Java语法疑问：String.toUpperCase()源码中scan相关陌生语法解析

Hey there! Let's break down your questions one by one, since I’ve dug into the OpenJDK source for String.toUpperCase() plenty of times.

1. Is scan a Java keyword? #

Absolutely not. Java has a fixed set of reserved keywords (like class, int, if, etc.) that carry special meaning for the compiler. scan is just a regular identifier—developers chose this name here because it describes exactly what the variable does: scanning through the string. It’s no different from naming a variable count or index; it’s just a descriptive label.

2. What does scan do in String.toUpperCase()? #

The scan variable exists purely for performance optimization. Here’s the core logic from the JDK source, simplified to make it clear:

• It starts at index 0 and traverses the string’s internal char array, checking each character to see if it already doesn’t need uppercase conversion (i.e., it’s already uppercase, or a character that doesn’t have an uppercase variant).
• It stops at the first character that actually needs to be converted to uppercase.
• If scan reaches the end of the string (meaning every character is already in a valid uppercase state), the method returns the original String object immediately—no need to create a new array or waste memory on unnecessary copies.
• If there are characters to convert, the scan index tells the method where to start modifying the string: it copies the unchanged prefix of the string up to scan, then converts the remaining characters to uppercase and creates a new String for the result.

Here’s a trimmed-down code snippet showing this flow:

public String toUpperCase() {
    int length = value.length;
    int scan = 0;

    // Scan for the first character that needs conversion
    while (scan < length) {
        if (Character.isLowerCase(value[scan]) || Character.isTitleCase(value[scan])) {
            break;
        }
        scan++;
    }

    // No conversions needed? Return the original string
    if (scan == length) {
        return this;
    }

    // Convert the rest of the characters and return a new string
    char[] result = Arrays.copyOf(value, length);
    for (int i = scan; i < length; i++) {
        result[i] = Character.toUpperCase(result[i]);
    }
    return new String(result, true);
}

3. Why did your test program with scan run fine? #

Since scan is just a regular variable name, you can use it in your own code without any conflicts. The compiler treats it like any other identifier—no special restrictions apply. Your program worked because you weren’t using a reserved keyword, just a descriptive name for a variable (likely mimicking the scanning logic from the JDK source).

内容的提问来源于stack exchange，提问作者Kihats