后端面试-笔试题

1、字符串：String host=”http://www.idawa.cn/”;请解析出www.idawa.cn 并统计a 出现的次数

public static void main(String[] args) {
    String host="http://www.idawa.cn/";
    int start = host.lastIndexOf("//")+2;
    int end = host.lastIndexOf("/");
    String str = host.substring(start,end);
    System.out.println(str);
    byte[] bytes = host.getBytes();
    int num=0;
    char a='a';
    for (byte b : bytes) {
        if (a==b){
            num++;
        }
    }
    System.out.println(num);
}

2、数组[9,4,7,8,3,4,6] 先定义数组先去重复，最后降序排列

public static void main(String[] args) {
    Integer[] intArr={9,4,7,8,4,6};
    List<Integer> intOldList = Arrays.asList(intArr);
    List<Integer> intNewList = new ArrayList<>();

    //使用set 进行去重处理
    Set<Integer> intSet = new HashSet<>(intOldList);
    for (Integer integer : intSet) {
        intNewList.add(integer);
    }
    Integer [] toArray = new Integer[intNewList.size()];
    for (int i = 0; i < intNewList.size(); i++) {
        toArray[i] = intNewList.get(i);
    }
    Integer[] sort = sort(toArray);
    for (Integer integer : sort) {
        System.out.println(integer);
    }
}

//进行冒泡排序
public static Integer[] sort(Integer[] s){
    int length = s.length;
    for (int i = 0; i < length; i++) {
        for (int j = 0; j < length-i-1; j++) {
            if (s[j+1] < s[j]){
                Integer b=s[j];
                s[j]=s[j+1];
                s[j+1]=b;
            }
        }
    }
    return s;
}

 

3、定义一个Hashmap<String,String> 放入3个个元素[(“1”,“2”),(“2”,“2”),(“3”,“3”)] 根据value值去重后遍历打印

public static void main(String[] args) {
    Map<String,String> map=new HashMap();
    map.put("1","2");
    map.put("2","2");
    map.put("3","3");
    quchong(map);
}

public static void quchong(Map<String, String> map){
    Map<String,String> endMap=new HashMap();
    Set<String> temp = new HashSet<>();
    Set<String> stringSet = map.keySet();
    for (String key : stringSet) {
        //根据set是否添加进去值 判断是否为重复值 set添加时如果遇到重复值则返回false
        boolean add = temp.add(map.get(key));
        if (add){
            endMap.put(key,map.get(key));
        }
    }

    System.out.println(endMap.size());
    Set<Map.Entry<String, String>> entries = endMap.entrySet();
    for (Map.Entry<String, String> entry : entries) {
        System.out.println("key："+entry.getKey()+"；value："+entry.getValue());
    }
}

 4、递归实现：列出D盘所有的文件名后缀为doc的文件

　 static List<String> result=new ArrayList<>();
    public static void main(String[] args) {
        File file=new File("D://");
        searchFile(file);
        for (String fileName : result) {
            System.out.println(fileName);
        }
    }
    public static void searchFile(File file){
        File[] files = file.listFiles();
        if (files != null && files.length > 0){
            for (File file1 : files) {
                if (file1.isDirectory()){
                    searchFile(file1);
                }else if (file1.isFile() && file1.getName().endsWith("doc")){
                    result.add(file1.getName());
                }
            }
        }
    }

 

5、sql笔试题

EXPLAIN select st.id,sum(score) s  from student st INNER JOIN score so on st.id=so.id where sex="女"  GROUP BY st.id HAVING s>350 order by s asc LIMIT 20,10