在添加Java之前检查对象是否在列表中

Question

Ok so I am having problems with my add method on how to implement checking if an object is already in the arraylist before adding a new one.

I would also like to create a remove method but everytime I attempt to use it I end up with a null pointer exception.

import java.util.*;

public class ContactsArrayList extends AbstractList{

    private Contact[] contacts;
    private int size;

    public ContactsArrayList(){
    //initializes contactArray with a capacity of 1
        contacts = new Contact[1];
        size = 0;
    }
    public ContactsArrayList(int capacity){
        if(capacity < 0){
            try {
                throw new Exception("Capacity: must be greater than zero.");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        contacts = new Contact[capacity];
        size = 0;
    }
    public int size(){
        //returns size of list
        return size;
    }
    public boolean isEmpty(){
        return size == 0;
    }
    private void listRangeCheck(int index){
        //checks if index is within range as built in debugging method
        if(index >= size || index < 0){
            throw new IndexOutOfBoundsException("Index: " + index + " is not within the list.");
        }
    }
    public Contact get(int index){
        //returns index at Contact at specified index
        listRangeCheck(index);
        return contacts[index];
    }
    public void capacityCheck(int minCapacity){
        //checks current capacity if capacity is less than required,
        // array copies its current values over to an array double the size
        int oldCapacity = contacts.length;
        if(minCapacity > oldCapacity){
            int newCapacity = (oldCapacity * 2);
            if(newCapacity < minCapacity)
                newCapacity = minCapacity;
            contacts = Arrays.copyOf(contacts, newCapacity);
        }
    }
    public boolean add(Contact contact){
    //appends the specified element to the end
        capacityCheck(size + 1);
        contacts[size++] = contact;
        return true;
    }
    public int capacity(){
    //returns ContactArray size
        return contacts.length;
    }
    public void sort() {
        //sorts the specified contact array list
        List<Contact> c_list = new ArrayList();
        c_list = Arrays.asList(contacts);
        Collections.reverse(c_list);
        c_list.toArray(contacts);
    }
}

Answer 1

Simple as always:

private boolean contains(Contact contact){  
    for (int i=0; i<contacts.length; i++){      
        if (<condition to know if the 'contact' object exists>) return true;        
    }
    return false;
}

But if you will consider using ArrayList<Contact>() you can simply use it's contains()

Answer 2

Please clarify. Will you be checking if two separately created objects have the same contents? Or will you be checking if you have added that object before?

All objects have a .equals() method, which checks that both references are the same object.

To extend Roey:

private boolean contains(Contact contact) {
    for (int i = 0; i < contacts.length; i++) {
        if (contacts[i].equals(contact)) {
            return true;
        }
    }
    return false;
}

In your add method, make a call to the contains method:

if contains(contact)
    return true
else
    add object

As to your remove method, you can also use the contains method. General pseudo code:

if contains(contact)
    shift all elements above contact down one slot
    decrement size
    return true
else
    return true

Answer 3

如果要比较对象，建议您提供一个属性进行比较，否则，如果存在的对象与您要添加的当前对象之间存在一些差异，则最终将再次插入该对象。