toUpperCase() 方法什么时候创建一个新对象?

问题描述

public class Child{

    public static void main(String[] args){
        String x = new String("ABC");
        String y = x.toUpperCase();

        System.out.println(x == y);
    }
}

输出:true

那么 toUpperCase() 总是创建一个新对象吗?

So does toUpperCase() always create a new object?

推荐答案

toUpperCase() 调用 toUpperCase(Locale.getDefault())，这会创建一个新的 String 对象仅在必须时.如果输入的String 已经是大写，则返回输入的String.

toUpperCase() calls toUpperCase(Locale.getDefault()), which creates a new String object only if it has to. If the input String is already in upper case, it returns the input String.

不过，这似乎是一个实现细节.我在 Javadoc 中没有发现它.

This seems to be an implementation detail, though. I didn't find it mentioned in the Javadoc.

这是一个实现:

public String toUpperCase(Locale locale) {
    if (locale == null) {
        throw new NullPointerException();
    }

    int firstLower;
    final int len = value.length;

    /* Now check if there are any characters that need to be changed. */
    scan: {
        for (firstLower = 0 ; firstLower < len; ) {
            int c = (int)value[firstLower];
            int srcCount;
            if ((c >= Character.MIN_HIGH_SURROGATE)
                    && (c <= Character.MAX_HIGH_SURROGATE)) {
                c = codePointAt(firstLower);
                srcCount = Character.charCount(c);
            } else {
                srcCount = 1;
            }
            int upperCaseChar = Character.toUpperCaseEx(c);
            if ((upperCaseChar == Character.ERROR)
                    || (c != upperCaseChar)) {
                break scan;
            }
            firstLower += srcCount;
        }
        return this; // <-- the original String is returned
    }
    ....
}

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