面試題總結 —— 摩羯

選擇題

1.下列說法正確的有()
A.class中的constructor不可省略

B.constructor必須與class同名,但方法不能與class同名

C.constructor在一個物件被new時執行

D.一個class只能定義一個constructor

解析: 
    A：無參構造器是可以省略的
    B：constructor不一定與class同名,因為存在內部類的原因
    D：方法的過載

2.下列程式碼的輸出結果是()
public class Test{
    public static void main(){
        People ab = new Child();    
    }
}

class People{
    static{
        System.out.println("1");
    }
    {
        System.out.println("2");
    }
    public People(){
        System.out.println("3")
    }
}

class Child extends People{
    static{
        System.out.println("a");
    }
    {
        System.out.println("b");
    }
    public People(){
        System.out.println("c")
    }
}
A:cab312    B:312cab    C:a123bc    D:1a23bc

解析:首先入口是main()方法,運用了多型,既然涉及到了多型那麼就涉及到了
例項化物件時候順序的問題,先執行父類的方法,在執行子類的方法,其中靜態代
碼塊是最先初始化的,它是在類載入到記憶體的時候就可以執行的,其次是程式碼塊
所以,通過前面的分析我們可以知道前面兩步的執行順序為:1a，之後再次進行
分析靜態程式碼塊之後是程式碼塊

3.下面程式碼輸出結果()
public static void main(){

    Integer a = new Integer(3);
    Integer b = 3;
    int c = 3;
    System.out.println(a == b);
    System.out.println(a == c);

}

A:true true  B:true false   C:false true    D:false false
分析:Integer是一個物件,int是8個基本資料型別裡面的也就是常量
    第一條語句:生成了一個物件
    第二條語句:Integer b = 3 其實在底層呼叫了Integer.valueOf()方法
              其中Integer.valueOf()返回一個Integer物件
    且==就是比較的是地址 所以第一條為 false
    第三條語句:int c = 3;在與Integer a = 3;相比較的時候會自動拆箱
    比較的是數值,所以返回true

    所以選擇C
4.在第三行中生成的object在第幾行執行後會成為garbage collection的物件

    1.public class MyClass{

    2.  public StringBuffer aMethod(){
        3.  StringBuffer sf = new StringBuffer("Hello");
        4.  StringBuffer[] sf_arr = new StringBuffer[1];
        5.  sf_arr[0] = sf;
        6.  sf = null;
        7.  sf_arr[0] = null;
        8.  return sf;
        9.  }

    }
A: 第五行  B：第六行   C：第七行   D：第八行

分析:被垃圾回收機制回收的物件存在以下的特點:
    1.沒有被其他物件所引用
    2.當前物件為null
所以正確答案:C

5.下面程式的輸出結果是()
public static void main(String[] args){

    Thread t = new Thread(){
        public void run(){
            pong();
        }
    };
    t.run();
    System.out.println("ping");
}

Static void pong(){
    System.out.println("pong");
}
A:pingpang B:pongping   C:pingpang和pangping都用可能 D：d都不輸出

分析：順序執行沒有什麼好說的B
 

解答題

1.整型數的反轉.例如輸入123,返回321,輸入 -1234返回-4321超過最大值返回-1

    public class InverseString {

    public static String InverseMethod(String strValue){

        char[] charArrayofString = strValue.toCharArray();
        String strInverse;
        String charBuffer = new String();
        for(int i = charArrayofString.length - 1; i >= 0; i--){
            strInverse = String.valueOf(charArrayofString[i]);
            charBuffer +=strInverse;
        }
        return charBuffer;
    }
    public static void main(String[] args){

        Scanner cin = new Scanner(System.in);
        int integerValue = cin.nextInt();
        String inverseStringValue = new String();
        if(integerValue > 0){
            String stringValue = integerValue + "";
            inverseStringValue = InverseMethod(stringValue);
        }else if(integerValue < 0){
            integerValue = Math.abs(integerValue);
            String stringValue = integerValue + "";
            inverseStringValue = InverseMethod(stringValue);
            inverseStringValue = "-"+inverseStringValue;
        }
        System.out.println("結束:" + inverseStringValue);
    }
}

2.有兩張表
學生基本資訊表student_info(student_no,name,age,sex)
學生成績表score_info(student_no,subject,score)
問題:請用sql查詢出沒有掛科的學生的姓名以及該學生的平均成績

分析:完整解答過程如下
CREATE TABLE student_info(

      student_no INT PRIMARY KEY,
      student_name VARCHAR(20),
      student_age INT,
      student_sex VARCHAR(20)
    )
    COMMIT;

    CREATE TABLE score_info(

      student_no INT,
      score_subject VARCHAR(20),
      score_score INT
    )
    COMMIT;

    DROP TABLE score_info;

    DESC student_info;
    DESC score_info;



    INSERT INTO student_info(student_no,student_name,student_age,student_sex) VALUES(1001,'baidu',18,'male');
    INSERT INTO student_info(student_no,student_name,student_age,student_sex) VALUES(1002,'TX',19,'male');
    INSERT INTO student_info(student_no,student_name,student_age,student_sex) VALUES(1003,'ali',20,'female');
    INSERT INTO student_info(student_no,student_name,student_age,student_sex) VALUES(1004,'google',21,'female');
    INSERT INTO student_info(student_no,student_name,student_age,student_sex) VALUES(1005,'windows',22,'male');
    COMMIT;

    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1001,'java',80);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1001,'php',81);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1001,'ui',82);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1001,'python',83);

    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1002,'java',59);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1002,'php',58);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1002,'ui',61);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1002,'python',53);


    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1003,'java',61);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1003,'php',62);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1003,'ui',50);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1003,'python',53);

    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1004,'java',80);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1004,'php',90);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1004,'ui',100);
    INSERT INTO score_info(student_no,score_subject,score_score) VALUES(1004,'python',99);

    COMMIT;



    -- 巢狀子查詢
    SELECT st.`student_name`,AVG(so.`score_score`)  FROM score_info so,student_info st  WHERE st.`student_no` = so.`student_no` GROUP BY so.`student_no` HAVING AVG(so.`score_score`) > 60;
    SELECT st.`student_name`,AVG(so.`score_score`)  FROM score_info so,student_info st 

    -- 一會再用左聯接的方式進行查詢
    SELECT  st.`student_name`,AVG(so.`score_score`)  FROM score_info so
    LEFT JOIN student_info st  ON so.`student_no`=st.`student_no`
    GROUP BY so.`student_no`
    HAVING AVG(so.`score_score`) > 60;

    -- 只要是有一科不及格就GG的
    -- 請用sql查詢出沒有掛科的學生的姓名
    -- 以及該學生的平均成績

    SELECT * FROM student_info st WHERE st.`student_no`  NOT IN (SELECT so.`student_no` FROM score_info so WHERE so.`score_score` < 60);

    -- 巢狀子查詢
    SELECT st.`student_name`,AVG(so.`score_score`)  FROM score_info so,student_info st  WHERE st.`student_no` = so.`student_no` GROUP BY so.`student_no` HAVING AVG(so.`score_score`) > 60;

    SELECT st.`student_name`,AVG(so.`score_score`)  FROM score_info so,student_info st  WHERE st.`student_no`  NOT IN (SELECT so.`student_no` FROM score_info so WHERE so.`score_score` < 60) AND  so.`student_no`=st.`student_no` GROUP BY so.`student_no`;