我有一个ArrayList，其中包含一些重复值和三次出现的值，我想将三次出现的值专门收集到另一个ArrayList中，例如
Arraylist<String> strings; //contains all strings that are duplicates and that occur thrice
在这里，我只想获取在另一个数组列表中三次出现的字符串。
Arraylist<String> thrice; //contains only elements that occur three times.
当前，我有一个solution用于处理重复项，但我不能仅将其复制三次就不能扩展它，这请帮我找出来。
答案 0 :(得分：3)
您可以通过流执行以下操作：
List<String> result = strings.stream()
.collect(Collectors.groupingBy(Function.identity(), counting()))
.entrySet().stream()
.filter(e -> e.getValue() == 3) // keep only elements that occur 3 times
.map(Map.Entry::getKey)
.collect(Collectors.toList());
您也可以按照以下步骤进行操作，但是我建议您使用上面的方法，因为它更可取。
List<String> result = new HashSet<>(strings).stream()
.filter(item -> strings.stream()
.filter(e -> e.equals(item)).limit(3).count() == 3)
.collect(Collectors.toList());
答案 1 :(得分：2)
基本上，您现在不想使用 HashSet 来使用 HashMap 来计算每个字符串的出现次数。< / p>
此外，您无需编写一种方法来查找出现三次的特定字符串，而是可以编写一个方法，该方法采用一个参数n来查找出现 N 的字符串。时间：
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
class StackOverflowQ {
static List<String> getStringsThatOccurNTimes(List<String> stringList, int n) {
if (stringList == null || stringList.size() == 0) {
return stringList;
}
Set<String> stringsThatOccurNTimesSet = new HashSet<>();
Map<String, Integer> stringCounts = new HashMap<>();
for (String s : stringList) {
int currentStringCount = stringCounts.getOrDefault(s, 0) + 1;
stringCounts.put(s, currentStringCount);
if (currentStringCount == n) {
stringsThatOccurNTimesSet.add(s);
} else if (currentStringCount == n + 1) {
stringsThatOccurNTimesSet.remove(s); // We use a set so this operation is O(1)
}
}
return new ArrayList<>(stringsThatOccurNTimesSet);
}
public static void main(String[] args) {
List<String> stringsList = new ArrayList<>(Arrays.asList("a", "b", "c", "d", "e", "b", "c", "c", "d", "d", "d", "e"));
List<String> stringsThatOccurTwoTimes = getStringsThatOccurNTimes(stringsList, 2);
List<String> stringsThatOccurThreeTimes = getStringsThatOccurNTimes(stringsList, 3);
List<String> stringsThatOccurFourTimes = getStringsThatOccurNTimes(stringsList, 4);
System.out.println("Original list: " + stringsList);
System.out.println("Strings that occur two times: " + stringsThatOccurTwoTimes);
System.out.println("Strings that occur three times: " + stringsThatOccurThreeTimes);
System.out.println("Strings that occur four times: " + stringsThatOccurFourTimes);
}
}
输出：
Original list: [a, b, c, d, e, b, c, c, d, d, d, e]
Strings that occur two times: [b, e]
Strings that occur three times: [c]
Strings that occur four times: [d]
答案 2 :(得分：1)
最好的方法是.....
制作所需的数组列表.........
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.motion.MotionLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/motionLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@color/white"
android:fitsSystemWindows="true"
app:layoutDescription="@xml/scene_01"
tools:context=".MainActivity">
<android.support.constraint.ConstraintLayout
android:id="@+id/materialCardView2"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintVertical_weight="2.5"
android:background="@color/white"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/imageView4">
<android.support.design.card.MaterialCardView
android:id="@+id/materialCardView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginTop="32dp"
android:layout_marginEnd="16dp"
app:cardBackgroundColor="@color/white"
app:cardCornerRadius="4dp"
app:cardElevation="0dp"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:strokeColor="@color/colorPrimary"
app:strokeWidth="2dp">
<android.support.constraint.ConstraintLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/num"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginTop="16dp"
android:text="Phone Number"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/textView4"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:text=":"
app:layout_constraintBottom_toBottomOf="@+id/num"
app:layout_constraintStart_toEndOf="@+id/num"
app:layout_constraintTop_toTopOf="@+id/num"
app:layout_constraintVertical_bias="0.0" />
<TextView
android:id="@+id/textView5"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="16dp"
android:text="Email ID"
app:layout_constraintEnd_toEndOf="@+id/num"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="@+id/num"
app:layout_constraintTop_toBottomOf="@+id/num" />
<TextView
android:id="@+id/textView6"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text=":"
app:layout_constraintEnd_toEndOf="@+id/textView4"
app:layout_constraintStart_toStartOf="@+id/textView4"
app:layout_constraintTop_toTopOf="@+id/textView5" />
<TextView
android:id="@+id/textView7"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginTop="16dp"
android:layout_marginBottom="16dp"
android:text="Batch"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="@+id/textView5"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="@+id/textView5"
app:layout_constraintTop_toBottomOf="@+id/textView5" />
<TextView
android:id="@+id/textView8"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text=":"
app:layout_constraintEnd_toEndOf="@+id/textView6"
app:layout_constraintStart_toStartOf="@+id/textView6"
app:layout_constraintTop_toTopOf="@+id/textView7" />
<TextView
android:id="@+id/textView9"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginTop="16dp"
android:layout_marginEnd="16dp"
