public final class java.lang.System extends java.lang.Object
{
        public static PrintStream err;
        public static InputStream in;
        public static PrintStream out;
        public static void arraycopy(Object src,int src_position,
                                     Object dst,int dst_position,int length);
        public static long currentTimeMillis();
        public static void exit(int status);
        public static void gc();
        public static Properties getProperties();
        public static String getProperty(String key);
        public static String getProperty(String key,String def);
        public static SecurityManager getSecurityManager();
        public static void load(String filename);
        public static void loadLibrary(String libname);
        public static void runFinalization();
        public static void setProperties(Properties props);
        public static void setSecurityManager(SecurityManager s);
}
System类中出现了两个getProperty方法，这是怎么回事？

public final class java.lang.System extends java.lang.Object
{
        public static PrintStream err;
        public static InputStream in;
        public static PrintStream out;
        public static void arraycopy(Object src,int src_position,
                                     Object dst,int dst_position,int length);
        public static long currentTimeMillis();
        public static void exit(int status);
        public static void gc();
        public static Properties getProperties();
        public static String getProperty(String key);
        public static String getProperty(String key,String def);
        public static SecurityManager getSecurityManager();
        public static void load(String filename);
        public static void loadLibrary(String libname);
        public static void runFinalization();
        public static void setProperties(Properties props);
        public static void setSecurityManager(SecurityManager s);
}
System类中出现了两个getProperty方法，这是怎么回事？