Java StringBuilder Demystified

saurabhpro profile image Saurabh Kumar Updated on ・2 min read

Let's understand what makes every lead developer say that you should know how to create your own StringBuilder if you plan to modify your strings.
And most of all - what is the difference between string concatenation vs using this pattern.

Simply put what we want to do is - an efficient management of space and performance.

This is the simplest of all the implementations with core characteristics of a StringBuilder

  • .append()
  • .length()
  • .toString()


import java.util.Arrays;

public class MyStringBuilder {

    private static final int INIT_CAPACITY = 16;
    private static final float LOAD_FACTOR = 0.75f;
    private static final int MULTIPLIER = 2;

    private int currentCapacity = INIT_CAPACITY;

    // store the total number of characters in our string builder
    private int count;

    // storage area for our characters
    private char[] chars;

    public MyStringBuilder() {
        chars = new char[INIT_CAPACITY];

    public MyStringBuilder(String str) {

    public MyStringBuilder append(String newStr) {

        final int length = newStr.length();

        // check if resized or not
        boolean inflated = ensureCapacity(length);

        if (inflated) {
            final char[] newBuffer = new char[currentCapacity];
            // copy current content
            System.arraycopy(chars, 0, newBuffer, 0, count);
            // copy new content
            System.arraycopy(newStr.toCharArray(), 0, newBuffer, count, length);

            // update the store array
            chars = newBuffer;
        } else {
            // add the new content to the remaining array space
            System.arraycopy(newStr.toCharArray(), 0, chars, count, length);

        // update the count of chars
        count += length;

        return this;

    private boolean ensureCapacity(int newLength) {
        boolean isInflated = false;
        // ensure the new length is properly accommodated
        while (Float.compare((float) (count + newLength) / currentCapacity, LOAD_FACTOR) >= 0) {
            currentCapacity *= MULTIPLIER;
            isInflated = true;

        return isInflated;

    public int length() {
        return count;

    public String toString() {
        return new String(Arrays.copyOfRange(chars, 0, count));

public class MyStringBuilderDriver {

    public static void main(String[] args) {

        var sb = new MyStringBuilder();

        sb.append("Hi").append(" ");
        sb.append("Saurabh").append(",").append(" ");
        sb.append("how are you today ?");


If you feel it's cool - let me know which Java language framework object I should demystify.


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