String class

hello, Hello everyone , Today I will bring you String Class

stay C In language , No string class , stay Java There are string classes in , for instance ,String  ret="hello";    This is a variable of string type ,

Let's talk about it String Class has three forms when it is created

public class Test {
    public static void main(String[] args) {
        String  string="hello";
        String ret=new String("wyb520");
        
        
        char[] ch={'a','b','c'};
        String str=new String(ch);
    }
}

One is to write directly

The other is new An object

The last one is by creating an array , then new An array object

 

  Look at my screenshot , We should note that in fact String When creating a variable, this object has value and hash Two memories , Let's switch to String Take a look at the source code of

  You can see from this source code , Yes hash and value,value It's a reference

There must be some classmates right String direct new A new object has questions , In fact, the real reason is String Class inherited object class ,object Class is the parent of all classes by default , So you can directly new Instantiate objects

Now let's use the diagram to better understand String Class

 

string And ret The creation of two variables is shown in the figure above , The special one is the array , First create an array and then new  String Copy an array .

Java There is no such thing as \0 ending .

String Is a reference type , Strings are not stored internally , In fact, the string is saved in the array

String s1 = new String("hello");
        String s2 = new String("world");
        String s3 = s1;
        System.out.println(s1.length());
        System.out.println(s1.isEmpty());

When calculating the length of a string, you must .length(), This bracket cannot be forgotten

Let's continue to draw pictures to better understand the creation of these objects

 String String comparison

String sort .Java A total of 4 In the way

1. use == To judge . == Compare whether to reference the same object

For basic types ,== The comparison is the value in the variable ; For reference types == Compare the address in the reference .

public static void main(String[] args) {
        int a = 40;
        int b = 20;
        int c = 40;
    
System.out.println(a == b); // false
System.out.println(a == c); // true
    }

 public static void main(String[] args) {
        String s1=new String("wyb");
        String s2=new String("wyb");
        System.out.println(s1==s2);//false
    }

For this reference type

  The addresses of the two variables are different . So the answer is false

2. use equals Methods to compare

 public static void main(String[] args) {
        String s1 = new String("hello");
        String s2 = new String("hello");
        String s3 = new String("world");

        System.out.println(s1 == s2);   // false
        System.out.println(s1.equals(s2));//true


        System.out.println(s2 == s3);   // false
        System.out.println(s2.equals(s3)); // false

    }

3. Use compare   to

System.out.println(s1.compareTo(s2));


//compaer  to  Source code 
 public int compareTo(String anotherString) {
        int len1 = value.length;
        int len2 = anotherString.value.length;
        int lim = Math.min(len1, len2);
        char v1[] = value;
        char v2[] = anotherString.value;

        int k = 0;
        while (k < lim) {
            char c1 = v1[k];
            char c2 = v2[k];
            if (c1 != c2) {
                return c1 - c2;
            }
            k++;
        }
        return len1 - len2;
    }

compare  to    The return is int Type value , Look at the source code, you can got it , Equal length returns 0, Unequal return the difference between two lengths

4.int compareToIgnoreCase

This function is associated with compare   to    It's the same thing , Just ignore the case

public static void main(String[] args) {
        String s1 = new String("WYB");
        String s2 = new String("wyb");
        System.out.println(s1.compareToIgnoreCase(s2));//0
    }

 

String Class provides string lookup methods

public static void main(String[] args) {

String s = "aaabbbcccaaabbbccc";
System.out.println(s.charAt(3)); // 'b'
System.out.println(s.indexOf('c')); // 6
System.out.println(s.indexOf('c', 10)); // 15
System.out.println(s.indexOf("bbb")); // 3
System.out.println(s.indexOf("bbb", 10)); // 12
System.out.println(s.lastIndexOf('c')); // 17
System.out.println(s.lastIndexOf('c', 10)); // 8
System.out.println(s.lastIndexOf("bbb")); // 12
System.out.println(s.lastIndexOf("bbb", 10));// 3
    }

char charAt(int index)： return index Characters in position , If index Negative or out of bounds , Throw out IndexOutOfBoundsException abnormal
int indexOf(int ch) return ch First occurrence , No return -1

