1.Collection collection
1.1 collective Architecture [memory]
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Characteristics of set classes
A storage model with variable storage space is provided, and the stored data capacity can be changed at any time
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System diagram of collection class
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1.2Collection set overview and basic usage [ application ]
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Collection collection overview
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Is the top-level interface of a singleton Collection. It represents a set of objects, which are also called Collection elements
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JDK does not provide any direct implementation of this interface. It provides more specific implementation of sub interfaces (such as Set and List)
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Collection basic usage
public class CollectionDemo01 { public static void main(String[] args) { //Create an object for the Collection Collection<String> c = new ArrayList<String>(); //Add element: Boolean add (E) c.add("hello"); c.add("world"); c.add("java"); //Output collection object System.out.println(c); } }
1.3 common methods of collection [application]
| Method name | explain |
|---|---|
| boolean add(E e) | Add element |
| boolean remove(Object o) | Removes the specified element from the collection |
| void clear() | Empty elements in collection |
| boolean contains(Object o) | Determines whether the specified element exists in the collection |
| boolean isEmpty() | Determine whether the collection is empty |
| int size() | The length of the set, that is, the number of elements in the set |
1.4 traversal of collection [application]
- Introduction to iterators
- Iterator, special traversal mode of collection
- Iterator iterator(): returns the iterator of the elements in this collection, which is obtained through the iterator() method of the collection
- The iterator is obtained through the iterator() method of the collection, so we say that it depends on the collection
- Traversal of Collection
public class IteratorDemo {
public static void main(String[] args) {
//Create collection object
Collection<String> c = new ArrayList<>();
//Add element
c.add("hello");
c.add("world");
c.add("java");
c.add("javaee");
//Iterator < E > iterator(): returns the iterator of the elements in this collection, which is obtained through the iterator() method of the collection
Iterator<String> it = c.iterator();
//Using while loop to improve the judgment and acquisition of elements
while (it.hasNext()) {
String s = it.next();
System.out.println(s);
}
}
}
1.5 illustration of collective use steps [understanding]
- Use steps
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1.6 case of Collection - the Collection stores student objects and traverses [application]
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Case requirements
Create a collection of student objects, store 3 student objects, and use the program to traverse the collection on the console
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code implementation
- Student class
public class Student { private String name; private int age; public Student() { } public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } }- Test class
public class CollectionDemo { public static void main(String[] args) { //Create Collection collection object Collection<Student> c = new ArrayList<Student>(); //Create student object Student s1 = new Student("Lin Qingxia", 30); Student s2 = new Student("Zhang Manyu", 35); Student s3 = new Student("Wang Zuxian", 33); //Add students to collection c.add(s1); c.add(s2); c.add(s3); //Traversal collection (iterator mode) Iterator<Student> it = c.iterator(); while (it.hasNext()) { Student s = it.next(); System.out.println(s.getName() + "," + s.getAge()); } } }
2.List set
2.1 list set overview and features [memory]
- List collection overview
- An ordered set (also known as a sequence), in which the user can accurately control the insertion position of each element in the list. Users can access elements through integer indexes and search for elements in the list
- Unlike Set collections, lists usually allow duplicate elements
- List set features
- Indexed
- Duplicate elements can be stored
- Element access order
2.2 unique method of list set [application]
| Method name | describe |
|---|---|
| void add(int index,E element) | Inserts the specified element at the specified location in this collection |
| E remove(int index) | Deletes the element at the specified index and returns the deleted element |
| E set(int index,E element) | Modify the element at the specified index and return the modified element |
| E get(int index) | Returns the element at the specified index |
2.3 case of the set - the List set stores the student object and traverses the [application]
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Case requirements
Create a collection of student objects, store 3 student objects, and use the program to traverse the collection on the console
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code implementation
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Student class
public class Student { private String name; private int age; public Student() { } public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } } -
Test class
public class ListDemo { public static void main(String[] args) { //Create a List collection object List<Student> list = new ArrayList<Student>(); //Create student object Student s1 = new Student("Lin Qingxia", 30); Student s2 = new Student("Zhang Manyu", 35); Student s3 = new Student("Wang Zuxian", 33); //Add students to collection list.add(s1); list.add(s2); list.add(s3); //Iterator pattern Iterator<Student> it = list.iterator(); while (it.hasNext()) { Student s = it.next(); System.out.println(s.getName() + "," + s.getAge()); } System.out.println("--------"); //for loop mode for(int i=0; i<list.size(); i++) { Student s = list.get(i); System.out.println(s.getName() + "," + s.getAge()); } } }
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2.4 concurrent modification exception [ application ]
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Causes of occurrence
In the process of iterator traversal, the elements in the collection are modified through the collection object, resulting in the inconsistency between the expected modified value and the actual modified value in the element obtained by the iterator: ConcurrentModificationException
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Solutions
Use the for loop to traverse, and then use the collection object to do the corresponding operation
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Sample code
