Work with Java StringBuilder

Question

good day

As many people know, it is undesirable to use such a construction in Java

 "Hello" + "Name" + "Surname" + etc.; //или же String text = "Hello"; text += "Name"; text += "Surname"; text += etc...;

And it is worthwhile to use StringBuilder in these cases, although I had a question about this, I would like to know if I use it correctly?

 public String toString() { StringBuilder builder = new StringBuilder(); builder.append("Name: " + this.getName()); builder.append("\nSurname: " + this.getSurname()); builder.append("\nAge: " + this.getAge()); builder.append(this.isSex() ? "\nWoman " : "\nMan "); builder.append("\nCourse: " + this.course); builder.append("\nFaculty: " + this.faculty); builder.append("\nGroup: " + this.group); String result = new String(builder); return result; }

Looking at the code, I think that I use it incorrectly ... If so, tell me how to use it, and what is desirable to use in such situations?

and on the return builder.toString() replace - Senior Pomidor

Ep1demic Ep1demic 2,581 13 25 · Accepted Answer · 2016-10-03T20:05:50

The concatenation operations in append are replaced by the continuation of the call chain of the append itself, in order to minimize the creation of new String strings:

 builder.append("\nAge: ").append(this.getAge());

etc. But in general - everything is fine. And the question of optimization in this case is questionable, although it is not excluded ...

@KryTer_NexT and do you think that your code creates any other variables?) The code I gave does exactly the same as yours;) I meant replacing concatenation (+ sign, creating a new String type) The append () method in your code.
In your version it happens like this: "\ nAge:" + this.getAge () - creates an extra new string String.

Answer 2 · 2016-10-03T20:43:42

 "Hello" + "Name" + "Surname" + etc.; // все ок, будет преобразовано в sb.append(...).append(...). ... А сумма строк-констант заранее склеется еще на этапе перехода в байт-код. Я бы не сказал, что данный код плохой. String text = "Hello"; text += "Name"; text += "Surname"; text += etc...; // крайне не эффективный код, перед каждой операцией сложения будет выполняться преобразование в StringBuilder, а затем снова в String. Также, заранее не можем сделать никаких вычислений.

The second block (directly with your code) has already been disassembled.

Answer 3 · 2016-10-04T10:28:41

In fact, you can combine strings with + if these are regular fields, and not a method call with some side-effect'ами . The compiler is smart enough, and it replaces such places with java.lang.StringBuilder .

You can verify this by considering the following example:

 public class Man { private String name; private String surname; public String firstMethod() { return name + surname; } public String secondMethod() { return new StringBuilder() .append(name) .append(surname) .toString(); }}

Let's see what is generated byte code. You can not delve into the details, the most important thing here is the explanations to it.

 public class ru.izebit.Man
   minor version: 0
   major version: 52
   flags: ACC_PUBLIC, ACC_SUPER
 Constant pool:
    # 1 = Methodref # 9. # 25 // java / lang / Object. "" :() V
    # 2 = Class # 26 // java / lang / StringBuilder
    # 3 = Methodref # 2. # 25 // java / lang / StringBuilder. "" :() V
    # 4 = Fieldref # 8. # 27 // en / izebit / Man.name: Ljava / lang / String;
    # 5 = Methodref # 2. # 28 // java / lang / StringBuilder.append: (Ljava / lang / String;) Ljava / lang / StringBuilder;
    # 6 = Fieldref # 8. # 29 // en / izebit / Man.surname: Ljava / lang / String;
    # 7 = Methodref # 2. # 30 // java / lang / StringBuilder.toString: () Ljava / lang / String;
    # 8 = Class # 31 // en / izebit / Man
    # 9 = Class # 32 // java / lang / Object
   # 10 = Utf8 name
   # 11 = Utf8 Ljava / lang / String;
   # 12 = Utf8 surname
   # 13 = Utf8               
   # 14 = Utf8 () V
   # 15 = Utf8 Code
   # 16 = Utf8 LineNumberTable
   # 17 = Utf8 LocalVariableTable
   # 18 = Utf8 this
   # 19 = Utf8 Lru / izebit / Man;
   # 20 = Utf8 firstMethod
   # 21 = Utf8 () Ljava / lang / String;
   # 22 = Utf8 secondMethod
   # 23 = Utf8 SourceFile
   # 24 = Utf8 Man.java
   # 25 = NameAndType # 13: # 14 // "": () V
   # 26 = Utf8 java / lang / StringBuilder
   # 27 = NameAndType # 10: # 11 // name: Ljava / lang / String;
   # 28 = NameAndType # 33: # 34 // append: (Ljava / lang / String;) Ljava / lang / StringBuilder;
   # 29 = NameAndType # 12: # 11 // surname: Ljava / lang / String;
   # 30 = NameAndType # 35: # 21 // toString: () Ljava / lang / String;
   # 31 = Utf8 en / izebit / Man
   # 32 = Utf8 java / lang / Object
   # 33 = Utf8 append
   # 34 = Utf8 (Ljava / lang / String;) Ljava / lang / StringBuilder;
   # 35 = Utf8 toString
 {
   public ru.izebit.Man ();
     descriptor: () V
     flags: ACC_PUBLIC
     Code:
       stack = 1, locals = 1, args_size = 1
          0: aload_0
          1: invokespecial # 1 // Method java / lang / Object. "" :() V
          4: return
       LineNumberTable:
         line 4: 0
       LocalVariableTable:
         Start Length Slot Name Signature
             0 5 0 this Lru / izebit / Man;

