Finding the name of a given constant using reflection

Question

The code below comes from the Arm_const class of my Android disassembler project:

// ARM condition code

public static final int ARM_CC_INVALID = 0;
public static final int ARM_CC_EQ = 1;
public static final int ARM_CC_NE = 2;
public static final int ARM_CC_HS = 3;
public static final int ARM_CC_LO = 4;
public static final int ARM_CC_MI = 5;
public static final int ARM_CC_PL = 6;
public static final int ARM_CC_VS = 7;
public static final int ARM_CC_VC = 8;
public static final int ARM_CC_HI = 9;
public static final int ARM_CC_LS = 10;
public static final int ARM_CC_GE = 11;
public static final int ARM_CC_LT = 12;
public static final int ARM_CC_GT = 13;
public static final int ARM_CC_LE = 14;
public static final int ARM_CC_AL = 15;

public static String getCCName(int cc)
{
    Class clazz=Arm_const.class;
    Field[] fields=clazz.getFields();
    for(Field f:fields)
    {
        String s=f.getName();
        if(s.contains("ARM_CC_"))
        try
        {
            if (((int)f.get(null))==cc)
            {
                return s.replace("ARM_CC_","");
            }
        }
        catch (IllegalAccessException e)
        {
            Log.e("arm","",e);
        }
        catch (IllegalArgumentException e)
        {}
    }
    return "";
}

The method getCCName returns the name of a constant from the declared public static final ints.

Examples:

• 3 → "HS"

• 12 → "LT"

Any suggestions to improve its performance (speed) are appreciated!

Answer 1

If performance is your concern I would implement getCCName as a lookup in a map that's filled when the class is loaded, e.g. like

private static final Map<Integer, String> _int2string;

static
{
    final Map<Integer, String> int2string = new HashMap<>();

    try
    {
        for (Field field: Arm_const.class.getFields())
        {
            final int mod = field.getModifiers();

            if (!int.class.equals(field.getType()))
                continue;

            if (!Modifier.isStatic(mod) || !Modifier.isPublic(mod))
                continue;

            if (!field.getName().startsWith("ARM_CC_"))
                continue;

            int2string.put(field.getInt(null),
                           field.getName().substring("ARM_CC_".length()));
        }
    }
    catch (IllegalAccessException l_e)
    {
        throw new RuntimeException(l_e); // should not occur
    }

    _int2string = Collections.unmodifiableMap(int2string);
}

public static String getCCName(int cc)
{
    return _int2string.get(cc);
}

Answer 2

Why are you using reflection at all? You should be taking advantage of enums. Also, the huge ARM_const class should be broken up into smaller classes. (Call ARMConstants.Condition.nameForCode(cc) instead of ARM_const.getCCName(cc).)

public class ARMConstants {
    private ARMConstants() {}

    public static enum Condition {
        INVALID, EQ, NE, HS, LO, MI, PL, VS, VC, HI, LS, GE, LT, GT, LE, AL;

        public int code() {
            return this.ordinal();
        }

        public static String nameForCode(int cc) {
            try {
                return values()[cc].name();
            } catch (ArrayIndexOutOfBoundsException noSuchCode) {
                Log.e("Disassembler", "", noSuchCode);
                return "";
            }
        }
    }

}

Answer 3

I tried performance test to determine which to accept as an answer and got a surprising result.

Exexution time gap gets smaller as I execute the test again.

