About java String class (source code, etc.) analysis

String class

• String is declared final and cannot be inherited

• String implements the Serializable interface, indicating that the string is serializable

• String implements the Compareble interface, indicating that String can be compared in size.

• String internally defines final char[] value value for storing string data, indicating the immutable character characteristics---- 1. When reassigning a string, the specified memory area assignment needs to be rewritten, and the original value cannot be used The value is assigned. 2. When concatenating existing strings, you also need to re-specify the memory area assignment. 3. When calling the replace method of String to modify the specified character or string, you also need to re-specify the memory area assignment.

• To assign a value to a string using a literal (different from new), the string value is declared in the string constant pool. Strings with the same content will not be stored in the constant pool.

  package StringTT;

  import org.junit.Test;

  public class Stringlei {

    @Test
    public void test1(){
    	String s1 = "abc"; // 字面量的定义方式
    	String s2 = "abc";
    
    	System.out.println(s1 == s2); // 对象比较的是地址值
    	s1 = "hello";
    
    
    
    	System.out.println(s1);
    	System.out.println(s2);
    
    	System.out.println("*****************");
    	String s3 = "abc";
    	s3 += "def" ;
    	System.out.println(s3);
    }
  

  }

Create String object

new is created in the heap space

String s = new String("abc"); method to create objects, how many objects are created in memory? Two: one is the new structure of the heap space, and the other is the data in the char[] object constant pool: "abc"

package StringTT;

import org.junit.Test;

public class StringTest2 {




	@Test
	public void test(){
	//方式一：通过字面量定义
		String s1 = "javaEE";
		String s2 = "javaEE";

	// 方式二：用new 在堆空间中创建
	
		String s3 = new String("javaEE");
		String s4 = new String("javaEE");
	
		System.out.println(s1 == s2);  // true
		System.out.println(s3 == s4);  // false
	
	}
}	

The splicing result of constants and constants is in the constant word, and the splicing of constants and variables will result in the heap

@Test
public void test1(){
	String s1 = "javaEE";
	String s2 = "hadoop";
	
	String s3 = "javaEEhadoop";
	String s4 = "javaEE" + "hadoop";
	String s5 = s1 + "hadoop";
	String s6 = "javaEE" + s2;
	String s7 = s1 + s2;
	
	System.out.println(s3 == s4); // true
	System.out.println(s3 == s5); // false
	System.out.println(s3 == s6); // false
	System.out.println(s3 == s7); // false
	System.out.println(s5 == s4); // fasle
	System.out.println(s5 == s6); // fasle
	System.out.println(s5 == s7); // fasle
	
	String s8 = s5.intern(); // 得到的是在常量池
	System.out.println(s8 == s3); // true 
}

String method

Conversion between String and char[]

• String to char[] using String.toCharArray() method

• Convert char[] to String and call the constructor of String

  package StringTT;

  import org.junit.Test;

  public class StringTest1 {
  // String 和 byte的转换
  @Test
  public void test123(){
  String str1 = “abc123”;
  char[] charArray = str1.toCharArray();
  for (int i = 0;i < 100; i++){
  System.out.println(charArray[i]);
  }
  }

    @Test
    public void test2(){
    	char[] ch1 = new char[]{'h','e','l','l','o'};
    	String str2 = new String(ch1);
    	System.out.println(str2);
    }
  

  }

Conversion between String and byte[]

If you have Chinese, you can't convert a number.
Encoding: string --> byte (understandable --> unintelligible) String to byte[] Use String's getBytes() method to
decode: byte --> string ( Unintelligible binary -> understandable) byte[] -->
During the String decoding process, it must be consistent with the character set used for encoding

package StringTT;

import java.io.UnsupportedEncodingException;
import java.util.Arrays;

import org.junit.Test;

public class StringTest1 {
	// String  和 byte的转换
	@Test
	public void test123(){
		String str1 = "abc123";
		char[] charArray = str1.toCharArray();
		for (int i = 0;i < str1.length(); i++){
			System.out.println(charArray[i]);
		}
	}


	@Test
	public void test2(){
		char[] ch1 = new char[]{'h','e','l','l','o'};
		String str2 = new String(ch1);
		System.out.println(str2);
	}


	@Test
	public void test3() throws UnsupportedEncodingException{
		String str1 = "abc123中国";
		byte[] b1 = str1.getBytes(); // 使用默认的字符集进行转换
		System.out.println(Arrays.toString(b1)); //输出
	
		byte[] b2 = str1.getBytes("gbk"); //使用gbk字符集进行编码
		System.out.println(Arrays.toString(b2));
	
		//解码 byte[] --> String
		String s1 = new String(b1); // 默认的字符集进行解码
		System.out.println(s1);
	}
}

Similarities and differences between StringBuffer, StringBuilder and String

• String is an immutable character sequence, and the bottom layer is stored using char[]
• StringBuffer is a variable sequence of characters, thread-safe and inefficient. The bottom layer uses char[] for storage.
• StringBuilder variable character sequence, new in JDK5.0, thread-unsafe, high efficiency, the bottom layer uses char[] for storage

Source code analysis

String str = new String();//char[] value = new char[0];
String str = new String(“abc”); // char[] value = new char[]{‘a’,‘b’,‘c’};

StringBuffer str = new StringBuffer();//char[] value = new char[16]; The underlying layer creates a character array with a length of 16

StringBuffer str1 = new StringBuffer("abc"); // char[] value = new char("abc".length + 16) creates a character array of length 19

Question 1: What is the length of printed str1? Or 3
questions 2: If the underlying data cannot hold the data to be added, then the underlying array needs to be expanded. By default, the expansion is twice the original capacity + 2, and the elements of the original array are copied to the new array at the same time

Guiding significance: It is recommended to use StringBuffer(int capacity) or StringBuilder(int capacity) during development, depending on whether security needs to be considered

package StringTT;

import org.junit.Test;

public class StringBufferBuiderTest {
	@Test
	public void test(){
	StringBuffer sb1 = new StringBuffer("abc");
	sb1.setCharAt(0, 'm');
	System.out.println(sb1);

	String s1 = "abc";
	s1.replace('a','m');
	System.out.println(s1); //s1不变
	}
}

public String substring – Returns a substring of a left-closed right-open interval starting from end and ending at index

Arranged from high to low efficiency: StringBuilder > StringBuffer > String

Date and time API before JDK8.0

@Test
public void test(){
	long time = System.currentTimeMillis();
	System.out.println(time);  //1970/1/1 到我们现在的时间差 毫秒数，通常叫一个时间戳
}