JavaSE基础面试题（二）

1、用循环控制语句打印输出：1+3+5+...+99=?的结果

答案一：
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i <= 99; i+=2) {
            sum += i;
        }
        System.out.println("sum = " + sum);
    }

答案二：
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i < 100; i++) {
            if (i % 2 != 0) {
                sum += i;
            }
        }
        System.out.println("sum = " + sum);
    }

2、请写一个冒泡排序，实现{5,7,3,9,2}从小到大排序

答案一：
        int[] arr = { 5, 7, 3, 9, 2 };
        for (int i = 1; i < arr.length; i++) {
            for (int j = 0; j < arr.length - i; j++) {
                if (arr[j] > arr[j + 1]) {
                    int temp = arr[j];
                    arr[j] = arr[j + 1];
                    arr[j + 1] = temp;
                }
            }
        }

答案二：
        int[] arr = { 5, 7, 3, 9, 2 };
        for (int i = 1; i < arr.length; i++) {
            for (int j = arr.length-1; j >= i; j--) {
                if (arr[j] < arr[j - 1]) {
                    int temp = arr[j];
                    arr[j] = arr[j - 1];
                    arr[j - 1] = temp;
                }
            }
        }

3、编写方法实现：求某年某月某日是这一年的第几天

提示：闰年（1）能被4整除不能被100整除（2）能被400整除

public static int daysOfYear(int year, int month, int day){
        int[] daysOfMonth = {31,28,31,30,31,30,31,31,30,31,30,31};
                               补充代码                        
    }

答案一：
    public static int daysOfYear(int year, int month, int day){
        int[] daysOfMonth = {31,28,31,30,31,30,31,31,30,31,30,31};
        int sum = day;
        for(int i=0;i< month-1; i++){
            sum += daysOfMonth[i];
        }
        if(month>2){
            if(year%4==0 && year%100!=0 || year%400==0){
                sum++;
            }
        }
        return sum;
    }

答案二：
    public static int daysOfYear(int year, int month, int day){
        int[] daysOfMonth = {31,28,31,30,31,30,31,31,30,31,30,31};
        int sum = day;
        for(int i=0;i< month-1; i++){
            sum += daysOfMonth[i];
            if(i==1){
                if(year%4==0 && year%100!=0 || year%400==0){
                    sum++;
                }
            }
        }
        return sum;
    }

4、通项公式如下：f(n)=n + (n-1) + (n-2) + .... + 1，其中n是大于等于5并且小于10000的整数，例如：f(5) = 5 + 4 + 3 + 2 + 1，f(10) = 10 + 9 + 8 + 7+ 6 + 5 + 4 + 3 + 2 + 1，请用非递归的方式完成方法long f( int n)的方法体。

答案一：非递归
    public static long f(int n) {
        long sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += i;
        }
        return sum;
    }

5、求1+2！+3！+...+20！的和

    public static void main(String[] args)  {
        long sum = 0;
        for (int i = 1; i <= 20; i++) {
            long temp = 1;
            for (int j = 1; j <=i; j++) {
                temp *= j;
            }
            sum += temp;
        }
        System.out.println("sum = " + sum);
    }

6、输出一个如下图形，一共有n行，第n行有2n-1个*，完成方法public void printStar(int n)的方法体

    *
   ***
  *****
 *******
*********

public void printStar(int n) {
        for (int i = 1; i <= n; i++) {
            for (int j = 0; j < n - i; j++) {
                System.out.print(" ");
            }
            for (int j = 0; j < 2 * i - 1; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }

7、请编写代码把一个字符串反转，例如：hello1234，反转后：4321olleh。

答案一：
public class Test {
    public static void main(String[] args) {
        String str = "hello1234";
        StringBuilder s = new StringBuilder(str);
        s.reverse();
        str = s.toString();
        System.out.println(str);
    }
}

答案二：
public class Test {
    public static void main(String[] args) {
        String str = "hello1234";
        char[] array = str.toCharArray();
        for (int i = 0; i < array.length / 2; i++) {
            char temp = array[i];
            array[i] = array[array.length - 1 - i];
            array[array.length - 1 - i] = temp;
        }
        str = new String(array);
        System.out.println(str);
    }
}