Test code:

Main:

import java.util.*;
public class Main
{
    public static void main(String[] args)
    {
        long time;
        long lapse1,lapse2;
        time=System.currentTimeMillis();
        //lapse1=time;
        //System.out.println("time"+time);
        for(int i=0;i<15;++i)
            System.out.println(ARMConstants.Condition.nameForCode(i));
        lapse1=System.currentTimeMillis()-time;
        time=System.currentTimeMillis();
        //System.out.println("lapse1"+System.currentTimeMillis());
        for(int i=0;i<15;++i)
            System.out.println(MapConst.getCCName(i));
            lapse2=System.currentTimeMillis()-time;
        //System.out.println("time"+System.currentTimeMillis());
        System.out.println("lapse1: "+lapse1+"lapse2:"+lapse2);
    }
}

ARMConstants:

public class ARMConstants {
    private ARMConstants() {}

    public static enum Condition {
        INVALID, EQ, NE, HS, LO, MI, PL, VS, VC, HI, LS, GE, LT, GT, LE, AL;

        public int code() {
            return this.ordinal();
        }

        public static String nameForCode(int cc) {
            try {
                return values()[cc].name();
            } catch (ArrayIndexOutOfBoundsException noSuchCode) {
               // Log.e("Disassembler", "", noSuchCode);
                return "";
            }
        }
    }

}

MapConst:

import java.lang.reflect.*;
import java.util.*;

public class MapConst
{
    private static final Map<Integer, String> _int2string;
    public static final int ARM_CC_INVALID = 0;
    public static final int ARM_CC_EQ = 1;
    public static final int ARM_CC_NE = 2;
    public static final int ARM_CC_HS = 3;
    public static final int ARM_CC_LO = 4;
    public static final int ARM_CC_MI = 5;
    public static final int ARM_CC_PL = 6;
    public static final int ARM_CC_VS = 7;
    public static final int ARM_CC_VC = 8;
    public static final int ARM_CC_HI = 9;
    public static final int ARM_CC_LS = 10;
    public static final int ARM_CC_GE = 11;
    public static final int ARM_CC_LT = 12;
    public static final int ARM_CC_GT = 13;
    public static final int ARM_CC_LE = 14;
    public static final int ARM_CC_AL = 15;

    static
    {
        final Map<Integer, String> int2string = new HashMap<>();

        try
        {
            for (Field field: MapConst.class.getFields())
            {
                final int mod = field.getModifiers();

                if (!int.class.equals(field.getType()))
                    continue;

                if (!Modifier.isStatic(mod) || !Modifier.isPublic(mod))
                    continue;

                if (!field.getName().startsWith("ARM_CC_"))
                    continue;

                int2string.put(field.getInt(null),
                               field.getName().substring("ARM_CC_".length()));
            }
        }
        catch (IllegalAccessException l_e)
        {
            throw new RuntimeException(l_e); // should not occur
        }

        _int2string = Collections.unmodifiableMap(int2string);
    }

    public static String getCCName(int cc)
    {
        return _int2string.get(cc);
    }
}

Result:

Odd!

And more odd thing happened!

I added my code.

for(int i=0;i<15;++i)
            System.out.println(MapConst.getCCNameMine(i));
        lapse3=System.currentTimeMillis()-time;

        //System.out.println("time"+System.currentTimeMillis());
        System.out.println("lapse1: "+lapse1+"lapse2:"+lapse2+"lapse3:"+lapse3);

public static String getCCNameMine(int cc)
    {
        Class clazz=MapConst.class;
        Field[] fields=clazz.getFields();
        for(Field f:fields)
        {
            String s=f.getName();
            if(s.contains("ARM_CC_"))
                try
                {
                    if (((int)f.get(null))==cc)
                    {
                        return s.replace("ARM_CC_","");
                    }
                }
                catch (IllegalAccessException e)
                {
                    //Log.e("Disassembler","",e);
                }
                catch (IllegalArgumentException e)
                {
                    //Log.e("Disassembler","",e);
                }
        }
        return "";
    }

Mine is fastest...

Edit

When I change System. currentTimeMillis to nanoTime:

lapse1: 5432129lapse2:3326416lapse3:9307861

So using enum is fastest.

Nono, I didn't check working performance nicely.

I may have test a lot more times;;

100 iteration:

lapse1: 293518068 lapse2:281768798 lapse3:433898927