Java operator `+` and `StringBuilder.append`
It was said that when I use the operator +
for concat String
it is later converted to StringBuilder.append
, but when I ran the Java Decompiler GUI and opened it MainClass.class
, there was
public class MainClass
{
public static void main(String[] paramArrayOfString)
{
String str1 = "abc";
String str2 = "def";
String str3 = str1 + str2;
}
}
when the original was
public class MainClass {
public static void main(String[] args) {
String val1 = "abc";
String val2 = "def";
String res = val1 + val2;
}
}
What's wrong? I compiled it withjavac MainClass.java
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Most decompilers will automatically convert compiler-generated sequences StringBuilder
to string concatenation using +
.
One of the few that is not there is Krakatau . If you decompile the class with Krakatau, you end up with something like
public class MainClass {
public MainClass()
{
super();
}
public static void main(String[] a)
{
new StringBuilder().append("abc").append("def").toString();
}
}
So, as you can see, there is a code StringBuilder
. You can also check dismantling.
.version 51 0
.source MainClass.java
.class super public MainClass
.super java/lang/Object
.method public <init> : ()V
; method code size: 5 bytes
.limit stack 1
.limit locals 1
aload_0
invokespecial java/lang/Object <init> ()V
return
.end method
.method static public main : ([Ljava/lang/String;)V
; method code size: 26 bytes
.limit stack 2
.limit locals 4
ldc 'abc'
astore_1
ldc 'def'
astore_2
new java/lang/StringBuilder
dup
invokespecial java/lang/StringBuilder <init> ()V
aload_1
invokevirtual java/lang/StringBuilder append (Ljava/lang/String;)Ljava/lang/StringBuilder;
aload_2
invokevirtual java/lang/StringBuilder append (Ljava/lang/String;)Ljava/lang/StringBuilder;
invokevirtual java/lang/StringBuilder toString ()Ljava/lang/String;
astore_3
return
.end method
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