Tidying Up Java Basics--string class

Java String class
Create a string
The simplest notation: string i = "I am a string";
The string class is immutable, and once a string object is created, it is fixed

String i = "123";System.out.println("i = " + i);i = "abc";System.out.println("i = " + i);输出结果：123abc

equivalent to :

String i = new String("123");System.out.println("i = " + i);String i = new String("abc");System.out.println("i = " + i);i & 123 & abc 需要分配3个单位的内存单元 程序自上而下执行，只是先后把123 & abc 赋值给i 并输出

From the output, I is changing the fact that 123 ABC has not changed in memory. And I just reference the object itself has not changed
Connection string
1.concat method joins: String1.concat (string2)
2. Use + Join

class TestA{    public static void main(String[] args){        String string1 = "123";        String string2 = "abc";        String string3 = "llll";        String string4;        String string5;        string4 = string1.concat(string2).concat(string3);        string5 = "123" + "abc" + "llll";        System.out.println("string4 = " + string4);        System.out.println("string5 = " + string5);    }}输出结果：string4 = 123abcllllstring5 = 123abcllll

Common methods of the string class
1. Length of the string
public int Length ()

String str = new String("adzzbzz");int len = str.length();//len = 7

2. Characters at the string index
public char charAt (int index)

String str = new String("adzzbzz");char ch = str.charAt(5);//ch = b

3. String comparisons
int CompareTo (Object o)
Or
int CompareTo (String anotherstring)

String str1 = new String("abc");String str2 = new String("ABC");int a = str1.compareTo(str2);//a>0int b = str1.compareTo(str2)//b=0

4. String connection
public string concat (string s)

String str = "111".concat("222").concat("333");String str = "111"+"222"+"333";

5. Extracting substrings
Public String substring (int beginindex)
Public String substring (int beginindex, int endIndex)

String str1 = new String("adzzbzz");String str2 = str1.substring(2);//str2 = "zzbzz"String str3 = str1.substring(2,5);//str3 = "zzb"

6. Single character lookup in a string
public int indexOf (int ch/string str)
public int indexOf (int ch/string str, int fromIndex)
public int lastIndexOf (int ch/string str)
public int lastIndexOf (int ch/string str, int fromIndex)

String str = "I am a good student";int i1 = str.indexOf(‘a‘);//i1= 2int i2 = str.indexOf("good");//i2 = 7int i3 = str.indexOf("w",2);//i3= -1int i4 = str.lastIndexOf("a");//i4= 5int i5 = str.lastIndexOf("a",3);//i5= 2

7. Case conversion of characters in a string
Public String toLowerCase ()
Public String toUpperCase ()

String str = new String("abCD");String str1 = str.toLowerCase();//str1 = "abcd"String str2 = str.toUpperCase();//str2 = "ABCD"

8. Substitution of characters in a string
Public String replace (char OldChar, char Newchar)
public string Replacefirst (string regex, string replacement)
public string ReplaceAll (string regex, string replacement)

String str = "adzzzad";String str1 = str.replace(‘a‘,‘b‘);//str1 = "bdzzzbd"String str2 = str.replace("ad","bp");//str2 ="bpzzzbp"String str3 = str.replaceFirst("ad","bp");//str3 = bpzzzad"String str4 = str.replaceAll("ad","bp");//str4 = "bpzzzbp"

Organizing Java Basics--string class