day14 Java common class String class

Keywords: Java

In my most sad time
Give me warm eyes

character string:
A string of data composed of multiple characters is called a string
It can also be regarded as a character array
Is a constant whose value cannot be changed once initialized

1, Construction method

1.1 public String()

Create an empty string with a length of 0, which is meaningless, so it is generally not used

String s1 = new String();
System.out.println(s1);
System.out.println(s1 + " + " + s1.length());
//	 + 0

1.2 public String(byte[] bytes)

Converts a byte array into a string
Because the array needs to be converted into ascii code, the members in the array should be legal

byte[] a = {97, 98, 99, 100, 101};
String s2 = new String(a);
System.out.println(s2 + " + " + s2.length());
//	abcde + 5

1.3 public String(byte[] bytes,int offset,int length)

Truncate a part of the byte array into a string
offset is the starting index position and length is the interception length

byte[] b = {97, 98, 99, 100, 101};
String s3 = new String(a,1,3);
System.out.println(s3 + " + " + s3.length());
//	bcd + 3

1.4 public String(char[] value)

Converts a character array into a string

char[] c = {'f','g','h','Leek','food','box','son'};
String s4 = new String(c);
System.out.println(s4 + " + " + s4.length());
//	fgh leek box + 7

1.5 public String(char[] value,int offset,int count)

Truncate a part of the character array as a string
offset is the starting index position and count is the intercept length

char[] d = {'f','g','h','Leek','food','box','son'};
String s5 = new String(d,3,4);
System.out.println(s5 + " + " + s5.length());
//	Leek box + 4

1.6 public String(String original)

Converts a string to a new string

String e = "fgh Leek box ";
String s6 = new String(e);
System.out.println(s6 + " + " + s6.length());
//	fgh leek box + 7

2, The String value cannot be changed

String s = "hello";
s += "world";
System.out.println(s);		
//	helloworld

At this time, helloworld is output because:
The variable s here is a reference type and points to the address of "hello" in the constant pool
When the + = operation is performed on it in the second step, space will be opened up in the constant pool to store "helloworld"
Then point s to the address of "Hello world"

2, The difference between String and new String

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

s1 points to the address of "hello" in the constant pool
s2 is an object created in heap memory and points to the address of "hello" in the constant pool
One is the address of the constant pool and the other is the address of the heap memory, so the addresses are different
The equals() method is overridden by default in the String class. At this time, the addresses in the two final constant pools are compared

3, String addition

1. If the string is a variable addition, it will open up space first, and then splice
2. If the string is a constant addition, add it first, and then look for it in the constant pool. If it is found, it will be returned. If it is not found, it will be created

String a = "hello";
String b = "world";
String c = "helloworld";
String d = a + b;
String e = "helloworld";
String f = a + b;
//Get string address value
System.out.println(System.identityHashCode(c));		//	1163157884
System.out.println(System.identityHashCode(d));		//	1956725890
System.out.println(System.identityHashCode(e));		//	1163157884
System.out.println(System.identityHashCode(f));		//	356573597

System.out.println(d == a + b);		//false
System.out.println(d == "helloworld");		//false

4, Judgment function of String class

4.1 boolean equals(Object obj)

Compares whether the contents of the string are the same, and is case sensitive

String a = "helloworld";
String b = "helloworld";
System.out.println(a.equals(b));		//	true

4.2 boolean equalsIgnoreCase(String str)

Compares whether the contents of the string are the same, ignoring case

String a = "helloworld";
String c = "Helloworld";
System.out.println(a.equalsIgnoreCase(c));		//	true

4.3 boolean contains(String str)

Judge whether the large string contains the string str. if so, it returns true, otherwise it is false
Case sensitive

String a = "helloworld";
String d = "world";
System.out.println(a.contains(d));		//	true

4.4 boolean startsWith(String str)

Tests whether this string starts with the specified prefix
Case sensitive

String a = "helloworld";
String e = "hel";
String f = "ello";
System.out.println(a.startsWith(e));		//	true
System.out.println(a.startsWith(f));		//	fales

##4.5 Boolean endswith (string STR) > test whether the string ends with the specified suffix > case sensitive ` ` ` java String a = "helloworld"; String e = "rld"; String f = "Rld"; System.out.println(a.startsWith(e)); // true System.out.println(a.startsWith(f)); // false ```

4.6 boolean isEmpty()

Determine whether the string is empty

String g = "";
String h = null;
System.out.println(g == h);		//	false
System.out.println(g.isEmpty());		//	true
System.out.println(h.isEmpty());		//	true

5, Judgment function of String class

5.1 int length()

Gets the length of the string

String s = "helloworld";
System.out.println(s.length());		//	10

5.2 char charAt(int index)

Returns the value at the index specified by char

String s = "helloworld";
System.out.println(s.charAt(1));		//	e
//An error will be reported when crossing the boundary

5.3 int indexOf(int ch)

Returns the index within the string where the specified character first appears

String s = "helloworld";
System.out.println(s.indexOf('e'));		//	1
//Return - 1 not found

5.4 int indexOf(String str)

Returns the index within the string where the specified substring first appears
Index of the first character of the string

String s = "helloworld";
System.out.println(s.indexOf("ello"));		//	1
//Return - 1 not found

5.5 int indexOf(int ch,int fromIndex)

Returns the index within the string where the specified character first appears, starting the search with the specified index

String s = "helloworld";
System.out.println(s.indexOf('o',4));		//	4
//Return - 1 not found

5.6 int indexOf(String str,int fromIndex)

Returns the index within the string where the specified string first appears, and starts the search with the specified index

String s = "helloworld";
System.out.println(s.indexOf("wo",3));		//	5
//Return - 1 not found

5.6 String substring(int start)

Returns a string that is a substring of this string
A substring starts with a character at the specified index and extends to the end of the string

String s = "helloworld";
System.out.println(s.substring(5));		//	world
//An error will be reported when crossing the boundary

5.7 String substring(int start,int end)

Returns a string that is a substring of this string
The substring starts at the specified index start and extends to the index end - 1
Left closed right open [start, end)

String s = "helloworld";
System.out.println(s.substring(5,10));		//	world
//An error will be reported when crossing the boundary

summary

boolean equals(Object obj)

Compare whether the contents of the string are the same
Case sensitive

boolean contains(String str)