java: use StringBuilder to insert at the beginning

问题

I could only do this with String, for example:

String str="";
for(int i=0;i<100;i++){
    str=i+str;
}

Is there a way to achieve this with StringBuilder? Thanks.

回答1:

StringBuilder sb = new StringBuilder();
for(int i=0;i<100;i++){
    sb.insert(0, Integer.toString(i));
}

Warning: It defeats the purpose of StringBuilder, but it does what you asked.

Better technique (although still not ideal):

Reverse each string you want to insert.
Append each string to a StringBuilder.
Reverse the entire StringBuilder when you're done.

This will turn an O(n²) solution into O(n).

回答2:

you can use strbuilder.insert(0,i);

回答3:

Maybe I'm missing something but you want to wind up with a String that looks like this, "999897969594...543210", correct?

StringBuilder sb = new StringBuilder();
for(int i=99;i>=0;i--){
    sb.append(String.valueOf(i));
}

回答4:

As an alternative solution you can use a LIFO structure (like a stack) to store all the strings and when you are done just take them all out and put them into the StringBuilder. It naturally reverses the order of the items (strings) placed in it.

Stack<String> textStack = new Stack<String>();
// push the strings to the stack
while(!isReadingTextDone()) {
    String text = readText();
    textStack.push(text);
}
// pop the strings and add to the text builder
String builder = new StringBuilder(); 
while (!textStack.empty()) {
      builder.append(textStack.pop());
}
// get the final string
String finalText =  builder.toString();

回答5:

This thread is quite old, but you could also think about a recursive solution passing the StringBuilder to fill. This allows to prevent any reverse processing etc. Just need to design your iteration with a recursion and carefully decide for an exit condition.

public class Test {

    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        doRecursive(sb, 100, 0);
        System.out.println(sb.toString());
    }

    public static void doRecursive(StringBuilder sb, int limit, int index) {
        if (index < limit) {
            doRecursive(sb, limit, index + 1);
            sb.append(Integer.toString(index));
        }
    }
}

回答6:

I had a similar requirement when I stumbled on this post. I wanted a fast way to build a String that can grow from both sides ie. add new letters on the front as well as back arbitrarily. I know this is an old post, but it inspired me to try out a few ways to create strings and I thought I'd share my findings. I am also using some Java 8 constructs in this, which could have optimised the speed in cases 4 and 5.

https://gist.github.com/SidWagz/e41e836dec65ff24f78afdf8669e6420

The Gist above has the detailed code that anyone can run. I took few ways of growing strings in this; 1) Append to StringBuilder, 2) Insert to front of StringBuilder as as shown by @Mehrdad, 3) Partially insert from front as well as end of the StringBuilder, 4) Using a list to append from end, 5) Using a Deque to append from the front.

// Case 2    
StringBuilder build3 = new StringBuilder();
IntStream.range(0, MAX_STR)
                    .sequential()
                    .forEach(i -> {
                        if (i%2 == 0) build3.append(Integer.toString(i)); else build3.insert(0, Integer.toString(i));
                    });
String build3Out = build3.toString();


//Case 5
Deque<String> deque = new ArrayDeque<>();
IntStream.range(0, MAX_STR)
                .sequential()
                .forEach(i -> {
                    if (i%2 == 0) deque.addLast(Integer.toString(i)); else deque.addFirst(Integer.toString(i));
                });

String dequeOut = deque.stream().collect(Collectors.joining(""));

I'll focus on the front append only cases ie. case 2 and case 5. The implementation of StringBuilder internally decides how the internal buffer grows, which apart from moving all buffer left to right in case of front appending limits the speed. While time taken when inserting directly to the front of the StringBuilder grows to really high values, as shown by @Mehrdad, if the need is to only have strings of length less than 90k characters (which is still a lot), the front insert will build a String in the same time as it would take to build a String of the same length by appending at the end. What I am saying is that time time penalty indeed kicks and is huge, but only when you have to build really huge strings. One could use a deque and join the strings at the end as shown in my example. But StringBuilder is a bit more intuitive to read and code, and the penalty would not matter for smaller strings.

Actually the performance for case 2 is much faster than case 1, which I don't seem to understand. I assume the growth for the internal buffer in StringBuilder would be the same in case of front append and back append. I even set the minimum heap to a very large amount to avoid delay in heap growth, if that would have played a role. Maybe someone who has a better understanding can comment below.

来源：https://stackoverflow.com/questions/5931261/java-use-stringbuilder-to-insert-at-the-beginning

标签

java

string

insert

append

stringbuilder