String class

String type

Catalog :

1. Common methods

stay java in , String is a reference type ,String There are many methods in class that are convenient for us to use , Its functions are applicable to various scenarios , Now let's introduce these common methods .

1.1 Construction method

You can enter the official help manual to search String class , We can see String Class in java.lang The package is imported by default , And it was final The modified .

There are many ways to construct strings , There are three common construction methods

1. String s1 = "hello";// Abbreviation 
   

2. String s1 = new String("hello");
   

3. char[] ch = new char[]{
         'a','b','c'};
   String s1 = new String(ch);
   

The first method is used most , The most simple .

Let's analyze the memory distribution of these construction methods

1.2 String lookup (indexOf)

Method	usage
string.charAt(index)	It is used to get a character in the string directly through subscript
string.indexOf(char ch)	Used to find the first character in the string ch The location of
string.lastindexOf(char ch)	Find characters from back to front ch First occurrence
string.indexOf(string ch)lastindexof It's fine too	Used to find strings ch First occurrence
string.indexOf(char ch ,int fromindex)last It's fine too	The front is the same ,fromindex It means to start looking from this position

    public static void main5(String[] args) {
    
        String s1 = "ababacabcdaseasq";
        String s2 ="www.baidu.com";
        System.out.println(s2.lastIndexOf('.',4));
        System.out.println(s2.lastIndexOf('.'));
        System.out.println(s1.lastIndexOf('ab'));
    }
    public static void main4(String[] args) {
    
        String s1 = "hello";
        for (int i = 0; i < s1.length(); i++) {
    
            System.out.println(s1.charAt(i));
        }
        System.out.println("====================");
        System.out.println(s1.indexOf('h'));// Find the first character 
        System.out.println(s1.indexOf('l',3));
        System.out.println("====================");
        String s2 = "abaabcabcdabsaea";
        System.out.println(s2.indexOf("abcd"));
    }

1.3 String comparison (equals,compareTo)

Generally, we use == To express , For basic types == The comparison is whether the content is the same , But for reference types == The comparison is whether the same object is referenced , This cannot meet our requirements to compare content .

For reference types, we use equals Compare ,equals The return value of the boolean,java Case insensitive equalsIgnoreCase Method .

String s1 = "hello";
String s2 = "hello";
String s3 = "HELLO";
System.out.println(s1.equals(s2));//true
System.out.println(s1.equalsIgnoreCase(s3));//true

equals You can only compare whether two strings are the same , To compare the size, we need to use compareTo Method because String Realized CompareTo Interface ,compareTo The return value type of is int, Again java A comparison that ignores case is also provided in compareToIgnoreCase Method .

String s1 = "hello";
String s2 = "hello";
String s3 = "HELLO";
System.out.println(s1.compareTo(s2));//0
System.out.println(s1.compareToIgnoreCase(s3));//0

1.4 transformation

1. toggle case

Method	function
toUpperCase()	Change all lowercase strings to uppercase （ Letter ）
toLowerCase()	Change all uppercase strings to lowercase （ Letter ）

        String s1 = "hello";
        String s2 = "HELLO";
        System.out.println(s1.toUpperCase());//HELLO
        System.out.println(s2.toLowerCase());//hello
        System.out.println(s1);//hello
        System.out.println(s2);//HELLO

Chinese regardless

2. String to array (toCharArray)

String s1 = "hello";
char[] ch = s1.toCharArray();
for (char x : ch) {
    
    System.out.println(x);
}

3. Numeric conversion string (valueOf)

Method	function
string.valueOf( Basic types i)	Put the basic type i Convert to string type
Basic types .valueOf(String s)	take string Convert type to base type
Basic types .parse***(String s)( This conversion is generally used )	take string Convert type to base type
Basic types .valueOf(String s radix x) radix Express x Base number （parse*** Also have ）	take String The value of type is pressed x Conversion from decimal to decimal

        String s1 = String.valueOf(123);
        String s2 = String.valueOf(12.5);
        System.out.println(s1);
        System.out.println(s2);

        String s3 = String.valueOf(new Student(12));// It can also be an object 
        System.out.println(s3);

        int a = Integer.valueOf("0101",2);//5
        System.out.println(a);
        int b = Integer.parseInt("1000");//1000
        System.out.println(b);

1.5 String replacement (replace)

Method	function
String replaceAll(String regex, String replacement)	Replace all specified contents
String replaceFirst(String regex, String replacement)	Replace the first content

        String s1 = "ababcabcdeaaweqggh";
        System.out.println(s1);
        System.out.println(s1.replace('a', 'k'));//kbkbckbcdekkweqggh
        System.out.println(s1.replace("ab", "wt"));//wtwtcwtcdeaaweqggh
        System.out.println(s1.replaceAll("ab","zc"));//zczcczccdeaaweqggh
        System.out.println(s1.replaceFirst("ab","1122"));//1122abcabcdeaaweqggh

1.6 String splitting (split)

Divide the string into several strings according to the specified separator

Method	function
String[] split(String regex)	Split all strings
String[] split(String regex, int limit)	The string in the specified format , Split into limit Group

        String s1 = "ni hao ma zui jin";
        String[] s2 = s1.split(" ",3);
        for (String x : s2) {
    
            System.out.println(x);
            //ni
            //hao
            //ma zui jin
        }

Escape the specified special separator, such as :,.

