Android兼容性例外初始化错误

我正在使用兼容类构建用户代理字符串：

public abstract class Compatibility {

    private static int sdkInt = 0;

    private static Compatibility instance;

    public static Compatibility getInstance(){
        if (instance!=null) return instance;

        try { sdkInt = Integer.parseInt(Build.VERSION.SDK); } catch (Exception e) {
            sdkInt = 3;
        }
        if (sdkInt<4) {
            instance = new CompatibilityAPI3();
        } else {
            instance = new CompatibilityAPI4();
        }
        return instance;
    }

    public int getSdkInt(){
        return sdkInt;
    }

    public abstract String getUa();

}


public class CompatibilityAPI3 extends Compatibility {
    @Override
    public String getUa() {
        return "unknown|unknown|"+Build.DEVICE+"|"+Build.PRODUCT+"|"+Build.BRAND+"|"+Build.VERSION.SDK;     
    }
}


public class CompatibilityAPI4 extends Compatibility {
    @Override
    public String getUa() {
        return Build.MANUFACTURER+"|"+Build.MODEL+"|"+Build.DEVICE+"|"+Build.PRODUCT+"|"+Build.BRAND+"|"+Build.VERSION.SDK;
    }
}

我做错了什么？

使用反射。请参阅：确定平台的API级别<1.6

使用反射。请参阅：确定<1.6平台的API级别

java.lang.ExceptionInInitializerError
....
Caused by: java.lang.NoSuchFieldError: ?
at ...CompatibilityAPI4.getUa(CompatibilityAPI4.java:8)