Collections常用方法梳理

Collections (java.util.Collections) 工具類包含了很多有關集合操作的靜態方法，使用這些方法能幫我們簡化代碼。

1. 獲取List中的最小值

Java代碼

List<Integer> intList = Arrays.asList(33, 24, 18, 6, 9, 99);
// 6
System.out.println(java.util.Collections.min(intList));

2. 獲取List中的最大值

Java代碼

List<Integer> intList = Arrays.asList(33, 24, 18, 6, 9, 99);
// 99
System.out.println(java.util.Collections.max(intList));

3. Shuffle

Shuffle方法可以使一個集合的元素亂序化。比如，一副牌的集合爲cardList (類型List<Card>), 使用Collections.shuffle(cardList)就能使一副牌處於亂序，達到洗牌的目的。

Java代碼

List<Integer> intList = Arrays.asList(33, 24, 18, 6, 9, 99);
Collections.shuffle(intList);
// 一次測試的結果
// [6, 18, 33, 24, 99, 9]
System.out.println(intList);

4. nCopies

該方法用於返回一個不可變列表組成的n個拷貝的指定對象。

Java代碼

// 生成一個由10個100組成的整數列表
List<Integer> nCopiesList = Collections.nCopies(10, 100);
//[100, 100, 100, 100, 100, 100, 100, 100, 100, 100]
System.out.println(nCopiesList);

5. sort

該方法用於對集合排序。

Java代碼

List<Integer> intList = Arrays.asList(33, 24, 18, 6, 9, 99);
Collections.sort(intList);

上述例子沒有包含Comparator參數,。我們也可以結合Comparator對對象集合進行排序。比如對存放Person類的對象集按照年齡進行排序。

Java代碼

package com.thecodesmaple.example.collection;
public class Person {
private int age;
private String firstName;
private String lastName;
public Person(int age, String firstName, String lastName) {
this.age = age;
this.firstName = firstName;
this.lastName = lastName;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String toString() {
return "Age: " + age + " FirstName " + firstName + " LastName: "
+ lastName;
}
}

Java代碼

List<Person> personList = Arrays.asList(new Person(21, "Eric", "W"),
new Person(17, "John", "A"), new Person(28, "Allan", "H"),
new Person(15, "Jonas", "B"));
// [Age: 21 FirstName Eric LastName: W, Age: 17 FirstName John LastName:
// A, Age: 28 FirstName Allan LastName: H, Age: 15 FirstName Jonas
// LastName: B]
System.out.println(personList);
Collections.sort(personList, new Comparator<Person>() {
@Override
public int compare(Person p1, Person p2) {
return p1.getAge() - p2.getAge();
}
});
// [Age: 15 FirstName Jonas LastName: B, Age: 17 FirstName John
// LastName: A, Age: 21 FirstName Eric LastName: W, Age: 28 FirstName
// Allan LastName: H]
System.out.println("按照年齡排序後:");
System.out.println(personList);

6. binarySearch

Java代碼

List<Integer> intList = Arrays.asList(33, 24, 18, 6, 9, 99);
// 2
System.out.println(Collections.binarySearch(intList, 18));

7. copy

用兩個參數，一個目標 List 和一個源 List, 將源的元素拷貝到目標，並覆蓋它的內容。目標 List至少與源一樣長。

幫助

Java代碼

List<String> listOne = Arrays.asList("A", "B", "C", "D");
List<String> listTwo = Arrays.asList("X", "Y", "Z");
Collections.copy(listOne, listTwo);
System.out.println(listOne);// [X, Y, Z, D]
System.out.println(listTwo);//[X, Y, Z]

Java代碼

List<Person> personList = Arrays.asList(new Person(21, "Eric", "W"),
new Person(17, "John", "A"), new Person(28, "Allan", "H"),
new Person(15, "Jonas", "B"));
// [Age: 21 FirstName Eric LastName: W, Age: 17 FirstName John LastName:
// A, Age: 28 FirstName Allan LastName: H, Age: 15 FirstName Jonas
// LastName: B]
System.out.println(personList);
Collections.sort(personList, new Comparator<Person>() {
@Override
public int compare(Person p1, Person p2) {
return p1.getAge() - p2.getAge();
}
});
// [Age: 15 FirstName Jonas LastName: B, Age: 17 FirstName John
// LastName: A, Age: 21 FirstName Eric LastName: W, Age: 28 FirstName
// Allan LastName: H]
System.out.println("按照年齡排序後:");
System.out.println(personList);

8. disJoint

用於檢查兩個集合有無相同的元素，如果沒有則返回true。

Java代碼

List<String> list3 = Arrays.asList("A", "B", "C", "D");
List<String> list4 = Arrays.asList("X", "Y", "Z");
boolean disJoint = Collections.disjoint(list3, list4);
// true
System.out.println(disJoint);

