java面试题1

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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<num;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
}
}

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