学ぶ StringBuilder

セクション 1. 章 34

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What is StringBuilder?

StringBuilder is a utility class for manipulating and editing string objects.

StringBuilder provides its own methods, and creating a StringBuilder object is straightforward and intuitive.

Main.java


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StringBuilder builder = new StringBuilder();

Inside the parentheses, we can also pass a string value, and our StringBuilder variable will hold that value:

Main.java


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StringBuilder builder = new StringBuilder("c<>definity");

To print our value to the console, we need to use one of the StringBuilder methods. It's the toString() method, which converts the value of the StringBuilder to a String type value. Let's look at an example:

Main.java


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package com.example;

public class Main {
    public static void main(String[] args) {
        StringBuilder builder = new StringBuilder("c<>definity");
        String string = builder.toString();
        System.out.println(string);
    }
}

StringBuilder in Action

StringBuilder provides a rich set of methods, with one of the most valuable and frequently used being append(String str). This method allows us to add a specified string to the existing content of a StringBuilder object.

Let's examine a code example:

Main.java


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package com.example;

public class Main {
    public static void main(String[] args) {
        // Initializing the new `String` with value "Hello"
        String hello = "Hello"; 
        System.out.println("Our string before using an append() method: " + hello);
        // Initializing a new `StringBuilder` with value of our already initialized string
        StringBuilder builder = new StringBuilder(hello); 
        // Using `append()` method, adding a string world to our `StringBuilder`
        builder = builder.append(" world!");
        // Creating a new `String` variable that has a `StringBuilder` value
        String result = builder.toString();
        System.out.println("Our string after using an append() method: " + result);
    }
}

Why not Use the + Operator?

We prefer StringBuilder's append method because it avoids creating multiple temporary objects, improving performance when modifying content repeatedly. Using the + operator creates a new object each time, which is costly in memory and time. StringBuilder modifies content directly, making it more efficient for repeated changes.

タスク

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You're building a product display for a webpage. Each product has a name and a price, and they are stored in a 2D array.
You need to format the entire list into a nicely structured string using a StringBuilder.

Create a method called buildProductList(String[][] products) that returns a formatted String.
Initialize a StringBuilder and append the header:
"Product List:\n"
Loop through the products array using a for-each loop.
For each product:
- Extract the name (product[0])
- Extract the price (product[1])
- Append a line to the StringBuilder in the format:
  ProductName - $Price\n
Return the full String from the StringBuilder.

解答

実践的な練習のためにデスクトップに切り替える下記のオプションのいずれかを利用して、現在の場所から続行する

すべて明確でしたか？

フィードバックありがとうございます！

セクション 1. 章 34

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