9. fill

使用指定元素替換指定列表中的所有元素

Java代碼

List<String> testList = Arrays.asList("A", "B", "C", "D");
Collections.fill(testList, "Z");
// [Z, Z, Z, Z]
System.out.println(testList);

10. frequency

獲取某個元素在集合中出現的次數。

Java代碼

List<String> testList = Arrays.asList("A", "B", "C", "D");
int freq = Collections.frequency(testList, "A");
// 1
System.out.println(freq);

11. indexOfSubList

返回指定源列表中第一次出現指定目標列表的起始位置

Java代碼

int index = Collections.indexOfSubList(Arrays.asList("A", "B", "C"),
Arrays.asList("B"));
// Print 1
System.out.println(index);

12. lastIndexOfSubList

返回指定源列表中最後一次出現指定目標列表的起始位置

Java代碼

int lastIndex = Collections.lastIndexOfSubList(
Arrays.asList("A", "B", "C", "B"), Arrays.asList("B"));
// Print 3
System.out.println(lastIndex);

13. emptyXXX

請參考之前的文章 https://www.cnblogs.com/qiumingcheng/p/7126281.html

14. checkedXXX

Java代碼

List<String> stringList = Arrays.asList("A", "B", "C", "D");
List<String> typeSafeList = Collections.checkedList(stringList, String.class);
//[A, B, C, D]
System.out.println(typeSafeList);

15. reverse

反轉列表中的元素順序。

Java代碼

List<String> reverseCandidate = Arrays.asList("A", "B", "C");
Collections.reverse(reverseCandidate);
// [C, B, A]
System.out.println(reverseCandidate);

16. replaceAll

Java代碼

List<String> replaceAllCandidate = Arrays.asList("A", "B", "C");
// 將A用Z代替
Collections.replaceAll(replaceAllCandidate, "A", "Z");
// [Z, B, C]
System.out.println(replaceAllCandidate);

17. swap

指定一個目標集合以及兩個元素的索引，交換這兩個指定位置元素的值。

Java代碼

List<String> swapCandidate = Arrays.asList("A", "B", "C");
// 首尾元素交換
Collections.swap(swapCandidate, 0, 2);
// [C, B, A]
System.out.println(swapCandidate);

18. synchronizedXXX

Java代碼

Collection<String> c = Collections
.synchronizedCollection(new ArrayList<String>());
List<String> list = Collections
.synchronizedList(new ArrayList<String>());
Set<String> set = Collections.synchronizedSet(new HashSet<String>());
Map<String, String> m = Collections
.synchronizedMap(new HashMap<String, String>());

19. unmodifiableXXX

Java代碼

List<String> unmodifiableList = Collections.unmodifiableList(Arrays
.asList("A", "B", "C"));
unmodifiableList.add("D");//此動作會拋異常
// Exception in thread "main" java.lang.UnsupportedOperationException
// at java.util.Collections$UnmodifiableCollection.add(Unknown Source)
// at
// com.thecodesmaple.example.collection.CollectionsExample.main(CollectionsExample.java:149)

20. singletonXXX

Java代碼

String init[] = { "One", "Two", "Three", "One", "Two", "Three" };
List list1 = new ArrayList(Arrays.asList(init));
List list2 = new ArrayList(Arrays.asList(init));
list1.remove("One");
// [Two, Three, One, Two, Three]
System.out.println(list1);
// [Two, Three, Two, Three]
list2.removeAll(Collections.singleton("One"));
System.out.println(list2);

21. rotate

根據指定的距離循環移動指定列表中的元素

Java代碼

List<String> rotateList = Arrays.asList("A", "B", "C", "D", "E", "F");
// [F, A, B, C, D, E]
// Collections.rotate(rotateList, 1);
// System.out.println(rotateList);
Collections.rotate(rotateList, 3);
// [D, E, F, A, B, C]
System.out.println(rotateList);

22. reverseOrder

Java代碼

List<String> reverseOrderTest = Arrays.asList("A", "B", "C", "D", "E",
"F");
Comparator c = Collections.reverseOrder();
Collections.sort(reverseOrderTest, c);
// [F, E, D, C, B, A]
System.out.println(reverseOrderTest);

Collections常用方法梳理

SQL優化-20231016

java存儲表情符號 Incorrect string value: '\xF0\x9F\x91\xBD' for column

mysql 增加字段

Git 常用操作命令

java 在項目中創建臨時文件夾，並稍後清除

git從遠程倉庫gitLab上拉取指定分支到本地倉庫

https://yachay.unat.edu.pe/blog/index.php?comment_area=format_blog&comment_component=blog&comment_co

linux以太網驅動總結