   public java.lang.String firstMethod ();
     descriptor: () Ljava / lang / String;
     flags: ACC_PUBLIC
     Code:
       stack = 2, locals = 1, args_size = 1
          0: new # 2 // class java / lang / StringBuilder
          3: dup
          4: invokespecial # 3 // Method java / lang / StringBuilder. "" :() V
          7: aload_0
          8: getfield # 4 // Field name: Ljava / lang / String;
         11: invokevirtual # 5 // Method java / lang / StringBuilder.append: (Ljava / lang / String;) Ljava / lang / StringBuilder;
         14: aload_0
         15: getfield # 6 // Field surname: Ljava / lang / String;
         18: invokevirtual # 5 // Method java / lang / StringBuilder.append: (Ljava / lang / String;) Ljava / lang / StringBuilder;
         21: invokevirtual # 7 // Method java / lang / StringBuilder.toString: () Ljava / lang / String;
         24: areturn
       LineNumberTable:
         line 9: 0
       LocalVariableTable:
         Start Length Slot Name Signature
             0 25 0 this Lru / izebit / Man;

   public java.lang.String secondMethod ();
     descriptor: () Ljava / lang / String;
     flags: ACC_PUBLIC
     Code:
       stack = 2, locals = 1, args_size = 1
          0: new # 2 // class java / lang / StringBuilder
          3: dup
          4: invokespecial # 3 // Method java / lang / StringBuilder. "" :() V
          7: aload_0
          8: getfield # 4 // Field name: Ljava / lang / String;
         11: invokevirtual # 5 // Method java / lang / StringBuilder.append: (Ljava / lang / String;) Ljava / lang / StringBuilder;
         14: aload_0
         15: getfield # 6 // Field surname: Ljava / lang / String;
         18: invokevirtual # 5 // Method java / lang / StringBuilder.append: (Ljava / lang / String;) Ljava / lang / StringBuilder;
         21: invokevirtual # 7 // Method java / lang / StringBuilder.toString: () Ljava / lang / String;
         24: areturn
       LineNumberTable:
         line 13: 0
         line 14: 11
         line 15: 18
         line 16:21
         line 13: 24
       LocalVariableTable:
         Start Length Slot Name Signature
             0 25 0 this Lru / izebit / Man;
 }

As we can see, both methods use java.lang.StringBuilder .

With regards to your code, it is quite correct, with the exception of excessive verbosity. StringBuilder allows StringBuilder to write more concisely using the fluent style

For the case of concatenation of 3 or more fields \ variables, will a single StringBuilder be generated in a single expression, or will a new one be created for each separate concatenation?
It does not depend on the number of lines, it depends on what you combine, for example, "hello" + getWorld () will not be optimized.
Is it possible to read somewhere in more detail, why does this not work with calling methods?
@ Roxio0 in this report Lesha Shipelev mentions this optimization youtube.com/watch?v=SZFe3m1DV1A

JAVAvladuxa JAVAvladuxa 1,177 four 18 39 · Answer 4 · 2016-10-03T20:29:54

Yes, the method in StringBuilder.append() will return an instance StringBuilder , that is, you can do as indicated by 'ep1demic'

 StringBuilder sb = new StringBuilder(); sb.append("") .append("") .append("");

it is even better not to use this.getSurname() , just write instead

 this.surname

and sb.getString(); instead of new String(sb);

There is no such getString() method for StringBuilder ... And about the methods this.getSurname, this.getName();
was due to the fact that I encapsulated in the parent class, which I had to write like this, but already removed the private modifier opened full access :)
you cannot remove the private modifier, or replace it with a protected one, or use the methods of the parent class.
Well, I used his methods, you wrote that remove this.getSurname();
if you can do this.name than this.getName (), so it’s more correct.

Work with Java StringBuilder

good day

4 answers 4

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