int indexOf(int ch, intfromIndex)： from fromIndex The location starts to look for ch First occurrence , No return -1

int indexOf(String str) return str First occurrence , No return -1
int indexOf(String str, intfromIndex)： from fromIndex The location starts to look for str First occurrence , No return -1
int lastIndexOf(int ch) Look backwards , return ch First occurrence , No return -1
int lastIndexOf(int ch, intfromIndex)： from fromIndex The location starts to look for , Look backwards ch First occurrence , No return -1
int lastIndexOf(String str) Look backwards , return str First occurrence , No return -1
int lastIndexOf(String str, intfromIndex)： from fromIndex The location starts to look for , Look backwards str First occurrence , No return -1

Numeric and string conversion

use valueof

 public static void main(String[] args) {
        // Put other data types   Become a string 
        String s = String.valueOf(123);
        String s2 = String.valueOf(12.5);
        System.out.println(s);
        System.out.println(s2);
    }

result

 

    public static void main(String[] args) {
        // String to number 
        int data1 = Integer.parseInt("199785");
    double data2 = Double.parseDouble("1997.85");
    System.out.println(data1);
    System.out.println(data2);
    }

 public static void main(String[] args) {
        int a = Integer.valueOf("64");//6*8^1 + 4*8^0 = 52// Default decimal 
        System.out.println(a);
        int a2 = Integer.parseInt("100");// Default decimal                                                 
        System.out.println(a2);
    }

public static void main(String[] args) {

            String s1 = "abondon";
            String ret = s1.toUpperCase();
            System.out.println(ret);
            System.out.println("s1-> " + s1);
            String s2 = "ABANDON";
            ret = s2.toLowerCase();
            System.out.println(ret);
    }

  Pay attention here , When using this case, remember that it has not changed

   public static void main(String[] args) {
     String s = "happy";
//  String to array 
     char[] ch = s.toCharArray();
    for (int i = 0; i < ch.length; i++) {
     System.out.print(ch[i]);
 }
  System.out.println();
//  Array to string 
  String s2 = new String(ch);
    System.out.println(s2);
    }

format

public static void main(String[] args) {
       
            String s = String.format("%d-%d-%d", 2003 ,2,14);
            System.out.println(s);//2003-2-14
        }

public static void main(String[] args) {

            String s1 = "ababcabcdabcde";

            String ret = s1.replaceFirst("ab","qquuuuu");// For qquuuuu Replace the first ab
            System.out.println(ret);

            System.out.println("s1->  "+s1);// Even if you replace , But it has not changed itself 
        }

 String splitting 

public static void main(String[] args) {

            String s1 = "hello wyb happy everyday";
            String[] ret = s1.split(" ",4);// Separate in the form of spaces , Divide into 4 Group 
            for (String s : ret) {
                System.out.println(s);// Print 
            }
        }

 

public static void main(String[] args) {
        String str = "192.168.1.1";
        String[] ret = str.split("\\.",4);
        for (String s : ret) {
            System.out.println(s);
        }
    }

With \. The forms of are divided into 4 Group printing ,\ You need two \\ To compile the , Then two \\ Need 4 strip \\\\ To compile the

  public static void main16(String[] args) {
        String str = "197\\168\\1\\1";
        String[] ret = str.split("\\\\");
        for (String s : ret) {
            System.out.println(s);
        }
    }

 

public static void main(String[] args) {

            String str = "abcdef";
            String ret = str.substring(1,4);//[1,4)// String interception // The result is bcd The default range is left closed and right open 
            System.out.println(ret);
            System.out.println("=============");
            String s = "    we are happy    ";
            System.out.println(s.trim());// Remove the left and right spaces of the current string 
//trim  Will remove the white space characters at the beginning and end of the string ( Space ,  Line break ,  Box drawings, etc ).
            System.out.println(s);


    }

String substitution

public static void main(String[] args) {
        String str="we%are%happy";
        String ret=str.replaceAll("%"," ");
        System.out.println(ret);
    }

Replace all with spaces %

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