为什么java.nio.charset.Charsets编译错误？

Question

在我的类UriCodec.java中，但是当我使用javac（jdk6）编译此类错误时。例如：javac UriCodec.java java代码：

import java.io.ByteArrayOutputStream;
import java.net.URISyntaxException;
import java.nio.charset.Charset;
import java.nio.charset.Charsets;


public abstract class UriCodec {

    protected abstract boolean isRetained(char c);

    public final String encode(String s, Charset charset) {
        // Guess a bit larger for encoded form
        StringBuilder builder = new StringBuilder(s.length() + 16);
        appendEncoded(builder, s, charset, false);
        return builder.toString();
    }

    public final void appendEncoded(StringBuilder builder, String s) {
        appendEncoded(builder, s, Charsets.UTF_8, false);
    }

    public final void appendPartiallyEncoded(StringBuilder builder, String s) {
        appendEncoded(builder, s, Charsets.UTF_8, true);
    }

    public static String decode(String s) {
        return decode(s, false, Charsets.UTF_8);
    }

    private static void appendHex(StringBuilder builder, String s, Charset charset) {
        for (byte b : s.getBytes(charset)) {
            appendHex(builder, b);
        }
    }

    private static void appendHex(StringBuilder sb, byte b) {
        sb.append('%');
        sb.append(Byte.toHexString(b, true));
    }
}

错误代码：

   import java.nio.charset.Charsets;
                           ^
    UriCodec.java:140: Can not find symbol
    symbol： Variable Charsets
    location： class com.android.exchange.utility.UriCodec
            appendEncoded(builder, s, Charsets.UTF_8, false);
                                      ^
    UriCodec.java:144: Can not find symbol
    symbol： Variable Charsets
    location： class com.android.exchange.utility.UriCodec
            appendEncoded(builder, s, Charsets.UTF_8, true);
                                      ^
    UriCodec.java:203: Can not find symbol
    symbol： Variable Charsets
    location： class com.android.exchange.utility.UriCodec
            return decode(s, false, Charsets.UTF_8);
                                    ^
    UriCodec.java:214: Can not find symbol
    symbol： Method toHexString(byte,boolean)
    location： class java.lang.Byte
            sb.append(Byte.toHexString(b, true));
                          ^
    5 error

类UriCodec.java在eclipse中正确编译，但是使用javac UriCodec.java不正确。谁能告诉我为什么？