java给我写一个base64算法工具类，不能调用其他类

好的，下面是 Java 实现的 Base64 工具类，不依赖其他类库：

public class Base64Util {

    private static final char[] BASE64_TABLE = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/".toCharArray();
    private static final int[] BASE64_INVERT_TABLE = new int[256];

    static {
        for (int i = 0; i < BASE64_INVERT_TABLE.length; i++) {
            BASE64_INVERT_TABLE[i] = -1; // 初始化为 -1
        }
        for (int i = 0; i < BASE64_TABLE.length; i++) {
            BASE64_INVERT_TABLE[BASE64_TABLE[i]] = i;
        }
    }

    public static String encode(String s) {
        byte[] data = s.getBytes(StandardCharsets.UTF_8);
        StringBuilder sb = new StringBuilder();
        int len = data.length;
        for (int i = 0; i < len; i += 3) {
            int b1 = data[i] & 0xFF;
            int b2 = i + 1 < len ? data[i + 1] & 0xFF : 0;
            int b3 = i + 2 < len ? data[i + 2] & 0xFF : 0;
            int c1 = b1 >>> 2;
            int c2 = ((b1 & 0x03) << 4) | (b2 >>> 4);
            int c3 = ((b2 & 0x0F) << 2) | (b3 >>> 6);
            int c4 = b3 & 0x3F;
            sb.append(BASE64_TABLE[c1]).append(BASE64_TABLE[c2]);
            sb.append(i + 1 < len ? BASE64_TABLE[c3] : "=");
            sb.append(i + 2 < len ? BASE64_TABLE[c4] : "=");
        }
        return sb.toString();
    }

    public static String decode(String s) {
        byte[] data = s.getBytes(StandardCharsets.UTF_8);
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        int len = data.length;
        for (int i = 0; i < len; i += 4) {
            int c1 = BASE64_INVERT_TABLE[data[i]];
            int c2 = i + 1 < len ? BASE64_INVERT_TABLE[data[i + 1]] : -1;
            int c3 = i + 2 < len ? BASE64_INVERT_TABLE[data[i + 2]] : -1;
            int c4 = i + 3 < len ? BASE64_INVERT_TABLE[data[i + 3]] : -1;
            if (c1 == -1 || c2 == -1 || c3 == -1 || c4 == -1) {
                throw new IllegalArgumentException("Invalid Base64 string");
            }
            int b1 = (c1 << 2) | (c2 >>> 4);
            int b2 = ((c2 & 0x0F) << 4) | (c3 >>> 2);
            int b3 = ((c3 & 0x03) << 6) | c4;
            bos.write(b1);
            if (c3 != -1) {
                bos.write(b2);
            }
            if (c4 != -1) {
                bos.write(b3);
            }
        }
        return new String(bos.toByteArray(), StandardCharsets.UTF_8);
    }
}

使用示例：

String text = "hello, world!";
String encoded = Base64Util.encode(text);
System.out.println(encoded); // aGVsbG8sIHdvcmxkIQ==
String decoded = Base64Util.decode(encoded);
System.out.println(decoded); // hello, world!

注意：这只是一个简单的示例，实际场景中还需要考虑编码格式、异常处理等问题。