Detailed introduction to the commonly used Java class String class

Commonly used class String

(Recommended tutorial: java introductory tutorial)

The String class represents a string. All string literals in Java programs (such as "abc" ) are implemented as instances of this class.

Strings are constants and their values ​​cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable, they can be shared.

String source code

public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
}

String member variables

//String的属性值
private final char value[];
//数组被使用的开始位置
private final int offset;
//String中元素的个数
private final int count;
//String类型的hash值
private int hash; // Default to 0
private static final long serialVersionUID = -6849794470754667710L;
private static final ObjectStreamField[] serialPersistentFields =
  new ObjectStreamField[0];

It can be seen from the source code that the bottom layer of String is maintained using a character array.

Member variables can know that the value of the String class is of final type and cannot be changed, so as long as a value changes, a new String type object will be generated, and storing String data does not necessarily start from the 0th position of the array. It starts with an element, but starts from the element pointed to by offset.

(Video tutorial recommendation: java video tutorial)

How to create a string object

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

"hello" of S1 and S3 and "hello" of s2 "world" exists in the constant pool of the method area.

As shown in the picture:

So s1==s3

s1 = new String ("hel1o") ;
s2 = new String ("hel1o") ;
System.out.println(s1 == s2) ; //false
System.out.println (s1.equals (s2) ) ; // true

s1 and s2 get two String objects through new and exist in the heap , the addresses are different. s1 and s2 are two references, so what is compared is the address, s1 != s2.

String's equals rewrites Object's equals. It does not compare addresses like Object, but compares values, so s1.equals (s2) is true.

3.char c[]= { 's', 'u' , 'n',',', 'j', 'a', 'v' ,'a'} ;
String s4 = new String(c) ;
String s5 = new String(c,4,4) ;
System.out.println(s4) ; //sun java
System.out.println(s5) ; //java

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