字符串对齐器（左对齐、居中、右对齐）

[java]代码库
package C5.src.book.string;
 
/**
 * 字符串对齐器。可以指定对齐格式和一行的最大字符数 对齐格式有左对齐、居中、右对齐
 */
public class StringAlign {
 
    /** 左对齐格式 */
    public static final int JUST_LEFT = 0;
    /** 居中格式 */
    public static final int JUST_CENTER = 1;
    /** 右对齐格式 */
    public static final int JUST_RIGHT = 2;
 
    /** 当前对齐格式 */
    private int just;
    /** 一行的最大长度 */
    private int maxChars;
 
    /**
     * 默认构造函数
     */
    public StringAlign() {
        // 默认为居中对齐
        this.just = JUST_CENTER;
        // 默认一行的最大长度为80
        this.maxChars = 80;
    }
 
    /**
     * 构造一个字符串对齐器，需要传入一行的最大长度和对齐的格式。
     * 
     * @param maxChars
     * @param just
     */
    public StringAlign(int maxChars, int just) {
        // 首先构造一个默认字符串对齐器
        this();
        // 根据传入参数修改字符串对齐器的属性
        this.setJust(just);
        this.setMaxChars(maxChars);
    }
 
    /**
     * 对齐一个字符串
     * 
     * @param obj
     *            待对齐的字符串
     */
    public String format(String s) {
        StringBuffer where = new StringBuffer();
        // 从待对齐的字符串中取出一段子字符串，子串的长度为行最大长度和s长度的较小值
        int wantedLength = Math.min(s.length(), this.maxChars);
        String wanted = s.substring(0, wantedLength);
        // 根据对齐模式，将空格放在合适的位置
        switch (this.just) {
        case JUST_RIGHT:
            // 如果是右对齐，那么将缺少的的字符用空格代替放在左边
            pad(where, maxChars - wantedLength);
            // 将字符串添加在右边
            where.append(wanted);
            break;
        case JUST_CENTER:
            // 居中对齐，将空格字符平均分在字符串两边。
            int startPos = where.length();
            pad(where, (maxChars - wantedLength) / 2);
            where.append(wanted);
            pad(where, (maxChars - wantedLength) / 2);
            // 调整舍入误差
            pad(where, maxChars - (where.length() - startPos));
            break;
        case JUST_LEFT:
            // 右对齐，将空格字符放在字符串右边。
            where.append(wanted);
            pad(where, maxChars - wantedLength);
            break;
        }
        // 如果原字符串的长度大于一行的最大长度，则将余下部分放入下一行
        if (s.length() > wantedLength) {
            String remainStr = s.substring(wantedLength);
            where.append("\n" + this.format(remainStr));
        }
        return where.toString();
    }
 
    /**
     * 在to的后面append howMany个空格字符。
     * 
     * @param to
     * @param howMany
     */
    protected final void pad(StringBuffer to, int howMany) {
        for (int i = 0; i < howMany; i++)
            to.append(" ");
    }
 
    public int getJust() {
        return just;
    }
 
    /**
     * 设置字符串对齐器的对齐格式
     * 
     * @param just
     */
    public void setJust(int just) {
        switch (just) {
        case JUST_LEFT:
        case JUST_CENTER:
        case JUST_RIGHT:
            this.just = just;
            break;
        default:
            System.out.println("invalid justification arg.");
        }
    }
 
    public int getMaxChars() {
        return maxChars;
    }
 
    /**
     * 设置字符串对齐器的一行最大字符数
     * 
     * @param maxChars
     */
    public void setMaxChars(int maxChars) {
        if (maxChars < 0) {
            System.out.println("maxChars must be positive.");
        } else {
            this.maxChars = maxChars;
        }
    }
 
    public static void main(String[] args) {
        // 一行最多70个字符，居中显示。
        StringAlign formatter = new StringAlign(20, StringAlign.JUST_CENTER);
        // 比如显示页码
        System.out.println(formatter.format("- i -"));
        System.out.println(formatter.format(Integer.toString(444)));
        System.out.println(formatter.format("kkkkkkkkkkkkkkkkkkkkkkkkkkkk"));
        // 左对齐
        System.out.println();
        formatter = new StringAlign(20, StringAlign.JUST_LEFT);
        System.out.println(formatter.format("- i -"));
        System.out.println(formatter.format(Integer.toString(444)));
        System.out.println(formatter.format("kkkkkkkkkkkkkkkkkkkkkkkkkkkk"));
        // 右对齐
        System.out.println();
        formatter = new StringAlign(20, StringAlign.JUST_RIGHT);
        System.out.println(formatter.format("- i -"));
        System.out.println(formatter.format(Integer.toString(444)));
        System.out.println(formatter.format("kkkkkkkkkkkkkkkkkkkkkkkkkkkk"));
    }
}
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