Detailed explanation of Java's StringBuffer class

1. The difference between String, StringBuffer and StringBuilder

String: an immutable character sequence; the bottom layer uses char[] to store
StringBuffer: a variable character sequence; thread-safe, low efficiency; the bottom layer uses char[] to store
StringBuilder: a variable character sequence; thread-unsafe, high efficiency ;The bottom layer is stored in char[]

[Note]: StringBuffer and StringBuilder are essentially the same, but StringBuffer is thread-safe, while StringBuilder is thread-unsafe; the basic operations of both are the same

2. Source code analysis

		// String和StringBuffer的操作对应底层源码操作
        String str = new String(); // char[] value = new char[0];
        String str1 = new String("abc"); // char[] value = new char[]{'a','b','c'};

        StringBuffer sb1 = new StringBuffer(); // char[] value = new char[16];
        StringBuffer sb2 = new StringBuffer("abc"); // char[] value = new char["abc".length()+16];

        System.out.println(sb1.length()); // 0
        sb1.append("a"); // value[0] = 'a';
        sb2.append('b'); // value[sb2.length()] = 'b';

3. StringBuffer method

3.1 StringBuffer append (xxx): Provides many append() methods for string concatenation

        StringBuffer sb1 = new StringBuffer("abc");
        sb1.append('a');
        sb1.append(3);
        sb1.append("def");
        System.out.println(sb1);

You can add data in any way, see the constructor for details:

3.2 StringBuffer delete (int start, int end): delete the content at the specified location

        StringBuffer sb1 = new StringBuffer("abca3def");
        sb1.delete(2, 4);
        System.out.println(sb1);

Output:
ab3def

3.3 StringBuffer replace (int start, int end, String str): replace [start, end] position with str

        StringBuffer sb1 = new StringBuffer("ab3def");
        sb1.replace(2, 4, "123");
        System.out.println(sb1);

ab123ef

3.4 StringBuffer insert (int offset, xxx): Insert xxx at the specified position

        StringBuffer sb1 = new StringBuffer("ab123ef");
        sb1.insert(0, 2.3);
        System.out.println(sb1);

2.3ab123ef

3.5 StringBuffer reverse (): reverse the current character sequence

        StringBuffer sb1 = new StringBuffer("2.3ab123ef");
        sb1.reverse();
        System.out.println(sb1);

fe321ba3.2

3.6 StringBuffer also contains String methods

• public int indexOf (String str)
• public String substring (int start,int end)
• public int length()
• public char charAt(int n)
• public void setCharAt (int n ,char ch)

For details, see Java's String class method explanation

4. Efficiency comparison of String, StringBuffer and StringBuilder

public class StringBufferTest {
    
    
    /**
     * String StringBuffer StringBuilder效率对比
     *
     */
    @Test
    public void test2() {
    
    
        long startTime = 0L;
        long endTime = 0L;
        String str = "";
        StringBuffer buffer = new StringBuffer("");
        StringBuilder builder = new StringBuilder("");

        startTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
    
    
            str += i;
        }
        endTime = System.currentTimeMillis();
        System.out.println("String执行时间：" + (endTime - startTime));

        startTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
    
    
            buffer.append(i);
        }
        endTime = System.currentTimeMillis();
        System.out.println("StringBuffer执行时间：" + (endTime - startTime));

        startTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
    
    
            builder.append(i);
        }
        endTime = System.currentTimeMillis();
        System.out.println("StringBuilder执行时间：" + (endTime - startTime));
    }
}

String execution time: 3603
StringBuffer execution time: 6
StringBuilder execution time: 3