This is the 10th day of my participation in the November Gwen Challenge. Check out the event details: The last Gwen Challenge 2021 “@TOC

🐱🏍

1: Collection classes in Java include ArrayList, LinkedList, HashMap, etc.

A ArrayList and LinkedList implement the List interface B ArrayList access is faster than LinkedList C ArrayList performs better when randomly adding and removing elements D HashMap implement the Map interface, which allows any type of key and value objects

💯 resolution: ArrayList is an array at the bottom, so deleting elements and randomly adding them doesn’t perform as well as LinkedList.

B) 🎖️

2: Read the following programs and choose which one is the correct output

class HelloA{
public HelloA(a)
{
System.out.println("I’m A class ");
}
static
{
System.out.println("static A"); }}public class HelloB extends HelloA{
public HelloB(a)
{
System.out.println("I 'm B class");
}
static{
System.out.println("static B");
}
public static void main (String[] args){
newHelloB(); }}Copy the code

A static A I’m A class static B I’m B class B I’m A class I’m B class static A static B C static A static B I’m A class I’m B class D I’m A class static A I’m B class static B

💯 resolution: call order; Parent classes take precedence over subclasses, static over non-static. So it should print the static code block of the parent class, then the static code block of the child class, then the parent class constructor, then the child class constructor.

C) 🎖️

3: The output of executing the following code is ()

public class Demo{
public static void main(String args[]){
int num = 10;
System.out.println(test(num));
}
public static int test(int b){
try
{
b += 10;
return b;
}
catch(RuntimeException e)
{
}
catch(Exception e2)
{
}
finally
{
b += 10;
returnb; }}}Copy the code

A 10 B 20 C 30 D 40

💯 parse: finally executes regardless of exceptions, so it should be b+=10, twice. I’m going to return 30

C) 🎖️

4: The output of the following code is _____

boolean b=true?false:true= =true?false:true;
System.out.println(b);
Copy the code

A true B false C null D Empty string

💯 : Boolean b =true? 💯 : Boolean b =true? false:true? False :true The second three-wood operator follows the right-to-left principle, true? Boolean b = true? false:false; The result of step 3 is false

B) 🎖️

🐱🏍

2.1 Converting a string to an integer

Link: HTTPS://www.nowcoder.com/questionTerminal/1277c681251b4372bdef344468e4f2eSource: Niuke.comCopy the code

To convert a string to an integer requires library functions that cannot use string to convert integers. Returns 0 if the value is 0 or the string is not a valid value

Data range: String length: meets advanced levels: space complexity and time complexity

Note: ① Any symbol may appear in the string, and output 0 directly when the symbol other than +/- appears. ② The string may appear +/- and only at the first part of the string.

The sample1The input"+ 2147483647"The output2147483647
Copy the code

The questions are linked here

3. Divide the discussion into three cases. 1: indicates that the first character starts with a ‘+’ or a ‘-‘. 2: indicates that the character string is a purely numeric character. 3: indicates that there are invalid characters

Code 1: Simple but tedious

public class Solution {
    public int StrToInt(String str) {
              //1. + 123-123 a123
        if (str == null || str.length () == 0) {
            return 0;
        }
        char ch[] = str.toCharArray ();
        int sum = 0;
        char first = ch[0];
        if (first == '+') {
            for (int i = 1 ; i < ch.length ; i++) {
                char tmp = ch[i];
                if (tmp < '0' || tmp > '9') {
                    return 0;
                }
                sum = sum * 10 + tmp - '0'; }}else if (first == The '-') {
            for (int i = 1 ; i < ch.length ; i++) {
                char tmp = ch[i];
                if (tmp < '0' || tmp > '9') {
                    return 0;
                }
                sum = sum * 10 + tmp - '0';
            }
            return sum*-1;
        } else if (first <= '0' || first <= '9') {
            for (int i = 0 ; i < ch.length ; i++) {
                char tmp = ch[i];
                if (tmp < '0' || tmp > '9') {
                    return 0;
                }
                sum = sum * 10 + tmp - '0'; }}else {
            return  0;
        }
        returnsum; }}Copy the code

Code 2: Streamlined and optimized version

public class Solution {
    public int StrToInt(String str) {
               char ch[]=str.toCharArray ();
        if(str.isEmpty ()){
            return 0;
        }
        int flg=1;
        if(ch[0] = =The '-'){
          flg=-1;
          ch[0] ='0';
        }else if(ch[0] = ='+'){
            flg=1;
            ch[0] ='0';
        }
        int sum=0;
        for (int i = 0 ; i <ch.length  ; i++) {
            if(ch[i]<'0'||ch[i]>'9'){
                sum=0;
                break;
            }
            sum=sum*10+ch[i]-'0';
        }
        returnflg*sum; }}Copy the code