Java（适用于Android）在类数组（类似C的结构）中定义和初始化类数组

Question

I am rewriting c code to Java to run on an Android-based BeagleBone Black. Since there are not struc types in java, I'm defining the data structures using public static class methods. However, in the structures are an array of sub-structures. As a model, say we want to define a class called Satellites. In each Satellite are Transponders and for each transponder is the channel information. I am trying to figure out how to declare and then initialize the sub-classes inside the class. My basic definition is:

public static final int SATELLITE_MAX = 50;
public static final int TRANSPONDER_MAX = 24;
public static final int CHANNEL_MAX = 36;

public static class satellite_struct {
    public string satId;
    public string satName;
    public string satLocation;

    public class transponder_struct {
        public int frequency;
        public char polarity;

        public class channels_struct {
            public string channel_name;
            public string encryption;
            public int sid;
        }
    }
}

I can instantiate the satellite structure class as follows:

 satellite_struct satellites[] = new satellite_struct[SATELLITE_MAX]

but, how do I initialize the x transponders and y channels inside this class?

Thanks.

Answer 1

The "easiest" solution would be:

public static class satellite_struct {
    public string satId;
    public string satName;
    public string satLocation;

    public class transponder_struct {
        public int frequency;
        public char polarity;

        public class channels_struct {
            public string channel_name;
            public string encryption;
            public int sid;
        }

        public channels_struct[] channels = new channel_struct[ CHANNEL_MAX ];
    }
    public transponder_struct transponders = new transponder_struct[ TRANSPONDER_MAX ];
}

Given that you defined

satellite_struct satellites[] = new satellite_struct[SATELLITE_MAX];

You can then access them by (eg):

satellites[x].transponders[y].channels[z].channel_name

However, you'd probably want to Java-fy it a bit:

public static class Satellite {
    public string satId;
    public string satName;
    public string satLocation;

    public class Transponder {
        public int frequency;
        public char polarity;

        public class Channel {
            public string channel_name;
            public string encryption;
            public int sid;
        }

        public Channel[] channels = new Channel[ CHANNEL_MAX ];
    }
    public Transponder transponders = new Transponder[ TRANSPONDER_MAX ];
}

Class names are customarily upper-case in Java. If this is a direct translation to an existing structure, the above should do, but you might want to use a List or SparseArray instead of arrays.

Answer 2

Declare transponders here;
Declare channels here;

public static class satellite_struct {
    public string satId;
    public string satName;
    public string satLocation;
    public []transponder t;
    public []channel c;
    public satellite_struct(int x,int y)
    {
       //here
       t=new transponder[x];
       for(....)
           t[i]=new transponder(params);
       c=new channel[y];
       for(....)
           c[i]=new channel(params);
    }

}

Answer 3

I got it to work as follows, and Java-ifying my syntax. The issue was with where to place the [] declaring an array of objects:

public static class Satellite {
    public String satId;
    public String satName;
    public String satLocation;

    public static class Transponder {
        public int frequency;
        public char polarity;

        public static class Channel {
            public String channel_name;
            public String encryption;
            public int sid;
        }
        public Channel channels[] = new Channel[CHANNELS_MAX];
    }
    public Transponder transponders[] = new Transponder[TRANSPONDERS_MAX];
}

Then, in my main code:

Structures.Satellite satellites[] = new Structures.Satellite[SATELLITES_MAX];

satellites[0].transponders[0].channels[0].channel_name="HELLO";

Update: It appears that while the IDE allows me to 'see' the transponders and channels when coding, when the code runs it returns error "all elements are null" for these sub-array values. How to initialize these?