8、编写代码实现，从一个标准url里取出文件的扩展名，尽可能高效。

public static void main(String[] args) {
        String str = fileExtNameFromUrl("http://localhost:8080/testweb/index.html");
        System.out.println(str);
    }
    
    public static String fileExtNameFromUrl(String url){
                            补充代码                  
    }

public static void main(String[] args) {
        String str = fileExtNameFromUrl("http://localhost:8080/testweb/index.html");
        System.out.println(str);
    }
    
    public static String fileExtNameFromUrl(String url){
        return url.substring(url.lastIndexOf('.')+1);
    }

9、有一个字符串String abc = “342567891”，请写程序将字符串abc进行升序，可以使用JDK API中的现有的功能方法。

参考答案一：
public class Test {
    public static void main(String[] args) {
        String str = "342567891";
        char[] arr = str.toCharArray();
        Arrays.sort(arr);
        str = new String(arr);
        System.out.println(str);
    }
}

10、编写一个懒汉式单例设计模式

答案一：懒汉式形式一
public class Singleton {
    private static Singleton instance;
    private Singleton(){
        
    }
    public static Singleton getInstance(){
        if(instance == null){
            synchronized (Singleton.class) {
                if(instance == null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

答案一：懒汉式形式二
public class Singleton{
    private Singleton(){
        
    }
    private static class Inner{
        private static final Singleton INSTANCE = new Singleton();
    }
    
    public static Singleton getInstance(){
        return Inner.INSTANCE;
    }
}

11、请编写一个饿汉式单例设计模式

答案一：饿汉式形式一
public class Singleton {
    public static final Singleton INSTANCE = new Singleton();
    private Singleton(){
    }
}

答案二：饿汉式形式二
public class Singleton {
    private static final Singleton INSTACNE = new Singleton();
    private Singleton(){
        
    }
    public static Singleton getInstance(){
        return INSTACNE;
    }
}

答案三：饿汉式形式三
public enum Singleton {
    INSTANCE
}

12、补充如下枚举类型的代码，使得如下代码达到运行效果

单词提示：monday,tuesday,wednesday,thursday,friday,saturday,sunday

import java.util.Scanner;

public class TestWeek {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("今天是星期几(1-7)：");
        int number = input.nextInt();//假设输入的是2
        Week w = Week.getByNumber(number);
        System.out.println("今天是：" + w);//今天是：TUESDAY(2,星期二)
    }

}
enum Week{
                        （1）                      
    
    private int number;
    private String decription;
    
    private Week(int number, String decription) {
        this.number = number;
        this.decription = decription;
    }
    
    public static Week getByNumber(int number){
                        （2）                   
    }

    @Override
    public String toString() {
        return super.toString()+"(" + number + ","+ decription + ")";
    }
}

答案：
enum Week{
    MONDAY(1,"星期一"),
    TUESDAY(2,"星期二"),
    WEDNESDAY(3,"星期三"),
    THURSDAY(4,"星期四"),
    FRIDAY(5,"星期五"),
    SATURDAY(6,"星期六"),
    SUNDAY(7,"星期日");
    
    private int number;
    private String decription;
    
    private Week(int number, String decription) {
        this.number = number;
        this.decription = decription;
    }
    
    public static Week getByNumber(int number){
        switch(number){
        case 1:
            return MONDAY;
        case 2:
            return TUESDAY;
        case 3:
            return WEDNESDAY;
        case 4:
            return THURSDAY;
        case 5:
            return FRIDAY;
        case 6:
            return SATURDAY;
        case 7:
            return SUNDAY;
        default:
            return null;
        }
    }

    @Override
    public String toString() {
        return super.toString()+"(" + number + ","+ decription + ")";
    }
}

13、写一段代码实现在遍历ArrayList时移除一个元素，例如：”java”？

import java.util.ArrayList;
import java.util.Iterator;

public class Test {

    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<String>();
        list.add("hello");
        list.add("java");
        list.add("world");