        String s1 = "255\\255\\255\\255";
        // Special characters need to be escaped 
        String[] s2 = s1.split("\\\\");// Because the string also needs escape , So there are two escape 
        for (String x : s2) {
    
            System.out.println(x);
        }
        String s1 = "255.255.255.255";
        // Special characters need to be escaped 
        String[] s2 = s1.split("\\.");
        for (String x : s2) {
    
            System.out.println(x);
        }

You can also give multiple delimiters to split multiple times

        String s1 = "zaiganma nihao&=nibuhao";
        String[] s2 = s1.split(" |&");
		for(Stiring x : s2){
    
                System.out.println(x);
        }
        // Split multiple times 
		s2 = s1.split(" ");
        for (String x : s2) {
    
            String[] s3 = x.split("=");
            for (String x1 : s3) {
    
                System.out.println(x1);
            }
        }

1.7 String interception (substring)

This is nothing to say simple

        String s1 = "liuyuhao";
        System.out.println(s1.substring(3));
        System.out.println(s1.substring(2, 5));//[2,5) Left closed right away 

1.8 Other operating

trim() Delete the space on the left and right sides of the string , Tabulation , Line break, etc

        String s1 = " liu yu ";
        System.out.println(s1);
        System.out.println(s1.trim());

intern() Manually enter the pool , The following is about constant pool

        char[] ch = {
    'a','b','c'};
        String s1 = new String(ch);
        s1.intern();// Manually enter the pool 
        String s2 = "abc";
        System.out.println(s1 == s2);

String Class method , If the return is basically new String object . because String The content in cannot be modified after it is created ,String Is an unchangeable object , So try to avoid direct modification of the string

        String s1 = "hello";
        s1 += " word";
        System.out.println(s1);

        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append("hello");
        stringBuilder.append(" word");

Because its bottom layer actually relies on StringBuilder class , Many temporary variables will be created in the middle , So for string modification, we directly use StringBuilder and StringBuffer.String The reason why the content of cannot be changed is String Fields in the class value By final modification , indicate value Cannot reference other character arrays , But you can modify the contents of the reference array .

1.9String Constant pool

String constant pool, also known as StringTable In fact, it is an efficient query data structure .

inter() Manually enter the pool

2.StringBuilder and StringBuffer

Be careful String,StringBuffer,StringBuilder Differences among the three

1. String The content of cannot be modified ,StringBuffer and StringBuilder The content of can be modified
2. StringBuffer and StringBuilder Both methods are roughly the same
3. StringBuffer Synchronous processing , Multithreading is very safe ,StringBuilder If not, do you handle , Unsafe when multithreading .

Here are some key points String There is no way ,StringBuffer The same way

        StringBuilder stringBuilder = new StringBuilder();
        String s1 = "hello";
        stringBuilder.append(s1);// Convert the string to StringBuilder
        stringBuilder.insert(0,"lyh ");// stay 0 The subscript position starts inserting ,lyh
        stringBuilder.insert(1,1);// stay 1 Insert at subscript position ,1
        stringBuilder.deleteCharAt(1);// Delete 1 Of subscript position 1
        stringBuilder.setCharAt(0,'z');// take 0 The character in the subscript position is changed to z
        stringBuilder.codePointAt(1);// return 1 Of subscript characters ASCII Code value 
        stringBuilder.reverse();// String reversal 
        String str = stringBuilder.toString();//StringBuilder turn String
        stringBuilder.append("how are you");//String turn StringBuilder
        int num = stringBuilder.capacity();
        System.out.println(num);// Get the total size of the underlying saved string space 
        System.out.println(stringBuilder);

Add ：

1. String format output

        String s1 = String.format("%d-%d-%d",2002,06,24);
        System.out.println(s1);

2. The length of the string is different from that of the array , String length is a method called , Array calls a field

        String s1 = "hello";
        System.out.println(s1.length());
        
        int[] array = {
    1,2,3,4};
        System.out.println(array.length);

3. Three algorithms for string matching ,BF,BM,KMP, Here is a brief introduction to ,BF The algorithm is a simple two-level cycle of violent matching ,BM The algorithm uses the idea of a good number pair and a bad number pair to process the substring ,KMP The algorithm cites a next Arrays handle substrings and next Optimization of arrays ,nextval Array , You can learn the specific related content .

Learning results of this section

1. I understand String class
2. Get the hang of String Various methods in the class
3. I understand String Constant pool
4. Get to know StringBuilder and StringBuffer Also mastered their various methods
5. master String,StringBuffer and StringBuilder The difference between