如何将数据输入到用户定义类的arraylist中？

Question

for(i=0;i<in.length;i++)
    {
        for(j=i+1;j<in.length;j++)
        {
            //System.out.println("for"+in[i].val);
            //System.out.println("for "+in[j].val);
            if(comp(in[i].bin,in[j].bin,n)!=null)
            {
                String bi=comp(in[i].bin,in[j].bin,n);
                //System.out.println("is it null "+co[k]);
                coin.a(in[i],in[j],bi);//taken input
                co.add(coin);//now passing to array list HERE LIES THE PROBLEM!!!
                System.out.println(in[i].val+","+in[j].val+"="+coin.bin);
                in[i].combo=true;
                in[j].combo=true;
                k++;
            }   

        }
    }

55f96302address of coin 55f96302 55f96302address of coin @55f96302 55f96302address of coin @55f96302 55f96302address of coin @55f96302 55f96302address of coin @55f96302 55f96302address of coin @55f96302 55f96302address of coin @55f96302 55f96302address of coin @55f96302 55f96302address of coin @55f96302

As you can see the reference of all elements in the list is same as that of "coin" what should i do to solve this?

Answer 1

I guess you wouldn't have any issue having a references build over a collection of couple class. This is the right way on dealing with elements storage in java.

Answer 2

If I understand the question correctly, your problem is that you do not want the item stored in the list to be the same element as the one stored in your variable coin . In this case, you should create an exact copy of the object stored in the variable coin and put the copy into the list.

In fact, you should always be careful when passing around mutable objects, eg, putting them into a list. Otherwise, your program can become hard to debug. Search for defensive copy in Java .

I can think of two ways of creating a defensive copy: cloning and copy constructor . However, note that . implementing Cloneable is not trivial .

Also, I would suggest you to think about whether and how you can make your class couple immutable. In most cases, the advantages of having an immutable class (ie, not having to worry about defensive copying) outweight the disadvanteges (ie, creating new objects, leading to a bit of a higher cost in memory and CPU cycles).

TL;DR: The only reason to not pass the same reference into the arraylist is that the item could potentially be modified by everyone that has access to it. Create a defensive copy or make your object immutable.

Edit: After you have updated your example, I see your problem (at least I think I do). I tried to reconstruct your example as a minimum working example. What you essentially have is:

public class Coin {

    int value;

    public Coin(int value) {
        this.value = value;
    }

    public void setValue(int value) {
        this.value = value;
    }

    public static void main(String[] args) {
        Coin myCoin = new Coin(0);
        List<Coin> bankAccount = new ArrayList<Coin>();
        for (int coinValue = 0; coinValue < 5; coinValue ++) {
            myCoin.setValue(coinValue);
            bankAccount.add(myCoin);
        }
        for(Coin coin : bankAccount)
            System.out.println(coin);
    }

    @Override
    public String toString() {
        return "Coin: " + value;
    }

}

If you run this code, then all coins in the list will have value 4. The reason is, that the method setValue (which corresponds to your method a ) mutates the Coin instance. The best way to solve this (if possible) is to make Coin immutable and return a new Coin instance when calling setValue . The code would look like this:

public class ImmutableCoin {

    final int value;

    public ImmutableCoin(int value) {
        this.value = value;
    }

    public ImmutableCoin setValue(int value) {
        return new ImmutableCoin(value);
    }

    public static void main(String[] args) {
        ImmutableCoin myCoin = new ImmutableCoin(0);
        List<ImmutableCoin> bankAccount = new ArrayList<ImmutableCoin>();
        for (int coinValue = 0; coinValue < 5; coinValue ++) {
            myCoin = myCoin.setValue(coinValue);
            bankAccount.add(myCoin);
        }
        for(ImmutableCoin coin : bankAccount)
            System.out.println(coin);
    }

    @Override
    public String toString() {
        return "ImmutableCoin: " + value;
    }

}

Try running this code. You will now not have the same object inserted multiple times in your list.

If however, your method a must modify the object and is not allowed to return a new object, then you must make a copy of the object yourself in the loop, modify this copy, and then insert the copy. The code could look something like this:

public class MutableCoin {

    int value;

    public MutableCoin(int value) {
        this.value = value;
    }

    // copy constructor
    public MutableCoin(MutableCoin toCopyFrom) {
        this.value = toCopyFrom.value;
    }

    public void setValue(int value) {
        this.value = value;
    }

    public static void main(String[] args) {
        MutableCoin myCoin = new MutableCoin(0);
        List<MutableCoin> bankAccount = new ArrayList<MutableCoin>();
        for (int coinValue = 0; coinValue < 5; coinValue ++) {
            // make a copy
            myCoin = new MutableCoin(myCoin);
            myCoin.setValue(coinValue);
            bankAccount.add(myCoin);
        }
        for(MutableCoin coin : bankAccount)
            System.out.println(coin);
    }

    @Override
    public String toString() {
        return "MutableCoin: " + value;
    }

}

This will also work. I hope this helps.