Do you know what string did to “+”

Time:2021-3-19

Here’s a simple piece of code

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

usejavap -c .\StringPlusTest.classDecompile 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 pushes items in the constant pool onto the stack

For more instructions, seeJVM instruction manual

For Java, the principle of this code should be:

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 principle of using “+” to splice strings in Java is to create a temporary StringBuilder object and call the methods of append and toString.

Make some changes 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 was the result?

The results are as follows

cccaaanullddd

View the source code of StringBuilder

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

This method calls the method inherited from 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;
}

The appendnull method simply means capacity + 4 and appending a null string.

So the result is “cccaaanullddd”.

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