android:text="90000000000"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toEndOf="@+id/textView4"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/textView10"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginEnd="16dp"
android:text="username@gmail.com"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="1.0"
app:layout_constraintStart_toEndOf="@+id/textView6"
app:layout_constraintTop_toTopOf="@+id/textView6" />
<TextView
android:id="@+id/textView11"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginTop="16dp"
android:layout_marginEnd="16dp"
android:layout_marginBottom="16dp"
android:text="Kolkata"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toEndOf="@+id/textView8"
app:layout_constraintTop_toBottomOf="@+id/textView10" />
</android.support.constraint.ConstraintLayout>
</android.support.design.card.MaterialCardView>
<android.support.design.button.MaterialButton
android:id="@+id/button5"
style="@style/Button"
android:layout_width="0dp"
android:layout_height="74dp"
android:layout_marginEnd="16dp"
android:text="Payment Status"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toEndOf="@+id/button2"
app:layout_constraintTop_toTopOf="@+id/button2" />
<android.support.design.button.MaterialButton
android:id="@+id/button2"
style="@style/Button"
android:layout_width="0dp"
android:layout_height="74dp"
android:layout_marginStart="16dp"
android:layout_marginTop="32dp"
android:layout_marginEnd="16dp"
android:text="Admission Details"
app:layout_constraintEnd_toStartOf="@+id/button5"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/materialCardView" />
<android.support.design.button.MaterialButton
android:id="@+id/button3"
style="@style/Button"
android:layout_width="0dp"
android:layout_height="74dp"
android:layout_marginTop="16dp"
android:text="Rules and Regulation"
app:layout_constraintEnd_toEndOf="@+id/button2"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="@+id/button2"
app:layout_constraintTop_toBottomOf="@+id/button2" />
<android.support.design.button.MaterialButton
android:id="@+id/button4"
style="@style/Button"
android:layout_width="0dp"
android:layout_height="74dp"
android:text="Leave"
app:layout_constraintEnd_toEndOf="@+id/button5"
app:layout_constraintStart_toStartOf="@+id/button5"
app:layout_constraintTop_toTopOf="@+id/button3" />
<android.support.design.button.MaterialButton
android:id="@+id/button6"
style="@style/Button"
android:layout_width="184dp"
android:layout_height="74dp"
android:layout_marginStart="64dp"
android:layout_marginTop="16dp"
android:layout_marginEnd="64dp"
android:text="Attendance"
app:layout_constraintEnd_toEndOf="@+id/button4"
app:layout_constraintStart_toStartOf="@+id/button3"
app:layout_constraintTop_toBottomOf="@+id/button3" />
<android.support.design.button.MaterialButton
android:id="@+id/button7"
style="@style/Widget.MaterialComponents.Button.UnelevatedButton"
android:layout_width="wrap_content"
android:layout_height="50dp"
android:layout_marginEnd="12dp"
android:text="Log out"
app:icon="@drawable/exit"
app:layout_constraintBottom_toTopOf="@+id/space3"
app:layout_constraintEnd_toEndOf="@+id/materialCardView" />
<Space
android:id="@+id/space3"
android:layout_width="1dp"
android:layout_height="15dp"
android:layout_marginStart="8dp"
app:layout_constraintBottom_toBottomOf="@+id/materialCardView"
app:layout_constraintEnd_toEndOf="@+id/materialCardView"
app:layout_constraintHorizontal_bias="0.882"
app:layout_constraintStart_toStartOf="@+id/button7"
app:layout_constraintTop_toTopOf="@+id/materialCardView"
app:layout_constraintVertical_bias="0.227" />
</android.support.constraint.ConstraintLayout>
<ImageView
android:id="@+id/imageView4"
android:layout_width="wrap_content"
android:layout_height="0dp"
app:layout_constraintVertical_weight="1"
android:scaleType="centerCrop"
app:layout_constraintBottom_toTopOf="@+id/materialCardView2"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.5"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.0"
app:srcCompat="@drawable/loginn" />
<Space
android:id="@+id/space"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintBottom_toBottomOf="@+id/imageView4"
app:layout_constraintEnd_toEndOf="@+id/imageView4"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="@+id/imageView4"
app:layout_constraintTop_toTopOf="@+id/imageView4"
app:layout_constraintVertical_bias="1.0" />
<TextView
android:id="@+id/textView"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginEnd="16dp"
android:layout_marginBottom="8dp"
android:text="Firstname Lastname"
android:textSize="20sp"
android:textStyle="bold"
android:textColor="@color/white"
app:layout_constraintBottom_toTopOf="@+id/materialCardView2"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/imageView4"
app:layout_constraintVertical_bias="1.0" />
<com.mikhaellopez.circularimageview.CircularImageView
android:id="@+id/profileImage"
android:layout_width="133dp"
android:layout_height="0dp"
android:layout_marginTop="16dp"
android:layout_marginBottom="8dp"
android:padding="16dp"
android:src="@drawable/loginn"
app:civ_border_color="@color/white"
app:civ_border_width="1dp"
app:civ_shadow="true"
app:civ_shadow_color="#000000"
app:civ_shadow_radius="0"
app:layout_constraintBottom_toTopOf="@+id/textView"
app:layout_constraintEnd_toEndOf="@+id/space"
app:layout_constraintStart_toStartOf="@+id/space"
app:layout_constraintTop_toTopOf="@+id/space"
app:layout_constraintVertical_bias="1.0" />
</android.support.constraint.motion.MotionLayout>
添加一些值进行检查....
