java面试题1

 import java.util.Random;
 /**
  *
  * @author Alexs
  */
 /*
  * Demo1
  */
 class Demo1 {
     public static void main(String[] args) {
         int num=2147483647;
         long temp=num+2L;
         System.out.println(temp);//2147483649
     }
 }
 /*
  * Demo2
  */
 class Demo2{
     public static void main(String[] args) {
         char c='A';
         int num=10;
         switch(c) {
         case 'B':
             num++;
         case 'A':
             num++;
         case 'Y':
             num++;
             break;
         default:
             num--;
         }
         System.out.println(num);//12
     }
 }
 /*
  * Demo3
  */
 class Demo3{
         public static void main(String[] args) {
             int x=10;
             double y=20.2;
             long z=10L;
             String str=""+x+y*z;
             System.out.println(str);//10202.0
         }
 }
 /*
  * Demo4
  */
 class Demo4{
     public static void main(String[] args) {
         int sum=0;
         for(int x=0;x<10;x++) {
             sum+=x;
             if(x%3==0) {
                 break;
             }
         }
         System.out.println(sum);//0
     }
 }
 /*
  * Demo5
  */
 class Demo5{
     public static void main(String[] args) {
         System.out.println(inc(10)+inc(8)+inc(-10));//35
     }
     public static int inc(int temp) {
         if(temp>0) {
             return temp*2;
         }
         return -1;
     }
 }
 /*
  * Demo6
  */
 class Demo6{
     public static void main(String[] args) {
         boolean flag=10%2==1 && 10/3==0 && 1/0==0;
         System.out.println(flag?"A":"B");//B
     }
 }
 /*
  * Demo7
  */
 class Demo7{
     public static void main(String[] args) {
         int num=50;
         num=num++*2;
         System.out.println(num);//100
     }
 }
 /*
  * Demo8
  */
 class Demo8{
     public static void main(String[] args) {
         int i=1;
         int j=i++;
         if(i==(++j)&&((i++)==j)) {
             i+=j;
             System.out.println("i="+i);//5
         }
     }
 }
 /*
  * Demo9
  */
 class Demo9{
     public static void main(String[] args) {
         String str="";
         for(int x=0;x<5;x++) {
             str+=x;
         }
         System.out.println(str);//01234
     }
 }
 /*
  * Demo10
  */
 class Demo10{
     public int a() {
         int i=0;
         i++;
         return i;
     }
     public static void main(String[] args) {
         Demo10 d=new Demo10();
         d.a();
         int j=d.a();
         System.out.println(j);//1
     }
 }
 /*
  * Demo11
  */
 class supers{
     String name;
     public supers(){};
     public supers(String name) {
         this.name=name;
     }
     public void fun1() {
         System.out.println("this is class super!"+name);
     }
 }

 class sub extends supers{

     public void fun1() {
         System.out.println("this is class sub!"+name);
     }
 }

 class Demo11{
     public static void main(String[] args) {
         supers s=new sub();
         s.fun1();//this is class sub!null
     }
 }
 /*
  * Demo12
  */
 class Demo12{
     public static void main(String[] args) {
         int i=9;
         switch (i) {
         default:
             System.out.println("default");
         case 0:
             System.out.println("zero");
         case 1:
             System.out.println("one");
         case 2:
             System.out.println("two");
         }
         /*
          * default
          * zero
          * one
          * two
          */
     }
 }
 /*
  * Demo13 写一个单例模式
  */
 class Demo13{
     //饿汉式
     private static final Demo13 d=new Demo13();
     private Demo13() {}
     public static Demo13 getInstance() {
         return d;
     }
 }
 class Demo13Test{
     public static void main(String[] args) {
         Demo13 d1=Demo13.getInstance();
         Demo13 d2=Demo13.getInstance();
         System.out.println(d1==d2);//true
     }
 }
 /*
  * Demo14
  * 生成一个随机密码，密码包含大小写字母及数字，并且可根据输入参数对密码长度进行调整
  */
 class Demo14{
     //定义一下字符串常量
     private static final String CHAR_ALL="0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
     public String getRandomPwd(int num) {
         String result="";
         //实例化Random对象
         Random r=new Random();
         for(int i=0;i
             //生成0-62之间的（小于62）int值
             int j=r.nextInt(CHAR_ALL.length());
             //截取下标跟跟int值对应的字符
             String singleChar=CHAR_ALL.substring(j, j+1);
             //拼接
             result+=singleChar;
         }
         return result;
     }
     public static void main(String[] args) {
         Demo14 d=new Demo14();
         System.out.println(d.getRandomPwd(6));
     }
 }
 /*
  * Demo15
  */
 class Demo15 {
      public static void main(String []args) {
             int i=9;
             int j=9;
             //System.out.println(i==j);//true
             //System.out.println(j==i++);//true
             //System.out.println(j==++i);//false
             //System.out.println(j++==i);//true
             //System.out.println(++j==i);//false
      }
 }