Collections
-
List (inherited from Collection Interface)
- ArrayList, Vector, LinkedList, Stack
-
Queue (inherited from Collection Interface)
- LinkedList, PriorityQueue
-
Set (inherited from Collection Interface)
- HashSet, LinkedHashSet, TreeSet
- Stack (inherited from Collection Interface)
-
Map
- HashMap, TreeMap, WeakHashMap, HashTable
Schematic Diagram
Collections
Map
Iterator
- Iterator, a light collection for those collections which implements Collection Interface.
- ListIterator, a light collection for those collections which implements List Interface, extends Iterator Interface, has more functions for traverse forward and backward with the ability to add, set, and remove an item.
| Iterator | ListIterator |
|---|---|
| hasNext() | hasNext() |
| next() | next() |
| hasPrevious() | |
| previous() | |
| nextIndex() | |
| previousIndex() | |
| add() | |
| set() | |
| remove() | remove() |
ArrayList, Vector, LinkedList
Commonality: dynamically increasing the count of elements
| Difference | ArrayList | Vector | LinkedList |
|---|---|---|---|
| Storage mode | Single List | Single List | Double List |
| Continuous storage space | Yes, extend 1.5x/time | Yes, extend 2x/time | No |
| Access element speed | Fast | Fast | Slow |
| Modify element(middle) speed | Slow | Slow | Fast |
| Thread security | Unsafe | Safe | Unsafe |
| Performance | High | Low | High |
HashMap, HashTable, TreeMap, and WeakHashMap
According to the hash code of the key to store the key-value pair, it will be easier to access the value by looking for the key.
| Difference | HashMap | HashTable |
|---|---|---|
| Allow null key | Yes, only one | No |
| Has contains() | No, but it has containsKey() and containsValue() | Yes |
| Thread security | No | Yes |
| Performance | High | Low |
| Traversable interface | Iterator | Enumeration |
| Hash array default, increase | 16, (old * 2) | 11, (old * 2 + 1) |
| Directly using hashCode | No | Yes |
TreeMap, implemented SortMap so that using for sorted structure
LinkedHashMap, stored by original input sequence
WeakHashMap, once the key is no longer be referenced, then it will be recycled immediately, but in HashMap, until the key is removed from the map it is still existing
What is synchronize? How to make the HashMap synchronized?
At the same time, there is only one thread can modify the map, any thread which is working on the update operation needs to request the lock of the object, and the others need to wait for the lock unlock.
Map map = Collections.synchronizedMap(new HashMap());
Customized Object as the key stored in Map
Insert a pair of key-value
-
Locate the key
key ---using hashCode() to calculate---> hash value
hash value ---search hash value in map---> list of key (theheadofaddresses)
list of key ---traverse all of the keys, and using equals() to exam the same---> one key -
Override
hashCode()andequals()
for example:
class Person {
String id;
String name;
public Person(String id, String name) {
this.id = id;
this.name = name;
}
public String toString() {
return "id = " + id + ", name = " + name;
}
public int hashCode() {
return id.hashCode();
}
public boolean equals(Object obj) {
Person person = (Person)obj;
return person.id.equals(this.id);
}
}
Collections.sort()
Create new Comparator<Object>(), and override compare method
import java.util.*;
//以下是学生类Student定义，有点类似C语言的结构体啊！^_^
class Student {
public int s_no;
public String s_name;
public int s_class;
}
public class compareTest {
public static void main(String[] args) {
//存放学生类的动态数组的初始化
ArrayList<Student> studentArr = new ArrayList<Student>();
Student s1 = new Student();
s1.s_no = 3;
s1.s_name = "a";
s1.s_class = 102;
studentArr.add(s1);
Student s2 = new Student();
s2.s_no = 2;
s2.s_name = "b";
s2.s_class = 101;
studentArr.add(s2);
Student s3 = new Student();
s3.s_no = 1;
s3.s_name = "c";
s3.s_class = 103;
studentArr.add(s3);
//初始化之后先打印以下这个动态数组
System.out.println("排序前：");
for (int i = 0; i < studentArr.size(); i++) {
System.out
.println("我是" + studentArr.get(i).s_class + "班的"
+ studentArr.get(i).s_name + "学号是"
+ studentArr.get(i).s_no);
}
//对于Comparator接口的重写
//这个接口就一个抽象函数，给出的参数与返回值都是定死的。
Collections.sort(studentArr, new Comparator<Object>() {
public int compare(Object o1, Object o2) {
//你首先设置你要比较的东西
//具体是把参数中的Object强制转换成你要比较的东西，这里是两个Student类
//这里的s1,s2与上面的s1,s2一点关系都没有，只是抽象的前者与后者的关系
Student s1 = (Student) o1;
Student s2 = (Student) o2;
//如果前者的学号大于后者的学号，就是前者大于后者，返回1系统就会识别是前者大于后者
if (s1.s_no > s2.s_no) {
return 1;
}
//小于同理
if (s1.s_no < s2.s_no) {
return -1;
}
//如果返回0则认为前者与后者相等
return 0;
}
});
//比较完毕再输出以学号排序之后的结果
System.out.println("按学号升序排序后：");
for (int i = 0; i < studentArr.size(); i++) {
System.out
.println("我是" + studentArr.get(i).s_class + "班的"
+ studentArr.get(i).s_name + "学号是"
+ studentArr.get(i).s_no);
}
//以下是以班级排序的过程
Collections.sort(studentArr, new Comparator<Object>() {
public int compare(Object o1, Object o2) {
Student s1 = (Student) o1;
Student s2 = (Student) o2;
if (s1.s_class > s2.s_class) {
return 1;
}
if (s1.s_class < s2.s_class) {
return -1;
}
return 0;
}
});
System.out.println("按班级升序排序后：");
for (int i = 0; i < studentArr.size(); i++) {
System.out
.println("我是" + studentArr.get(i).s_class + "班的"
+ studentArr.get(i).s_name + "学号是"
+ studentArr.get(i).s_no);
}
}
}
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