08. Java basic data structure

Catalog introduction

• 1. Array of objects
• 2. Collection
  • 2.1 The origin of sets
  • 2.2 The difference between an array and a collection
  • 2.3 Collection inheritance system diagram
  • 2.4 Overview of Collection Functions
  • 2.5 Traversal of collections
  • 2.6 The principle of iterators
• 3. Data structure
  • 3.0 How to choose a suitable List collection
  • 3.1 Arrays and linked lists of data structures
  • 3.2 Stacks and Queues of Data Structures
• 4. Collection Summary
  • 4.1 Collection Summary

1. Array of objects

• 1.1 Drawing object array memory map
  • Requirement: Store the information [name, age] of the three students into an object array
  • Plot the infield diagram as follows:

2. Collection

• 2.1 The origin of sets

  • Object-oriented languages ​​reflect things in the form of objects, so in order to facilitate the operation of multiple objects, Java provides a collection class.

• 2.2 The difference between an array and a collection

  • (1): Length difference: the length of the array is fixed and the length of the collection is variable
  • (2): The difference between storage data types: Arrays can store basic data types and reference data types; collections can only store reference data types
  • (3): Content difference: Arrays can only store elements of the same data type, and sets can store elements of different types

• 2.3 Collection inheritance system diagram

• 2.4 Overview of Collection Functions

• a: add function

  • boolean add(Object obj): add an element
  • boolean addAll(Collection c): add an element of a collection

• b: delete function

  • void clear(): remove all elements
  • boolean remove(Object o): removes an element
  • boolean removeAll(Collection c): removes an element of a collection (one or all)

• c: Judgment function

  • boolean contains(Object o): Determines whether the set contains the specified element
  • boolean containsAll(Collection c): Determine whether the collection contains the specified collection element (one or all)
  • boolean isEmpty(): Determine if the collection is empty

• d: get function

  • Iterator<E> iterator() (emphasis)

• e: length function

  • int size(): the number of elements
  • Interview question: Does an array have a length() method? Does a string have a length() method? Does a collection have a length() method?

• f: intersection function

  • boolean retainAll(Collection c): elements that both collections have? Think about where the elements go, and what does the returned boolean mean?

• g: Convert the collection to an array

  • Object[] toArray()

• 2.5 Traversal of collections

• 2.5.1 Iterator traversal [less used]

public class CollectionDemo {
    public static void main(String[] args) {
        // Iterator iterator():     使用迭代器进行遍历(重点*********)
        // 创建Collection集合对象
        Collection col = new ArrayList() ;
        // 添加元素
        col.add("张三") ;
        col.add("李四") ;
        col.add("王五") ;
        col.add("赵六") ;
        // 使用迭代器对其进行遍历
        // 1. 获取迭代器对象
        Iterator it = col.iterator() ;   // 返回时实现了Iterator这个接口的子类对象
        // 2. 调用迭代器对象中的方法完成遍历
        // E next(): 获取元素的同时,将指针向后移动一位
        while(it.hasNext()) {
            System.out.println(it.next());
        }
    }
}

• 2.5.2 Collection to array traversal

public class CollectionDemo {
    public static void main(String[] args) {
        // 1. 创建集合对象
        Collection col = new ArrayList() ;
        // 2. 添加元素
        col.add("鸣人") ;
        col.add("佐助") ;
        col.add("佩恩") ;
        // 3. 把集合转换成数组
        // Object[] toArray()
        Object[] objs = col.toArray() ;
        // 4. 遍历数组
        for(int x = 0 ; x < objs.length ; x++) {
            // 向下转型
            String s = (String)objs[x] ;
            System.out.println(s.length());
        }
    }
}

• 2.6 The principle of iterators

3. Data structure

• 3.0 How to choose a suitable List collection

• 3.1 Arrays and linked lists of data structures

• 3.2 Stacks and Queues of Data Structures

4. Collection Summary

• 4.1 Collection Summary