Java学习笔记10

 

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java第十天
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(1) JavaSE平台:
集合: 是一个用来管理其他多个对象的对象.
分为两个接口:  Collection Map
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(2) collection:
每一个元素都是一个对象:
两个子接口:
List: 元素的存放是有顺序的,用下标表示,里面的元素是可以重复的.
Set: 无顺序的,不可重复.
子接口: SortedSet(排序的Set)
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(3) Map
每一个元素是一个Key-value,无顺序,不重复.
子接口: SortedMap(排序的Map)
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(4) List接口: 实现类:ArrayList() :查询效率高,增删的效率底.
ArrayList: 底层是用数组实现的.
public class ArrayList extends AbstractList :ArrayList 没有直接实现Iterator接口,而是通过缺省适配模式,ArrayList继承了AbstractList ,而AbstractList 的一个内部类实现了Iterator接口.然后里面也有一个方法,iterator().就是返回这个内部类.
小知识点: [一个接口的后面加一个s,例如: collection-->collections(一个工具类)里面的方法都是静态的,用类名调用.]
实现comparable接口:
compartTo(Object o) 如果当前对象小于o对像,那么返回负数,否则返回正数.(升序)
例子:
publicclass Employee implements Comparable { // 如果想把这个List排序,则要实现这个接口.

       String name;

       intage;

       doublesalary;

       public Employee(String name, int age, double salary) {

              this.name = name;

              this.age = age;

              this.salary = salary;

       }

       publicint compareTo(Object o) { // 用来比较两个类而关系.实现Comparable接口:

              Employee e = (Employee) o;

              if (this.salary > e.salary)

                     return -1;

              elseif (this.salary == e.salary) {

                     if (this.age > e.age)

                            return -1;

                     elseif (this.age == e.age) {

                            returnthis.name.compareTo(e.name);

                     } elseif (this.age < e.age)

                            return 1;

                     else

                            return 0;

              } else

                     return 1;

       }

       public String toString() {

              return"姓名:" + name + "    年龄:" + age + "   薪水:" + salary;

       }

}
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<1>摹拟,Comparable类的接口回调:
publicclass Collections2 {

       publicstaticvoid a(List list) {

              for (int i = 0; i < list.size(); i++) {

                     for (int j = i + 1; j < list.size(); j++) {

                            Object o1 = list.get(i);

                            Object o2 = list.get(j);

                            Comparable c1 = (Comparable) o1; // 接口的回调.

                            Comparable c2 = (Comparable) o2; // 接口的回调.

                            if (c1.compareTo(c2) > 0) {

                                   Collections.swap(list, i, j); //Collections里面的井台方法:

                            }

                     }

              }

       }

}
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<2> 用LinkList实现而栈;//因为栈牵涉而都是元素的增删因此要用到增删效率高的链表.publicclass TestMyStack { 

       publicstaticvoid main(String[] args) {

              MyStack stack = new MyStack();

              stack.push("ChenPy");

              stack.push("Chenzq");

              stack.push("Fanlb");

              stack.push("YangQing");

              System.out.println(stack.pop());

              System.out.println(stack.pop());

              System.out.println(stack.pop());

              System.out.println(stack.pop());

       }

}

 

class MyStack {

       private LinkedList ll = new LinkedList();

 

       publicvoid push(Object o) {

              ll.addFirst(o);

       }

 

       public Object pop() {

              return ll.removeFirst();

}

}

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(5) LinkedList 底层是用链表实现的. 增删效率高,查询效率低.ArrayList: 是轻量级的.消耗资源少.线程不安全
Vector: 是重量级的,消耗资源多,线程比较安全.(多线程访问的时候)
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(6) HashSet : Hash算法.保证元素不重复: 覆盖equals(), hashCode();
hastCode(): 覆盖他的时候尽量保证,返回的哈希码不同.用来保证效率.
例子:
import java.util.*;

publicclass student {// 不用继承Comparable接口因为set不要求排序,本来就是杂乱无章的但他要覆盖equals(),

                                          // hashCode(),这两个方法(object 类中的方法.),用来除去set中相同的元素.

// 元素定义;

       String name;

       intage;

       intcpp;

       intjava;

       intunix;

       inttotal;

       // 构造方法.

       public student(String name, int age, int cpp, int java, int unix) {

              this.name = name;

              this.age = age;

              this.cpp = cpp;

              this.java = java;

              this.unix = unix;

       }

       // 覆盖Object 类中的返回对象的方法.

       public String toString() {

              return"姓名: " + name + " 年龄: " + age + "总成绩: " + this.getTotal();

       }

       // 覆盖Object类中比较两个对象是否相同的方法.

       publicboolean equals(Object o) {

              if (o == this)

                     returntrue;

              if (o == null)

                     returnfalse;

              if (this.getClass() != o.getClass())

                     returnfalse;

              student s = (student) o;

              if (this.name.equals(s.name) && this.age == s.age && this.cpp == s.cpp

                            && this.java == s.java && this.unix == s.unix)

                     returntrue;

              else

                     returnfalse;

       }

       // 覆盖Object类中的返回hashCode的方法.

       publicint hashCode() {

              returnage + name.hashCode();

       }

       // 返回总分.

       publicint getTotal() {

              returncpp + java + unix;

       }

       publicint compareTo(Object o) {

              student s = (student) o;

              if (this.getTotal() < s.getTotal())

                     return 1;

              elseif (this.getTotal() > s.getTotal())

                     return -1;

              else

                     return 0;

       }

}