();
// Adding elements to List object
// Using add() method
list.add("Geeks");
list.add("Practice");
list.add("Contribute");
list.add("IDE");
list.add("Courses");
// Calling split method which return List of array
List[] lists = split(list);
// Printing specific elements of list by
// passing arguments with in
System.out.println(lists[0]);
System.out.println(lists[1]);
}
}
输出
[Geeks, Practice, Contribute]
[IDE, Courses]
方法4： 使用Google guava库
Guava是一个开源的基于Java的库，由Google公司开发。在Guava库中，我们可以使用Lists.partition()方法将列表分割成连续的子列表，每个列表都有指定的大小。为了将列表分割成两个子列表，在我们的例子中，我们可以传入等于我们列表一半大小的大小。
例子
// Java Program to Split a List into Two Sub-List
// Importing Guava library
import com.google.common.collect.Iterables;
// Importing required classes
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
// Main class
public class GFG {
// Method 1
// To split a list into two sublists in Java
public static List[] split(List<String> list)
{
// Partition the List into two sublists and
// getting iterator
Iterator<List<String> > itr
= Iterables.partition(list, (list.size()) / 2)
.iterator();
// Returning an array containing both lists
return new List[] { new ArrayList<>(itr.next()),
new ArrayList<>(itr.next()) };
}
// Method 2
// Main driver method
public static void main(String[] args)
{
// Creating an ArrayList of string type
List<String> list = new ArrayList<String>();
// Adding elements t oabove object
// Custom input elements
list.add("Geeks");
list.add("Practice");
list.add("Contribute");
list.add("IDE");
list.add("Courses");
// Calling split method which return List of array
List[] lists = split(list);
// Printing specific elements of list by
// passing arguments with in
System.out.println(lists[0]);
System.out.println(lists[1]);
}
}
输出
[Geeks, Practice]
[Contribute, IDE]