public class ListDemo { public static void main(String[] args) { //Create collection object List<String> list = new ArrayList<String>(); //Add element list.add("hello"); list.add("world"); list.add("java"); //Traverse the collection to get each element. See if there is a "world" element. If so, I will add a "javaee" element. Please write code to implement it // Iterator<String> it = list.iterator(); // while (it.hasNext()) { // String s = it.next(); // if(s.equals("world")) { // list.add("javaee"); // } // } for(int i=0; i<list.size(); i++) { String s = list.get(i); if(s.equals("world")) { list.add("javaee"); } } //Output collection object System.out.println(list); } }
2.5 list iterator [application]
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Introduction to ListIterator
- It is obtained through the listIterator() method of the List collection, so it is a unique iterator of the List collection
- A list iterator that allows the programmer to traverse in either direction, modify the list during the iteration, and get the current position of the iterator in the list
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Sample code
public class ListIteratorDemo { public static void main(String[] args) { //Create collection object List<String> list = new ArrayList<String>(); //Add element list.add("hello"); list.add("world"); list.add("java"); //Get list iterator ListIterator<String> lit = list.listIterator(); while (lit.hasNext()) { String s = lit.next(); if(s.equals("world")) { lit.add("javaee"); } } System.out.println(list); } }
2.6 enhanced for loop [application]
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Define format
for(Element data type variable name : array/Collection object name) { Circulatory body;} -
Sample code
public class ForDemo { public static void main(String[] args) { int[] arr = {1,2,3,4,5}; for(int i : arr) { System.out.println(i); } System.out.println("--------"); String[] strArray = {"hello","world","java"}; for(String s : strArray) { System.out.println(s); } System.out.println("--------"); List<String> list = new ArrayList<String>(); list.add("hello"); list.add("world"); list.add("java"); for(String s : list) { System.out.println(s); } System.out.println("--------"); //The internal principle is an Iterator iterator / * for (string s: list) {if (s.equals ("world")) {list. Add ("JavaEE"); / / concurrentmodificationexception}} * /}}
2.7 set case List set stores student objects and traverses [application] in three ways
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Case requirements
Create a collection of student objects, store 3 student objects, and use the program to traverse the collection on the console
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code implementation
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Student class
public class Student { private String name; private int age; public Student() { } public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; }} -
Test class
public class ListDemo { public static void main(String[] args) { //Create a list collection object list < student > List = new ArrayList < student > ()// Create student object S1 = new student ("Lin Qingxia", 30); Student S2 = new student ("Maggie Cheung", 35); Student S3 = new student ("Wang Zuxian", 33)// Add students to the set list add(s1); list. add(s2); list. add(s3); // Iterator: iterator < student > it = list iterator(); while (it.hasNext()) { Student s = it.next(); System.out.println(s.getName()+","+s.getAge()); } System. out. println("--------"); // Normal for: traversal method with index for (int i = 0; I < list. Size(); i++) { Student s = list.get(i); System.out.println(s.getName()+","+s.getAge()); } System. out. println("--------"); // Enhanced for: the most convenient traversal method for (student s: list) {system. Out. Println (s.getname() + "," + s.getage());}}}
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3. Data structure
3.1 stack and queue of data structure [memory]
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Stack structure
In and out
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Queue structure
First in first out
3.2 array and linked list of data structure [memory]
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Array structure
Fast query, slow addition and deletion
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Queue structure
Slow query, fast addition and deletion
4. Implementation class of list set
4.1 characteristics of list aggregation subclass [memory]
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ArrayList collection
The bottom layer is the implementation of array structure, with fast query and slow addition and deletion
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LinkedList collection
The bottom layer is the implementation of linked list structure, with slow query and fast addition and deletion
4.2 case of the collection - the ArrayList collection stores student objects and traverses [application] in three ways
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Case requirements
Create a collection of student objects, store 3 student objects, and use the program to traverse the collection on the console
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code implementation
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Student class
public class Student { private String name; private int age; public Student() { } public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; }} -
Test class
public class ArrayListDemo { public static void main(String[] args) { //Create ArrayList collection object ArrayList < student > array = new ArrayList < student > ()// Create student object S1 = new student ("Lin Qingxia", 30); Student S2 = new student ("Maggie Cheung", 35); Student S3 = new student ("Wang Zuxian", 33)// Add students to the set array add(s1); array. add(s2); array. add(s3); // Iterator: iterator < student > it = array iterator(); while (it.hasNext()) { Student s = it.next(); System.out.println(s.getName() + "," + s.getAge()); } System. out. println("--------"); // Normal for: traversal method with index for (int i = 0; I < array. Size(); i++) { Student s = array.get(i); System.out.println(s.getName() + "," + s.getAge()); } System. out. println("--------"); // Enhanced for: the most convenient traversal method for (student s: array) {system. Out. Println (s.getname() + "," + s.getage());}}}
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4.3 unique function of LinkedList set [application]
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Unique method
Method name explain public void addFirst(E e) Inserts the specified element at the beginning of the list public void addLast(E e) Appends the specified element to the end of this list public E getFirst() Returns the first element in this list public E getLast() Returns the last element in this list public E removeFirst() Removes and returns the first element from this list public E removeLast() Removes and returns the last element from this list