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There is a simple code below

public class StringPlusTest{
    public static void main(String[] args) {
        String s1 = "aaa";
        String s2 = "bbb";
        String s = "ccc" +s1 + s2 +"ddd";
    }}

Use javap -c .\StringPlusTest.class to decompile and get

Compiled from "StringPlusTest.java"public class com.epoint.codetuning.test.StringPlusTest {
  public com.epoint.codetuning.test.StringPlusTest();
    Code:
       0: aload_0       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String aaa
       2: astore_1       3: ldc           #3                  // String bbb
       5: astore_2       6: new           #4                  // class java/lang/StringBuilder
       9: dup      10: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      13: ldc           #6                  // String ccc
      15: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      18: aload_1      19: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      22: aload_2      23: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      26: ldc           #8                  // String ddd
      28: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      31: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      34: astore_3      35: return}

Among them,

ldc 把常量池中的項壓入棧
astore_1 將引用類型或returnAddress類型值存入局部變量1
astore_2 將引用類型或returnAddress類型值存入局部變量2
new 創(chuàng)建一個新對象
dup 復(fù)制棧頂部一個字長內(nèi)容
invokespecial 根據(jù)編譯時類型來調(diào)用實例方法
aload_1 從局部變量1中裝載引用類型值
aload_2 從局部變量2中裝載引用類型值
astore_3 將引用類型或returnAddress類型值存入局部變量3

For more instructions, please see the JVM instruction manual

For For Java, this code should be in principle:

public class StringPlusTest{
    public static void main(String[] args) {
        String s1 = "aaa";
        String s2 = "bbb";
        String s = new StringBuilder().append("ccc").append(s1).append(s2).append("ddd").toString();
    }}

It can be seen that the implementation principle of using " " to splice strings in Java is to create a temporary StringBuilder object and call the append and toString methods.

Made some modifications to the above code

public class StringPlusTest{
    public static void main(String[] args) {
        String s1 = "aaa";
        String s2 = null;
        String s = "ccc" +s1 + s2 +"ddd";
        System.out.println(s);
    }}

What is the result?

You can get the following when running:

cccaaanullddd

View the StringBuilder source code

@Overridepublic StringBuilder append(String str) {
    super.append(str);
    return this;}

This method calls the method that inherits the parent class AbstractStringBuilder, and then goes to the parent class to view

 public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;}private AbstractStringBuilder appendNull() {
    int c = count;
    ensureCapacityInternal(c + 4);
    final char[] value = this.value;
    value[c++] = 'n';
    value[c++] = 'u';
    value[c++] = 'l';
    value[c++] = 'l';
    count = c;
    return this;}

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