First:

Attention：Here STR ="test OK "is not changing the value but making STR refer to a new object 

knowlodge point：Assignment of a reference variablep57.

Secondary: 

Answe：false

Third: 

Knowledge point： charater's ordinary pool

public class String {    public static void main(String[] args) {     StringBuilder stringBuilder=new StringBuilder(); stringBuilder.append("hello"); stringBuilder.append("world"); java.lang.String s=stringBuilder.toString(); System.out.println(s);    }}

knowledge point: 

Knowledge point：

The append method always adds these characters to the end of the builder, so the contents of the StringBuilder can be modified

interview question：

1. The difference between String, StringBuffer, StringBuilder  

The contents of String cannot be modified. The contents of StringBuffer and StringBuilder can be modified.  

A StringBuffer is similar to a StringBuilder in most respects  

StringBuffer is a synchronous, thread-safe operation.  StringBuilder does not use synchronization, which is a thread-unsafe operation

2.How many String created in total?[Regardless of whether the constant pool existed before]

String str = new String("ab");  // 会创建多少个对象String str = new String("a") + new String("b");  // 会创建多少个对象

Answer:2 and 5

Fourth:

topic：

1.

    public class Pratice { public static void main(String[] args) {     String word="loveleetcode";     int ret= Find (word);     System.out.println(ret); } static int Find(String word){     int[]count=new int[26];     for (int i = 0; i <word.length(); i++) {  char ch=word.charAt(i);  count[ch-'a']++;     }     for (int i = 0; i <word.length(); i++) {  char ch=word.charAt(i);  if (count[i]==1){      return i;  }     }     return -1; }    }

2.Calculate the length of the last word in the string

    public class Pratice { public static void main(String[] args) {     String word="loveleetcode";     int ret= Calculate (word);     System.out.println(ret); } static int Calculate(String word) {     String s=word.substring(word.lastIndexOf(' ')+1);     int len=s.length();     return len; }    }

 Split by characte：unfamiliar

 Train of thought:

Attention:Conversion to lower-case

3. 

    public class Pratice { public static void main(String[] args) {     String word="A man, a plan, a canal: Panama";     word=word.toLowerCase();     boolean ret= Estimate (word);     System.out.println(ret); } static boolean Wheather(char ch){     if (ch>='a'&&ch='0'&&ch<='9'){  return true;     }     return false; } static boolean Estimate(String word) {     int left=0;     int right=word.length()-1;     while (left<right){  while (left<right&&!Wheather(word.charAt(left))){      left++;  }  while (left<right&&!Wheather(word.charAt(right))){      right--;  }  if (word.charAt(left)!=word.charAt(right)){      return false;  }else {      left++;      right--;  }     }     return true; }    }

Fifth:  

gitte address:2022-6-1 · 孙雨嘉/Practice the warehouse - 码云 - 开源中国 (gitee.com)