                          补充代码                         
    }
}

答案：
import java.util.ArrayList;
import java.util.Iterator;

public class Test {

    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<String>();
        list.add("hello");
        list.add("java");
        list.add("world");

        Iterator<String> iterator = list.iterator();
        while (iterator.hasNext()) {
            String next = iterator.next();
            if ("java".equals(next)) {
                iterator.remove();
            }
        }
    }
}

14、把如下信息添加到Map中，并遍历显示，请正确指定泛型

浙江省
    绍兴市
    温州市
    湖州市
    嘉兴市
    台州市
    金华市
    舟山市
    衢州市
    丽水市
海南省
    海口市
    三亚市
北京市
    北京市

参考答案一：
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map.Entry;
import java.util.Set;

public class Test {

    public static void main(String[] args) throws Exception {
        HashMap<String,List<String>> map = new HashMap<String,List<String>>();
        map.put("北京市", Arrays.asList("北京市"));
        map.put("海南省", Arrays.asList("海口市","三亚市"));
        map.put("浙江省", Arrays.asList("绍兴市","温州市","湖州市","嘉兴市","台州市","金华市","舟山市","衢州市","丽水市"));
    
        Set<Entry<String, List<String>>> entrySet = map.entrySet();
        for (Entry<String, List<String>> entry : entrySet) {
            System.out.println(entry.getKey());
            List<String> value = entry.getValue();
            for (String string : value) {
                System.out.println("\t" + string);
            }
        }
    }
}

参考答案二：
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map.Entry;
import java.util.Set;

public class Test {

    public static void main(String[] args) throws Exception {
        HashMap<String,ArrayList<String>> map = new HashMap<String,ArrayList<String>>();
        
        ArrayList<String> bj = new ArrayList<String>();
        bj.add("北京市");
        map.put("北京市", bj);
        
        ArrayList<String> hn = new ArrayList<String>();
        hn.add("海口市");
        hn.add("三亚市");
        map.put("海南省", hn);
        
        ArrayList<String> zj = new ArrayList<String>();
        zj.add("绍兴市");
        zj.add("温州市");
        zj.add("湖州市");
        zj.add("嘉兴市");
        zj.add("台州市");
        zj.add("金华市");
        zj.add("舟山市");
        zj.add("衢州市");
        zj.add("丽水市");
        map.put("浙江省", zj);
    
        Set<Entry<String, ArrayList<String>>> entrySet = map.entrySet();
        for (Entry<String, ArrayList<String>> entry : entrySet) {
            System.out.println(entry.getKey());
            ArrayList<String> value = entry.getValue();
            for (String string : value) {
                System.out.println("\t" + string);
            }
        }
    }
}

参考答案三：
import java.util.HashSet;
import java.util.HashMap;
import java.util.Map.Entry;
import java.util.Set;

public class Test {

    public static void main(String[] args) throws Exception {
        HashMap<String,HashSet<String>> map = new HashMap<String,HashSet<String>>();
        
        HashSet<String> bj = new HashSet<String>();
        bj.add("北京市");
        map.put("北京市", bj);
        
        HashSet<String> hn = new HashSet<String>();
        hn.add("海口市");
        hn.add("三亚市");
        map.put("海南省", hn);
        
        HashSet<String> zj = new HashSet<String>();
        zj.add("绍兴市");
        zj.add("温州市");
        zj.add("湖州市");
        zj.add("嘉兴市");
        zj.add("台州市");
        zj.add("金华市");
        zj.add("舟山市");
        zj.add("衢州市");
        zj.add("丽水市");
        map.put("浙江省", zj);
    
        Set<Entry<String, HashSet<String>>> entrySet = map.entrySet();
        for (Entry<String, HashSet<String>> entry : entrySet) {
            System.out.println(entry.getKey());
            HashSet<String> value = entry.getValue();
            for (String string : value) {
                System.out.println("\t" + string);
            }
        }
    }
}

