Setting a constructor and few objects in java

Question

So I have this class called memoryStructure which is supposed to represent support structure for object called Student which has firstName, lastName, age and university.

This is trouble part:

public class MemoryStructure { 

    private Student[] memoryArray;  
    private int currentSize; 
    private int arraySize; 

    // constructor goes here 

    /** 
     * Adds the object <code>student</code> to the collection right after the 
     * last element in the current collection. 
     * 
     * @param student 
     * Object to be added to the collection.
     */ 

    public void add(Student student) { 

    // to be done

Can someone at least go a bit through explaining what should I write here? It's my first touch with Java and I am having a bit of trouble.

Also, should student just be made as another class with his constructor and setters/getters for changing it through memoryarray in above class?

I am also supposed to make new array once currentarray size gets bigger than arraySize, it should be 2*arraySize and have all elements from previous array copied in. Is this also put in constructor?

I hope this question isn't too broad.

Answer 1

Most of the answers suggest you to use an ArrayList , which is a good idea but I suppose that it will defeat the purpose of your exercise which requires you to work with an array.

Can someone at least go a bit through explaining what should I write here? It's my first touch with Java and I am having a bit of trouble.

Firstly you'll have to define a constructor for this class (note that this is pseudo code).

MemoryStructure (size) {
   define the arraySize
   create the array with this size
   initialize the currentSize to 0 
}

Now take a look at the add method. What you need to do is to store the Student object you pass in parameter in your array. So it will roughly look like this:

public void add(Student student) {
    if(currentSize == arraySize)
         increase the capacity of the array
    storeStudent in the array at the right index (you can use currentSize since arrays are 0 base indexed)
    increment currentSize
}

Now you have a little idea about what you have to do.

Also, should student just be made as another class with his constructor and setters/getters for changing it through memoryarray in above class?

Yes it have to be another class.

I am also supposed to make new array once currentarray size gets bigger than arraySize, it should be 2*arraySize and have all elements from previous array copied in. Is this also put in constructor?

No, as I said above you should check for resizing your array when you want to add a Student in your array so it has to be in the add method and not in the constructor.

Notes:

1. The method copyOf will be useful when you'll want to increase the size of your array.
2. If you have a method to remove a student from the array, take a look at the arrayCopy method which will help you to resize it accordingly

Answer 2

epoch's answer to use a list is actually a better approach if you're not restricted to using arrays. Anyway, I hope I'm not spoiling the fun of learning the language by posting the code.

It should look something like this

public void add(Student student) {
   // assuming currentSize = 0 and arraySize = *something* in the constructor and that    memoryArray = new Student[*something*]
   if(currentSize == arraySize){
      arraySize = arraySize*2;
      memoryArray = Arrays.copyOf(memoryArray, arraySize);
   }
   memoryArray[currentSize] = student;
   currentSize++;
}

Answer 3

In constructor I would do nothing (all int variables get automatically initialized with zeros, exactly what we need).

In add() method I would create the first array, if memoryArray is null . Then I would check, if the size of the array memoryArray.length less or equal to currentSize . If so we have to extend the array. You can do it like this:

Student[] newMemoryArray = new Student[memoryArray.length * 2];

Then you have to copy the old array into the new one using System.arraycopy() method. I will leave mastering the right arguments to you. Then you just throw the old array away and replace it the the new one

memoryArray = newMemoryArray;

then you just add the new element to the array and increment the currentSize :

memoryArray[currentSize] = student;
currentSize++;

That's it. So, your add() method looks like this:

public void add(Student student) { 
    if (memoryArray == null) {
        memoryArray = new Student[10];
    }
    else if (memoryArray.length <= currentSize) {
        Student[] newMemoryArray = new Student[memoryArray.length * 2];
        // Copy old array into the new one
        memoryArray = newMemoryArray;
    }

    memoryArray[currentSize] = student;
    currentSize++;
}

PS. The arraySize is not really needed here, since the array itself knows it length.

Answer 4

Instead of Array you can use ArrayList.

If you want to add via constructor code will be

public class MemoryStructure { 

 private ArrayList Students = new ArrayList();  

 MemoryStructure(Student student){
          Students.add(student);
        }
}

Answer 5

You can use ArrayList instead, like:

ArrayList<Student> memoryArray = new ArrayList<Student>();

Then :

public void add(Student student) { 
    memoryArray.add(student);
}

Answer 6

import java.util.ArrayList;
import java.util.List;


public class MemoryStructure {

    private static List<Student> memoryArray; 

    public void add(Student student) { 
        if(memoryArray ==null) {
            memoryArray = new ArrayList<>();
        }
        memoryArray.add(student);
    }

    public int getCurrentSize() {
        if(memoryArray ==null) {
            return 0;
        }
        return memoryArray.size();
    }


}

public class Student {


}