ArrayList<String> thrice=new ArrayList<>();
ArrayList<String> one=new ArrayList<>();
映射完成任务.....
one.add("1");one.add("1");one.add("1");one.add("1");one.add("1");
one.add("2");one.add("2");one.add("2");one.add("2");one.add("2");
one.add("1");one.add("1");one.add("1");
one.add("3");one.add("3");
one.add("4");one.add("5");
one.add("2");one.add("2");one.add("2");
在列表视图中设置列表...
Map<String, Integer> duplicates = new HashMap<String, Integer>();
for (String str : one) {
if (duplicates.containsKey(str)) {
duplicates.put(str, duplicates.get(str) + 1);
} else {
duplicates.put(str, 1);
}
}
for (Map.Entry<String, Integer> entry : duplicates.entrySet()) {
if(entry.getValue()>=3){
thrice.add(entry.getKey());
}
}
输出= 1,2
已经检查了答案，您可以根据自己的方式更改条件
ArrayAdapter<String> ad=new ArrayAdapter<>
(this,android.R.layout.simple_list_item_1,thrice);
ListView lv=findViewById(R.id.new_l);
lv.setAdapter(ad);
上面最简单的答案...不需要为Android用户更改最低sdk的版本
答案 3 :(得分：0)
您可以为此使用computeIfAbsent。
List<String> strings;
final Map<String, BigInteger> stringCounts = new HashMap<>();
strings.stream().forEach(s -> {
BigInteger count = stringCounts.computeIfAbsent(s, k -> BigInteger.ZERO).add(BigInteger.ONE);
stringCounts.put(s, count);
});
现在使用stringCounts过滤value=3。
答案 4 :(得分：0)
您要尝试解决的两个问题中的通用工具将Collections.frequency用作：
/**
* @param input the list as your input
* @param n number of occurrence (duplicates:2 , triplets:3 etc..)
* @param <T> (type of elements)
* @return elements with such conditional occurrent in a Set
*/
static <T> Set<T> findElementsWithNOccurrence(List<T> input, int n) {
return input.stream()
.filter(a -> Collections.frequency(input, a) == n) // filter by number of occurrences
.collect(Collectors.toSet()); // collecting to a final set (one representation of each)
}
注意 ：这将是一种 O(n^2) 的方法，因为它使用了Collections.frequency来遍历整个集合再次获得频率。但建议针对您要的内容采用更具可读性和通用性的方法。此外，这有意将最终输出收集到Set，因为List毕竟可以再次具有重复项。
或者，您可以使用方法count the frequency of elements in Java-8并迭代创建的Map的条目，从而根据需要处理过滤并在同一迭代中收集输出：
/**
* @param input the list as your input
* @param n number of occurrence (duplicates :2 , triplets :3 etc)
* @param <T> (type of elements)
* @return elements in a set
*/
static <T> Set<T> findElementsWithNOccurrence(List<T> input, int n) {
return input.stream() // Stream<T>
.collect(Collectors.groupingBy(Function.identity(),
Collectors.counting())) // Map<T, Long>
.entrySet() // Set<Map.Entry<T,Long>>
.stream() // Stream<Map.Entry<T,Long>>
.filter(e -> e.getValue() == n) // filtered with frequency 'n'
.map(Map.Entry::getKey) // Stream<T>
.collect(Collectors.toSet()); // collect to Set
}