public class RandomChars {
  char[] chars;

  public RandomChars() {
    chars = new char[26];
    for (int i = 0; i < 26; i++) {
      chars[i] = (char) (i + 65);
    }
  }

  public char[] get4Chars() {
    char[] rlt = new char[4];
    for (int i = 0; i < rlt.length; i++) {
      int randomIndex = (int) (Math.random() * 26);
      rlt[i] = chars[randomIndex];
    }
    return rlt;
  }
}

    float unitWidth = (float) getWidth() / (float) chars.length;
    for (int i = 0; i < chars.length; i++) {
      String str = chars[i] + "";
      textPaint.getTextBounds(str, 0, str.length(), mRect);
      resetColor();
      int angel = (int) (Math.random()*(8-(-8)+1)+(-8));
      canvas.rotate(angel);//旋转字母，随机角度
      canvas.drawText(str, i * unitWidth + 5, getHeight() / 2 - mRect.centerY(), textPaint);
      /**
       * 很关键，旋转
       */
      canvas.save();//保存状态
      canvas.restore();//恢复
    }

/**
 * 重新设置随机颜色
 */
  private void resetColor() {
    int r = (int) (Math.random() * 230 - 30);
    int g = (int) (Math.random() * 230 - 30);
    int b = (int) (Math.random() * 230 - 30);
    textPaint.setColor(Color.rgb(r, g, b));
  }

    submit.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        String inputStr = input.getText().toString();
        inputStr = inputStr.toUpperCase();
        str = str.toUpperCase();
        if (str.equals(inputStr)) {
          Toast.makeText(MainActivity.this, "输入正确", Toast.LENGTH_SHORT).show();
        }else{
          Toast.makeText(MainActivity.this, "验证码输入错误", Toast.LENGTH_SHORT).show();
          char[] getchar = randomChars.get4Chars();
          str = new String(getchar);
          codeView.setChars(getchar);
        }
      }
    });