String类的方法里的Format方法如何做到格式化?如果是转义,转义后是否会变成其他数据类型?

Question

//这个转义之后是String类型的1?还是整数类型的1?
        String str = "hi";
        str=String.format("%d", 1);

Answer 1

先回答String.format("%d", 1)是个格式化方法，它返回的就是字符串String。第一参数的含义是格式化模板，第二是可变参数，表示要格式化的参数。
其格式化的过程：
1、现在格式化模板中找到特殊的格式化标记，并记录下标记所在模板的位置索引，把这些标记记录到一个list中
2、根据这个list顺序，将第二参数列表的参数进行格式化处理成想要的字符串，并替换到模板中标记的位置，拼接成最终要的字符串返回出去
于是我就去看了下jdk里源码，其实现思路跟我分析的过程很像，就节选了部分代码

 public Formatter format(Locale l, String format, Object ... args) {
        ensureOpen();

        // index of last argument referenced
        int last = -1;
        // last ordinary index
        int lasto = -1;
       //此方法就过程的第一步去找特殊标记
        FormatString[] fsa = parse(format);
        for (int i = 0; i < fsa.length; i++) {
            FormatString fs = fsa[i];
            int index = fs.index();
            try {
                switch (index) {
                case -2:  // fixed string, "%n", or "%%"
                    fs.print(null, l);
                    break;
                case -1:  // relative index
                    if (last < 0 || (args != null && last > args.length - 1))
                        throw new MissingFormatArgumentException(fs.toString());
                    fs.print((args == null ? null : args[last]), l);
                    break;
                case 0:  // ordinary index
                    lasto++;
                    last = lasto;
                    if (args != null && lasto > args.length - 1)
                        throw new MissingFormatArgumentException(fs.toString());
                    fs.print((args == null ? null : args[lasto]), l);
                    break;
                default:  // explicit index
                    last = index - 1;
                    if (args != null && last > args.length - 1)
                        throw new MissingFormatArgumentException(fs.toString());
                    fs.print((args == null ? null : args[last]), l);
                    break;
                }
            } catch (IOException x) {
                lastException = x;
            }
        }
        return this;
    }


    /**
     * Finds format specifiers in the format string.
     */
    private FormatString[] parse(String s) {
        ArrayList<FormatString> al = new ArrayList<>();
        Matcher m = fsPattern.matcher(s);
        for (int i = 0, len = s.length(); i < len; ) {
            if (m.find(i)) {
                // Anything between the start of the string and the beginning
                // of the format specifier is either fixed text or contains
                // an invalid format string.
                if (m.start() != i) {
                    // Make sure we didn't miss any invalid format specifiers
                    checkText(s, i, m.start());
                    // Assume previous characters were fixed text
                    al.add(new FixedString(s.substring(i, m.start())));
                }

                al.add(new FormatSpecifier(m));
                i = m.end();
            } else {
                // No more valid format specifiers.  Check for possible invalid
                // format specifiers.
                checkText(s, i, len);
                // The rest of the string is fixed text
                al.add(new FixedString(s.substring(i)));
                break;
            }
        }
        return al.toArray(new FormatString[al.size()]);
    }