15、完成在如下Map中查询城市信息

已知有省份Provice类型，有属性省份编号id和名称name，有城市City类型，有属性城市编号id和名称name,所属省份编号pid，以及所有信息现保存在一个Map中，现在要在map中，根据省份编号，查找这个省份下所有的城市。

1:北京市
    1:北京市
2:海南省
    1:海口市
    2:三亚市
3:浙江省
    1:绍兴市
    2:温州市
    7:舟山市
    8:衢州市
    9:丽水市
    5:台州市
    4:嘉兴市
    6:金华市
    3:湖州市

import java.util.HashSet;
import java.util.HashMap;
import java.util.Set;

public class AreaManager {
    private HashMap<Province,HashSet<City>> map;
    
    public AreaManager(){
        map = new HashMap<Province,HashSet<City>>();
                
        HashSet<City> bj = new HashSet<City>();
        bj.add(new City(1,"北京市",1));
        map.put(new Province(1,"北京市"), bj);
        
        HashSet<City> hn = new HashSet<City>();
        hn.add(new City(1,"海口市",2));
        hn.add(new City(2,"三亚市",2));
        map.put(new Province(2,"海南省"), hn);
        
        HashSet<City> zj = new HashSet<City>();
        zj.add(new City(1,"绍兴市",3));
        zj.add(new City(2,"温州市",3));
        zj.add(new City(3,"湖州市",3));
        zj.add(new City(4,"嘉兴市",3));
        zj.add(new City(5,"台州市",3));
        zj.add(new City(6,"金华市",3));
        zj.add(new City(7,"舟山市",3));
        zj.add(new City(8,"衢州市",3));
        zj.add(new City(9,"丽水市",3));
        map.put(new Province(3,"浙江省"), zj);
    }
    
    public HashSet<City> findCity(int pid){
                    补充代码                     
    }
}

参考答案：
    public HashSet<City> findCity(int pid){
        Set<Province> keySet = map.keySet();
        for (Province province : keySet) {
            if(province.getId() == pid){
                return map.get(province);
            }
        }
        return null;
    }

16、请编写代码读取一个项目根目录下info.properties文件

里面的内容有user=atguigu等，请获取user的value中，并在控制台打印

import java.io.FileInputStream;
import java.io.IOException;
import java.util.Properties;

public class Test{
    public static void main(String[] args) throws IOException {
        Properties pro = new Properties();
        
        //相对于bin
//        pro.load(ClassLoader.getSystemResourceAsStream("info.properties"));
        
        //相对于项目根目录
        pro.load(new FileInputStream("info.properties"));
    
        String username = pro.getProperty("user");
        System.out.println(username);
    }
}

17、请编写代码把一个GBK的文本文件内容读取后存储到一个UTF-8的文本文件中。

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;

public class Test {

    public static void main(String[] args) throws Exception {
        FileInputStream fis = new FileInputStream("test_gbk.txt");
        InputStreamReader isr = new InputStreamReader(fis, "GBK");

        FileOutputStream fos = new FileOutputStream("test_utf8.txt");
        OutputStreamWriter oos = new OutputStreamWriter(fos, "UTF-8");

        char[] data = new char[10];
        int len;
        while ((len = isr.read(data)) != -1) {
            oos.write(data, 0, len);
        }

        isr.close();
        fis.close();
        oos.close();
        fos.close();
    }
}

18、用实现Runnable接口的方式，启动一个线程完成在线程中打印1-100的数字

答案一：
public class Test {

    public static void main(String[] args) {
        PrintNumberRunnable p = new PrintNumberRunnable();
        Thread t = new Thread(p);
        t.start();
    }

}
class PrintNumberRunnable implements Runnable{

    @Override
    public void run() {
        for(int i=1; i<=100; i++){
            System.out.println("i=" + i);
        }
    }
    
}

答案二：
public class Test {

    public static void main(String[] args) {
        new Thread(new Runnable(){
                @Override
                public void run() {
                    for(int i=1; i<=100; i++){
                        System.out.println("i=" + i);
                    }
                }
        }).start();
    }
}