本文导读:
包含了集合:arraylist,hashmap等
数组、二进制、泛型、异常 |
LinkedList,ArrayList,HashMap,HashSet,Vector
import java.util.*;
arraylist al = new arraylist();
a1.add(0,Clerk);
a1.remove(1);
Clerk wo = (Clerk)a1.get(0);
HashMap hm = new HashMap();
Clerk c1 = new Clerk("wang", 100);
Clerk c2 = new Clerk("zhao", 200);
Clerk c3 = new Clerk("wu", 300);
hm.put("131", c1);
hm.put("132", c2);
hm.put("133", c3);
if(hm.containsKey("132"))
{
System.out.println("hashmap has 132!");
}
Iterator it = hm.keySet().iterator();
while(it.hasNext())
{
String key = it.next().toString();
Clerk ctClerk = (Clerk)hm.get(key);
System.out.println("name:"+ctClerk.getName());
System.out.println("money:"+ctClerk.getMoney());
}
Hashtable 是同步的 不可以放入NULL
HashMap 是异步的 线程安全性没有 可以放入 NULL
Vector 同步 双倍扩容 大量数据有优势
ArrayList 异步 扩容50%

用Map 去 接受HashMap 比较灵活
HashSet 无顺序,元素唯一。底层为HashMap
math.random();//随机产生0~1
int x = 1;
int []a = new int[x];
1.定义
int a[] = {1,2,3};
int a[] = new int[3];
int []a = new int[3];
2.a.length
3.对象数组
Dog dogs[] = new Dog[4];
dogs[0] = new Dog();
- java全为有符号数
- 计算机中均用补码显示
- 0的反码,补码均为0
- >>,<< 算数 补符号 >>> 逻辑 补0
- ^ 异或
- 80G中,1m=1024k,sell 1m=1000k
Demo<Dog> d1 = new Demo<Dog>(new Dog("w", 10));
d1.print();
class Demo<T>
{
private T i;
public Demo(T p) {
i = p;
}
public void print()
{
System.out.println(i.getClass().getName());
Method []methods = i.getClass().getDeclaredMethods();
for(int k = 0; k < methods.length;k++)
{
System.out.println(methods[k].getName());
}
}
}
- java.lang.Exception
- java.lang.RuntimeException
- java.lang.ERROR
try{
}catch(Exception e){
e.getMessage();
e.printStackTrace();
}finally{
}
- 只有try,finally也是可以的!
- 4种情况finnaly不会被执行:1.system.exit() 2.线程异常 3.finally 异常 4.CPU关闭
- 类throws Exception,谁调用谁管