替代算法

 老师布置的作业，哈，保存到网上。

package net.as.cn;

/**
 * 2013.3.22 am 替换算法
 * 
 * @author chenas
 * 
 */

public class MyTiHuan {

	public final int smalla = 97;
	public final int bigA = 65;
	public final int smallz = 122;
	public final int bigZ = 90;
	public final int space = 32;
	public final int smallZero = 96;
	public final int bigZero = 64;

	/**
	 * 判断是否为小写字母
	 * 
	 * @param a
	 * @return
	 */
	public boolean isSmall(char a) {
		if (a >= 97 && a <= 122) {
			return true;
		} else
			return false;
	}

	/**
	 * 判断是否为大写字母
	 * 
	 * @param A
	 * @return
	 */
	public boolean isBig(char A) {
		if (A >= 65 && A <= 90) {
			return true;
		} else
			return false;
	}

	/**
	 * 判断是否为空格
	 * 
	 * @param spa
	 * @return
	 */
	public boolean isSpace(char spa) {
		if (spa == space) {
			return true;
		} else
			return false;
	}

	/**
	 * 检查非法字符
	 * 
	 * @param a
	 * @return
	 */
	public boolean isPass(char a) {
		if (a == space) {
			return true;
		} else if (a >= smalla && a <= smallz) {
			return true;
		} else if (a >= bigA && a <= bigZ) {
			return true;
		}
		return false;
	}

	/**
	 * 
	 * 加密
	 * 
	 * @param mingwen
	 *            明文
	 * @param yiwei
	 *            右移的位数
	 * @return 密文
	 * 
	 */
	public String enCode(String mingwen, int yiwei) {
		StringBuffer sBuff = new StringBuffer();
		char[] mw = mingwen.toCharArray();
		for (int i = 0; i < mw.length; i++) {
			char temp = 'a';
			char c = mw[i];
			if (isPass(c)) {
				if (isSmall(c)) {
					temp = (char) (c + yiwei);
					if (!isPass(temp)) {
						temp = (char) (temp - smallz + smallZero);
					}
				} else if (isBig(c)) {
					temp = (char) (c + yiwei);
					if (!isPass(temp)) {
						temp = (char) (temp - bigZ + bigZero);
					}
				} else if (isSpace(c)) {
					temp = c;
				}
			} else {
				return sBuff.append("输入中带有非法字符！").toString();
			}
			sBuff.append(temp);
		}
		return sBuff.toString();
	}

	/**
	 * 解密
	 * 
	 * @param miwen
	 *            密文
	 * @param yiwei
	 *            左移的位数
	 * @return 明文
	 */
	public String deCode(String miwen, int yiwei) {
		StringBuffer sBuff = new StringBuffer();
		char[] mw = miwen.toCharArray();
		for (int i = 0; i < mw.length; i++) {
			char temp = 'a';
			char c = mw[i];
			if (isPass(c)) {
				if (isSmall(c)) {
					temp = (char) (c - yiwei);
					if (!isPass(temp)) {
						temp = (char) (smallz - smallZero + temp);
					}
				} else if (isBig(c)) {
					temp = (char) (c - yiwei);
					if (!isPass(temp)) {
						temp = (char) (bigZ - bigZero + temp);
					}
				} else if (isSpace(c)) {
					temp = c;
				}
			} else {
				return sBuff.append("密文中带有非法字符！").toString();
			}
			sBuff.append(temp);
		}
		return sBuff.toString();
	}

	public static void main(String[] args) {
		MyTiHuan mt = new MyTiHuan();
		String miw = mt.enCode("My name is Jack", 3);
		System.out.println(miw);
		String mingw = mt.deCode(miw, 3);
		System.out.println(mingw);
	}

}